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Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation (Enhanced Version for 286/386/486 Computers) (C) Copyright David H. Ransom, Jr., 1989-1992 All rights reserved. Version 9218 April 26, 1992 by David H. Ransom, Jr. Rancho Palos Verdes, California, USA BBS: (310) 541-7299 Program STSORBIT PLUS Satellite Orbit Simulation Page i TABLE OF CONTENTS ----------------- INTRODUCTION ......................................................1 HARDWARE AND SOFTWARE REQUIREMENTS ................................5 PROGRAM DESCRIPTION ...............................................6 STSORBIT PLUS FILES ...............................................8 PROGRAM SETUP AND USAGE NOTES .....................................10 Slow Computers and 80x87 Mach Coprocessor Chips .................10 Using a RAM Disk ................................................11 Starting Program STSORBIT PLUS ..................................12 Predicting Visible Satellite Passes .............................13 Known STSPLUS Problems and Bugs .................................14 PROGRAM OPERATION .................................................17 THE STSORBIT PLUS GROUND TRACK DISPLAY ............................19 World Maps ......................................................19 Quadrant Maps ...................................................19 Zoom Maps .......................................................20 Location Maps with Isocontours ..................................21 Tracking Station Maps with Isocontours ..........................21 Satellite Motion Maps ...........................................22 On-line Help (F1 Key) ...........................................22 Satellite Position and Orbit Projections ........................23 User's Circle of Visibility .....................................23 Spacecraft Circle of Visibility .................................24 TDRS Satellite Features .........................................24 Ground Tracking Stations and .TRK Files..........................25 STSORBIT PLUS MAIN MENU ...........................................28 F1 Program STSORBIT PLUS Demonstration .........................28 F2 Read NASA/NORAD 2-Line Elements .............................29 F3 Read Prior Mission Information from *.INI File ..............30 F4 Enter New Orbital Information ...............................31 F5 Adjust Orbital Parameters ...................................32 2-Line Elements Model .......................................32 Simple Orbital Model ........................................33 F6 Set Elapsed Time Option (2-line elements only)...............34 F7 Set FILENAMES and Paths .....................................35 F8 Set Program TIME and DATE....................................36 F9 DOS Shell ...................................................39 F10 Set STSORBIT PLUS Program Options and Features ..............39 ENTER Resume Mission ...........................................39 ESC Quit STSORBIT PLUS and Save Current Mission ..............40 PROGRAM OPTIONS AND FEATURES MENU .................................41 F1 Program STSORBIT PLUS Information ...........................41 F2 Set New Local Coordinates ...................................41 F3 Set Display Features ........................................43 F6 Set Map Center and Size .....................................43 F7 Set for SLOW COMPUTER or NO COPROCESSOR .....................44 F9 Set UTC Time Offset and Daylight Flag .......................44 F10 Enable/Disable Printer Logging ..............................45 SET DISPLAY FEATURES ..............................................47 Program STSORBIT PLUS Satellite Orbit Simulation Page ii F1 Display LOCAL Circles of Visibility .........................47 F2 Display TDRS Coverage .......................................47 F3 Enable/Disable Additional Map Grid Lines ....................48 F4 Display Tracking Stations ...................................48 F5 Show Ascending & Descending Node Data .......................48 F6 Display Spacecraft Circle of Visibility .....................48 F7 Display South Atlantic Anomaly Zone .........................49 F8 Display Terminator, Sun and Spacecraft Lighting .............49 F9 Select Distance Units: NM or KM .............................50 F10 Select Satellite Coordinates: RA/DEC, AltAz or XYZ ..........50 ACTIVE KEYS DURING GROUND TRACK DISPLAY ...........................51 STSORBIT PLUS'S ORBITAL MODELS ....................................54 ADJUSTING ORBITAL PARAMETERS (Simple Orbital Model) ...............56 STSORBIT PLUS Revision History ....................................58 Program STSORBIT PLUS Satellite Orbit Simulation Page 1 INTRODUCTION ------------ Program STSORBIT PLUS is an enhanced version of STSORBIT, my original orbital tracking and display program. As a general rule, a 286 or better computer (AT-class IBM compatible) is recommended, and a math coprocessor chip will significantly improve performance. See the section HARDWARE REQUIREMENTS for additional information and discussion. The program is intended for use during Space Shuttle missions, for simulating a Space Shuttle mission, and for general satellite tracking using NASA/NORAD 2-Line Orbital Elements. STSORBIT PLUS is copyrighted software; you are hereby granted a non- exclusive license for non-commercial or educational use only. Agencies of the U.S. Government are also hereby granted a non-exclusive license for internal use. Use STSORBIT PLUS if you like it, discard it if you don't. There are no warranties of any kind. If you wish to use STSORBIT PLUS commercially, write for license information. The only request I make of users is that they take the time to complete and return the confidential questionnaire in file README. The questionnaire gives users a chance to offer comments and suggestions, and lets me know that people use and appreciate STSORBIT PLUS. Registration of STSORBIT PLUS is inexpensive and optional -- but will be appreciated and will encourage me to continue supporting and enhancing the program. Program STSORBIT PLUS (which I will usually refer to as STSPLUS from here on) is intended to display the position and ground track of an orbiting satellite on a selection of maps ranging from a full map of the world to zoom maps showing considerable detail. The program has special features implemented at the request of NASA astronauts and others for use during a NASA Space Shuttle mission. When used with NASA/NORAD 2-Line Elements, the position and ground track of a variety of satellites, such as the Space Shuttle, the Hubble Space Telescope, the Gamma Ray Observatory, or the Soviet MIR Space Station, may be displayed. Special Location and Tracking Station displays show concentric isocontours, circles of equal satellite altitude; these special maps can be especially valuable for visual or amateur radio sightings. The initial premise for STSORBIT was to attempt to duplicate the wall map in NASA's Mission Control Center in Houston, Texas. Before I started this project, I had seen several other programs which tracked satellites but each fell short of my map and display objectives for one reason or another. I therefore set out to do the job myself. STSORBIT and now STSORBIT PLUS have been the result. Since then other programs have appeared which produce similar information, most notably Paul Traufler's excellent TRAKSAT (which was inspired by STSORBIT). It may be, of course, that others will judge this effort lacking as well. One problem is that of size and resolution: the wall map at Mission Control Center is some twenty feet wide with an impressive pixel resolution, very different from the typical personal computer and the size and resolution of its monitor. The NASA wall map shows essentially the entire globe in a cylindrical projection; STSORBIT uses a cylindrical projection but restricts the vertical display to latitudes from +85 degrees to -85 degrees in order to achieve reasonable proportions and vertical resolution while at the same time showing recognizable land features. Initially, and as a consequence of a lack of accurate orbital data for Space Shuttle missions while they were in progress, I did not try to be Program STSORBIT PLUS Satellite Orbit Simulation Page 2 especially precise with respect to the orbital mathematics. Additionally, mathematical complexity had to be held to a reasonable minimum if older computers not equipped with a math coprocessor were to be able to maintain the presentation in real time. My somewhat casual attitude toward mathematical precision changed with the launch of the Hubble Space Telescope (HST) and the regular availability of NASA/NORAD 2-Line Elements via modem from TS Kelso's Celestial BBS. Until HST, I had been content to update the orbital data occasionally during the course of a typical five day mission and live with the errors inherent in my original simple orbital model. The accuracy of that model degrades rapidly after five or ten orbits and, although it may be manually adjusted from time to time during a mission, more accurate data are now readily available prior to a launch and during a mission. The NASA SpaceLink BBS in Huntsville, Alabama began posting 2-line orbital elements for the Space Shuttle in early 1991 due in part to my persistent and continuing suggestions; Bill Anderson and Jeff Ehmen, sysops of the SpaceLink BBS, are continually upgrading the services available. Beginning in mid-1990, therefore, STSORBIT was extensively modified to read orbital data from these NASA 2-line elements and thereby maintain significantly improved accuracy over long periods of time. As an incidental benefit, the ground tracks of other satellites (such as the Russian space station MIR) could also be displayed. At present, the orbital model (SGP4) used with 2-line elements is accurate only for low Earth orbits. Deep space orbits, defined as orbits having an orbital period greater than or equal to 225 minutes, require a more complex orbital model (SDP4) which takes into account solar and lunar perturbations for best accuracy. STSPLUS calculates data and displays a ground track for deep space objects but the accuracy of these data has not been validated; it is believed to be "reasonably" accurate. At about the same time, STSORBIT also found its way to the NASA Johnson Space Center in Houston, Texas. Quite a few individuals from JSC sent me comments and suggestions for further improving the program, among them Ron Parise of the STS-35/ASTRO-1 crew. Ron suggested that I make modifications to allow the display of Mission Elapsed Time (MET) for shuttle missions while using the NASA 2-line elements. This would allow both the higher accuracy of the NASA 2-line orbital data and permit following the mission timeline using MET. Since launch time and date are not included in the 2-line elements but are required to compute MET, these data must be entered independently. Another suggestion from Ron and others was to include the Sun, Sun terminator (calculated at Mean Sea Level), and spacecraft lighting conditions to determine if the spacecraft is visible. Not satisfied with the somewhat rough map used with STSORBIT, I upgraded the maps to use a modified version of the World Data Base II. This had the desired effect, to the point where rivers and other landmarks could easily be recognized on the monitor and on downlinked orbiter television. As a side effect, however, the processor overhead increased dramatically -- to the point where some slower computers not equipped with a math coprocessor were unable to keep up. I have therefore essentially "frozen" the original STSORBIT program (except for minor updates) and created a new program, STSORBIT PLUS intended for the faster, more capable processors. Since mid-1991, STSPLUS has also spread throughout the various NASA Centers and around the world. In addition to NASA and individual users all over the world, STSPLUS and STSORBIT are also being used in an educational setting. As many as 1100 high schools participated in the Inspire Project, a VLF propagation test Program STSORBIT PLUS Satellite Orbit Simulation Page 3 flown on STS-45 and for which STSPLUS was one of the recommended tools. At a middle school in Kansas, the program is projected in the school auditorium from time to time during a mission to show the children graphically what is happening and to give them a sense of "real time" participation in our space program. At an Air Force training facility, STSORBIT is one of many tools used to prepare Air Force officers for their duties in the Air Force Space Command. The program was widely distributed at a recent National Association of Science Teachers convention and by radio amateurs at regional "ham fests". In perhaps its most prestigious installtion, STSPLUS is the software used by the NASA/JPL Multimission Computer Control Center in Pasadena, California, to display the ground track of Earth-orbiting satellites. A brief biographical note: I am a retired physicist and engineer who spent all of his professional life in the world of electronics, data communications and, more recently, computers. As a young man I was actively involved in the early American space program, including projects such as Ranger, Mariner, Mercury, Gemini, and Apollo; my interest in space has continued to this day. The desire to "keep in touch" with our Space Shuttle missions was one of the principal incentives in the development of this software. I continue to be astonished that a relatively inexpensive personal computer is sufficient to perform calculations that pushed the limits of our best mainframe computers only a decade or so ago. If STSORBIT PLUS also serves to help spark the interest of young people in science and technology or can be a learning tool at any level, I will have more than achieved my goal. For those who are interested in our space program and who have access to a modem, I recommend NASA's SpaceLink Bulletin Board System in Huntsville, Alabama, (205) 895-0028, available twenty four hours per day, 300 to 9600 baud. NASA SpaceLink, located at the NASA Marshall Space Flight Center, provides a wealth of information on NASA and its projects. 2-line orbital elements for a Space Shuttle mission are usually available while the mission is in progress. In addition to educational materials and software (including my programs STSORBIT PLUS, STSORBIT and JPLCLOCK), general information on NASA programs and plans, news releases, and graphics images from prior spacecraft missions such as Voyager, SpaceLink also devotes a complete section to current news and information on the Space Shuttle. I particularly appreciate the STS Mission Press Kit, posted about two weeks before each mission, which provides a great deal of information on the upcoming mission, payload and crew as well as broadcast schedules on NASA Select Television, Satellite F2-R, Transponder 13. Mission status reports are generated daily during the course of a mission. I regularly call SpaceLink and post files of interest on my own RPV ASTRONOMY BBS. Special thanks to Paul Traufler for his friendship and encouragement over the past several years. Our regular telephone conversations have generated many a new idea and the synergism has been beneficial to us both. Our two programs, STSORBIT and TRAKSAT, have engaged us in a friendly rivalry which has, I think, improved both programs many fold. I may have provided the initial spur to Paul to write TRAKSAT (in order to improve on my "sloppy orbital math", as Paul described it) but TRAKSAT has in turn kept my nose to the grindstone. The emphasis of the two programs is slightly different, with STSORBIT concentrating on the graphical display and TRAKSAT on high precision analytical and predictive techniques. I strongly recommend TRAKSAT for the serious satellite tracker. My thanks as well for Paul's help in upgrading STSORBIT to use the NASA/NORAD 2-Line Program STSORBIT PLUS Satellite Orbit Simulation Page 4 Elements. Thanks also to Rob Matson for offering comments and code to help me implement STSPLUS. Rob's fine program, SKYMAP, provides high accuracy sky/star maps with or without satellite tracks. Finally, my thanks to all those individuals who have taken the time to write or leave a message on my BBS with comments and suggestions. While I haven't implemented every suggestion, many are now included and the feedback is most welcome. For current orbital information (if a mission is in progress), current NASA/NORAD 2-line element sets and the most recent version of the program, call my RPV ASTRONOMY BBS at (310) 541-7299. If the BBS has not answered after the third ring, hang up, wait TWO MINUTES, then call back; the system has a power controller and if the system is off it takes that long for the computer to start up and do its housekeeping chores. The system has well over 1,000 regular users and is often busy, so please be patient. If you do not have access to a modem, you may send US$10.00 as a donation to cover materials, postage and handling for a copy of the current version of STSORBIT PLUS; please specify 5-1/4" 360K or 3-1/2" 720K disks. Please allow two to three weeks for shipment. David H. Ransom, Jr. 7130 Avenida Altisima Rancho Palos Verdes, CA 90274 Program STSORBIT PLUS Satellite Orbit Simulation Page 5 HARDWARE AND SOFTWARE REQUIREMENTS ---------------------------------- An AT-class computer equipped with a 286 processor (running at 8 MHz) and a 287 math coprocessor chip is the minimum system used for all program testing and development. While other systems may give acceptable performance, this minimum configuration assures that all features will execute as described and in real time. Performance with 386 and 486 systems will be considerably superior to 286 systems. Note that NO TESTING is performed on systems not equipped with a math coprocessor chip. The following minimum hardware is recommended: 286/386/486 IBM-compatible computer 287/387 math coprocessor chip VGA color display Hard disk RAM disk with at least 500K space The 287/387 math coprocessor chip is HIGHLY RECOMMENDED and is required for some processors to operate in real time. The calculations relating to orbital mechanics are very complex and STSPLUS will use the coprocessor chip if one is equipped; performance is improved by about an order of magnitude. Other "fast" processor and coprocessor combinations may yield acceptable performance. A SLOW mode is provided to accommodate slower machines. However, math coprocessor chips are now reasonably inexpensive, particularly for 286 systems, and the performance improvement is impressive and well worth the modest cost. As an example, my vintage Zenith laptop is equipped with an 80C88 processor and an 8087 math coprocessor and is just able to keep up in real time when running at a clock speed of 8 MHz (although map drawing times are very slow). However, an 8 MHz 286 (AT- class) computer without a math coprocessor is NOT able to execute the program correctly except in the SLOW mode and map drawing times are painfully slow. STSPLUS is intended to be used with an EGA or VGA video adapter and a color monitor; with these adapters, the display is in color. Because of its improved vertical resolution, the VGA is recommended over the EGA. A monochrome VGA display with shades of gray is also suitable for use with the program. Because of hardware limitations, CGA and HGC systems can only present graphics in monochrome; although those display adapters are supported in current versions of STSPLUS, that support may NOT continue in future versions. The original STSORBIT will continue to support CGA and HGC monitors. A hard disk is recommended for performance in program and file loading and for storage of orbital elements files. A RAM disk with sufficient space to hold the program and its various data files is also recommended for improved performace, especially for reduced map drawing times. However, the program will execute correctly on floppy disk based systems with 640K base memory provided the system is otherwise equipped as suggested. Although the program may execute properly on other software operating systems, STSPLUS has been designed and tested using standard configurations of Microsoft DOS 3.3 and 5.0. No optional Terminate and Stay Resident programs (TSR's) or "shell" programs have been tested. Third party memory management programs and Digital Research DRDOS 6.0 may experienc problems with internal memory allocation performed by the Microsoft BASIC Compiler; there is no known remedy. Program STSORBIT PLUS Satellite Orbit Simulation Page 6 PROGRAM DESCRIPTION ------------------- A typical Space Shuttle orbit is nearly, but almost never exactly, circular with an altitude of approximately 160 nautical miles to a maximum of approximately 300 nautical miles and an inclination of about 28 degrees through about 57 degrees. Occasional missions, especially military missions, fly at higher altitudes and/or inclinations and often use more elliptical orbits. Little of this information is known to very good accuracy by the casual listener. Initially, therefore, the interested would-be mission observer may have only the time and date of launch to initialize a tracking program. Given the geographical coordinates of the Kennedy Space Center, assuming a circular orbit, and using the typical altitude and inclination of a Space Shuttle orbit, the data should be sufficient to give at least a rough idea of the Shuttle's position for the first several orbits. After that, additional information is required if the position is going to be very close. Fortunately, NASA does announce the orbital altitude (for non-military missions at least) and this information is usually reported in the media. The reader is cautioned that the popular press and television often (but not always) convert the orbital altitude to statute miles with the inevitable confusion resulting. If a television viewer has access to NASA Select Television on Satcom F2R, Transponder 13, he will periodically see Mission Control Center's huge wall map on which the current position of the Space Shuttle is always displayed. Using this display, the careful viewer can make a visual estimate of the longitude of the ascending or descending node, the point at which the orbit crosses the Earth's equator in the Northbound or Southbound direction respectively. Not exactly high-tech data acquisition, but better than nothing at all! Given all of these uncertainties, I did not feel it necessary or worthwhile to try to hone the mathematics of my "simple" orbital model in program STSORBIT beyond that required to give an approximate position in real time. Coincident with the Hubble Space Telescope mission I added the J2 perturbation factor for improved accuracy. As an example, STSORBIT was used to track STS-31, Discovery and the Hubble Space Telescope, and gave an accurate position over more than 25 orbits. Within a day of launch, the NASA/NORAD 2-line elements are usually available and will yield a more accurate position over longer time periods provided no orbital maneuvers are made. Using NASA/NORAD 2-line elements is quite simple. No adjustment of orbital parameters is necessary when using the 2-line elements. All that is required is to obtain the current 2-line elements file (from my BBS or elsewhere), enter the name of the file (for example, NASA877.TXT) and the name of the desired satellite (i.e. "HST" for Hubble Space Telescope or "STS" for the Space Shuttle). STSPLUS takes care of all the rest. The 2-line element set available at the time of this release of STSPLUS is included in the program files. The data for each satellite included in the 2-line element set is referenced to a specific date and time, the "Epoch" of the data. As a general rule, orbital calculations will be relatively accurate for 10 to 20 days after the Epoch date; the lower the orbit, the greater the effect of factors such as atmospheric drag and the less accurate the calculations will be as time passes. Users who prepare their own 2-line element files are cautioned to make certain no extraneous lines are included, that the satellite name is on a single line, and that the two data lines exactly conform to the 69- Program STSORBIT PLUS Satellite Orbit Simulation Page 7 character/line NASA/NORAD format. STSPLUS displays a portion of the Earth using a cylindrical projection of the surface area. For the full world map, this extends almost from one pole to the other; a small area near each pole (approximately five degrees) is omitted to maintain optimum map proportions. The map shows most of the Earth's land boundaries, and continental areas and major seas and oceans are easily recognizable. Twelve quadrant maps display one quarter of the Earth, and zoom maps may be selected which display a field of view adjustable from 180 degrees to 45 degrees. With the quadrant and zoom maps, automatic map generation may be enabled to endure that the satellite is displayed. The display shows the selected satellite as a small symbol or icon, the projected low-Earth orbit ground track for approximately the next two orbits (three hours), and the ground track for the past orbit (one and a half hours). The program may be operated in real time, in simulated time, or in "fast time", which is ten or sixty times normal time. For the "simple orbital model", the orbital parameters may be manually entered or adjusted for the desired mission. A pre-programmed demonstration may be run to permit the user to become familiar with the program and its display. The lower section of the screen displays current data about the mission in progress. Program STSORBIT PLUS Satellite Orbit Simulation Page 8 STSORBIT PLUS FILES ------------------- STSORBIT PLUS is normally distributed in archived format using either the PAK or ZIP format. Note that all files for STSORBIT PLUS are called "STSPLUS" in order to conform to DOS filename requirements. The following files are usually included: STSPLUS.EXE Main program (required) STSPLUS.DOC Documentation (not required) STSPLUS.ICO Icon for WINDOWS 3 (optional) STSPLUS.MPT World map data (required) STSPLUS.TRK NASA Tracking Stations (optional) STSPLUS.CTY City coordinates (optional) STSPLUS.INI Initialization data (optional) MSHERC.COM Hercules driver (required for HGC) NASAnnn.TXT 2-Line Elements (optional) NASA.TRK NASA Tracking Stations (not required) USSR.TRK Soviet Tracking Stations (not required) INTELSAT.TRK INTELSAT Tracking Stns (not required) SPACENTR.TRK Other Tracking Stations (not required) README STSPLUS Confidential Questionnaire QUICK.DOC Quick Start Instructions Files noted as "(required)" must be in the current default directory for program operation. Files noted as "(optional)" do not need to be in the default directory when STSPLUS is operated but provide additional features or information if present. *** IMPORTANT NOTE *** File STSPLUS.INI contains initialization data from previous runs of the program. If file STSPLUS.INI is not present it will be created. Note that if STSPLUS.INI was written by a prior version of STSPLUS, all data will be ignored and the program must be initialized as if being run for the first time. File NASAnnn.TXT (where "nnn" will be a number such as "940") is a set of NASA/NORAD 2-line elements as of the date of the file. Note that the 2-line elements should only be used for ten to twenty days after the epoch date for each satellite if reasonable accuracy is to be maintained. Current orbital elements are posted on my bulletin board system two or three times per week. Other files with 2-line elements are also available; they typically have names like GSFCnnn.TXT or N2L-nnn.TXT for general satellites, and STSmmJnn.TXT for Space Shuttle missions. Space Shuttle orbital elements are usually posted at least daily during missions; because of orbital maneuvers, Space Shuttle elements more than 24 hours old may yield inaccurate positions. Files with filetype .TRK are tracking station locations or other locations of interest which may be plotted on the map. These files may be Program STSORBIT PLUS Satellite Orbit Simulation Page 9 created or edited with a standard ASCII editor. File NASA.TRK is the same as file STSPLUS.TRK; if you wish to use a different TRK file, use Function Key F7 from the Main Menu to select the desired file. Other files, such as 2-line elements for an upcoming Space Shuttle mission or a mission in progress, may be included from time to time. Files with filetype .TXT are normally 2-line orbital elements. Some common satellite name abbreviations are: "STS" = Space Shuttle; "HST" = Hubble Space Telescope; "GRO" = Gamma Ray Observatory; and, "MIR" = Soviet MIR Space Station. Program STSORBIT PLUS Satellite Orbit Simulation Page 10 PROGRAM SETUP AND USAGE NOTES ----------------------------- The following notes may prove helpful in setting up STSPLUS to operate most efficiently on your system or to provide hints in ways that some of the program features may be used to advantage. Slow Computers and 80x87 Math Coprocessor Chips ----------------------------------------------- STSPLUS has been designed for 80286/80287 or better computers eqipped with an EGA or VGA color display. While the program can be executed on older 8088 (XT-class) computers and/or computers without math coprocessor chips, performance is seriously degraded. But it would seem that warnings and suggestions can NEVER convince people that the calculations required for orbital mechanics are very complex and tax even a powerful computer. The best computers we had a decade or more ago had trouble doing what I now take for granted on my personal computer! Not all personal computers are created equal. Further, the 80x87 math coprocessor chip can do many of the calculations ten or twenty times faster than the main processor. For 8088 (XT-class) and 80286 (AT-class) computers, this makes a tremendous difference AND for a very modest cost, often under $100. I really don't want to hear from users how slow this program can be on older machines; I recommend the original STSORBIT if you want the best performance from an 8088 computer. If, in spite of these suggestions, you insist on using STSPLUS on your old clunker, here are a few cautions and reminders. 1. Especially at startup and when redrawing the maps, long time delays can be expected -- on the order of a minute or more in some cases. No special messages are presented on the screen during these delays since they are relatively short on faster computers. Note also that the .EXE file is compressed to save disk space and is decompressed at load time; this may cause a delay on any system, with or without a math coprocessor. 2. The original CGA display, even when equipped with a color monitor, can display reasonable resolution graphics (320x640) ONLY in monochrome. Your color CGA monitor buys you nothing for graphics that are quite inferior to the EGA and VGA. If you have a monochrome monitor, be sure to include the "/M" command line option to force monochrome. The program may otherwise fail with or without an error message. 3. If you don't have a math coprocessor or are using a slow computer, be SURE to enable the SLOW option using Function Keys F10+F7. Even a 286 or 386 computer may be unable to keep up with the calculations in the normal mode. The clue is to watch the LOCAL and SIMULATION times at the lower left of the screen; if these times are not within one or two seconds of each other, you must use the SLOW mode. The SLOW mode slows the screen update interval to once every 10 seconds from once every second to allow more time for calculations. 4. Even with the SLOW mode, you may not be able to turn on all the options and features you'd like to use and still keep up. Too bad, but only a math coprocessor chip can solve that problem for 286 computers Program STSORBIT PLUS Satellite Orbit Simulation Page 11 and there is no solution for 8088 computers running at the original 4.77 MHz. Some features, such as the solar terminator, may have to be disabled. Experiment with your computer to see what features permit proper operation. Using a RAM Disk ---------------- Because of program memory requirements, a RAM disk should be used only if your computer is equipped with expanded or extended memory. Using a RAM disk in conventional memory (the memory up to 640K) will probably use memory that STSPLUS (as well as most other programs) may need to operate correctly. The basic files required for operation of STSPLUS require a minimum of about 256K of RAM disk; additional map or satellite data files could as much as double that amount. A RAM disk of at least 500K is therefore recommended. The actual size RAM disk you can provide will be dependent upon how much memory is equipped in your computer. In order to use the RAM disk effectively, all essential files must be copied from your hard disk to the RAM disk. The following program files are recommended: STSPLUS.EXE STSPLUS Main Program file STSPLUS.INI STSPLUS Initialization file STSPLUS.MVF STSPLUS Map file STSPLUS.CTY STSPLUS City file (optional) STSPLUS.TRK STSPLUS Tracking Station file (optional) In addition, you must copy all 2-line orbital elements files you plan to use, files such as NASAnnn.TXT, GSFCnnn.TXT, and so forth. Remember that all RAM disk files are lost when you turn off your computer! I recommend that you build a batch file which copies all of the files from the hard disk to the RAM disk and, when you are finished with STSPLUS, copies the file STSPLUS.INI back to your hard disk. Saving STSPLUS.INI on the hard disk ensures that the next time you run STSPLUS, the previous settings will be restored. The following is a sample batch file which will copy the STSPLUS files from the hard disk (drive F: and subdirectory STSPLUS in this example) to the RAM disk (drive J: in this example), run program STSPLUS with the /R (resume) option, then copy the STSPLUS.INI file back to the original drive on the hard disk. Many variations are possible to suit individual needs. J: COPY F:\STSPLUS\STSPLUS.EXE COPY F:\STSPLUS\STSPLUS.INI COPY F:\STSPLUS\STSPLUS.MVF COPY F:\STSPLUS\STSPLUS.CTY COPY F:\STSPLUS\STSPLUS.TRK COPY F:\STSPLUS\NASA*.TXT COPY F:\STSPLUS\GSFC*.TXT COPY F:\STSPLUS\JSCEL*.TXT COPY F:\STSPLUS\STS*.TXT STSPLUS /R COPY STSPLUS.INI F:\STSPLUS Program STSORBIT PLUS Satellite Orbit Simulation Page 12 Starting Program STSORBIT PLUS ------------------------------ Before starting program STSPLUS for the first time, delete the file STSPLUS.INI if it has been created by a previous version of STSPLUS. The format of the .INI file changes periodically from version to version and it is safer to "start from scratch". To start program STSPLUS, enter one of the following commands: STSPLUS Automatic monitor, CGA/HGC/EGA/VGA STSPLUS /EGA Force EGA (or lower) monitor STSPLUS /CGA Force CGA monitor STSPLUS /M Force monochrome operation, EGA/VGA STSPLUS /R Resume last mission automatically Only one display option (/EGA or /CGA or /M) may be used. Options may be combined and entered in any order. For example, using the following command will resume the prior mission and force EGA mode: STSPLUS /R/EGA * * * * * Hercules Graphics Card USERS NOTE: ---------------------------------- Run the program MSHERC prior to running STSPLUS. This Microsoft program works with compiled BASIC programs to enable use of the Hercules Graphics Card. One user reported that setting the HGC to FULL and selecting Page Zero (using software supplied with the HGC) was sufficient for proper operation with program STSPLUS. At least one HGC "clone" user reported improper operation. * * * * * If you have already run STSPLUS (or if you have file STSPLUS.INI) and simply wish to resume that same mission, use the /R (resume) command line option: STSPLUS /R STSPLUS will sense the monitor type, enable color for EGA and VGA systems, read the map data as usual, then proceed directly to plotting the mission. The data from the last run, as saved in file STSPLUS.INI, is used to initialize the program. If 2-line elements were used, that file must also be present. Once started in this manner, pressing the ENTER key after plotting has started will return to the Main Menu. Program STSORBIT PLUS Satellite Orbit Simulation Page 13 Predicting Visible Satellite Passes ----------------------------------- One of the best uses for a satellite tracking program is to show when a satellite of interest will be visible from a specified location. Using STSPLUS, my wife and I have spotted three different Space Shuttle missions, the Hubble Space Telescope, the Russian MIR Space Station, and many others with the naked eye. The trick, of course, is knowing when and where to look for the satellite. There are a number of satellite tracking programs, most notably Paul Traufler's TRAKSAT, which can generate a tabular output for a given satellite or group of satellites telling when and where the satellite will be visible. STSPLUS does not have those tabular capabilities and relies instead on its graphics capabilities to make pass predictions quick and easy. Here are a series of suggestions to help you check for visible passes for a satellite of interest. 1. Set up STSPLUS for the desired satellite. First, use Function Keys F10+F3+F5 to set the satellite ground track to LINE or BOTH (rather than DOTS). This makes the ground track more visible on the zoom map displays. Then use Function Key F2 to select the satellite's 2-line orbital elements from the available satellites. Typical files have names like NASA971.TXT or GSFC037.TXT or, for the space shuttle, STS45N03.TXT. The numbers will change from time to time or mission to mission. Use one of the quadrant maps or the world map to make sure that the satellite is correctly displayed. 2. Remember that under most circumstances a "visible pass" means that the satellite is in full sunlight and the viewer is in darkness. Although there are exceptions in unusual situations, this restricts the times for visible passes to the several hours prior to dawn and the several hours after sunset. (Note, however, that "visible" to a ham radio operator simply means above his horizon!) In most cases, the Space Shuttle and satellites such as MIR Space Station and Hubble Space Telescope are visible with the naked eye given favorable lighting conditions. Satellites in higher altitude orbits will be visible sooner before dawn and longer after sunset. Satellites in very high orbits, no matter how large the satellite, are seldom visible without high power binoculars or a telescope. 3. The geometry of the pass and the attitude and geometry of the spacecraft is also important. The relative angles between the Sun, the satellite, and the viewer determine how light is reflected from the surfaces of the spacecraft to you, the viewer. A spacecraft passing between you and the Sun may not reflect much light to you and may therefore not be visible even at higher altitudes. On the other hand, a spacecraft nearer the horizon but on the other side of you from the Sun may appear brilliantly lighted. The kinds of surfaces on the spacecraft are important too; large solar panels can reflect enough light to appear the most brilliant objects in the sky while large but rounded spacecraft seem all but invisible. 4. Given otherwise good conditions and favorable weather, the single most important factor is spacecraft apparent altitude during a pass. This is the spacecraft's apparent elevation above your local horizion. Program STSORBIT PLUS Satellite Orbit Simulation Page 14 Depending upon local conditions, an altitude of at least 5 degrees will generally be necessary before a spacecraft can be seen even under the best lighting conditions. In the Los Angeles area, at least 20 or 30 degrees is a better number to use because of smog and haze (except when looking out over the Pacific Ocean). 5. After altitude, the azimuth is the number which describes the direction from the viewer to the spacecraft at any moment. This is given in the sense NESW, North to East to South to West, in degrees. For a good pass after sunset, for example, an azimuth ranging from 60 to 150 degrees would indicate a pass moving generally from the Northeast to the Southeast, ideal lighting conditions with the Sun in the West. 6. Now to look for a visible pass using STSPLUS. Use Function Keys F8+F3 to select a local time shortly after sunset. For example, if sunset will occur at 17:30, set the local time to 18:00 and select the date desired to test. Now begin the display and press the "L" key to obtain the Location Map display centered on your viewing location. (If you have a second location set into STSPLUS, press "L" again to toggle between the two locations.) Notice the concentric spacecraft altitude isocontour rings spaced at 10 degree intervals and centered on your location. The outermost ring is 0 degrees, the approximate point at which the spacecraft will appear over your horizon. Press "PgUp" or "PgDn" to select a different zoom factor for the map. The ground track will be drawn against this map background. If you see no ground track, there will be no visible passes that evening (or dawn)! A good visible pass will arc in toward your location and reach a peak altitude of 40 or more degrees, which should be easily visible even over most local obstructions. Since as many as three sequential ground tracks may be shown, be sure you know which one occurs at what time! 7. Once you have found a visible pass which looks promising, pause the display with Function Key F6. This freezes the satellite on your display while still generating the altitude ("Alt") and azimuth ("Azm") values. Move the satellite back and forth along its ground track with the "+" and "-" keys and adjust the time step (1, 10, or 60 seconds) with Function Key F4. Now you can position the satellite at the point you expect it to become visible, reach its highest altitude, and disappear from view and record the time, altitude, and azimuth for each of these events. The spacecraft lighting is shown as an asterisk ("*") next to the "Orbit #" if the satellite is in sunlight; for color displays, the satellite icon is bright white in sunlight, yellow in partial shadow, and dim white in full shadow (umbra). If the lighting conditions are favorable and you are using reasonably current 2-line orbital elements, this method is just about foolproof. Known STSPLUS Problems and Bugs ------------------------------- STSPLUS is being used on thousands of computers around the world without any significant problems. However, like almost any computer program and in spite of my best efforts to minimize such bugs, there are several known problems or "bugs" with STSPLUS. Some are the result of slow Program STSORBIT PLUS Satellite Orbit Simulation Page 15 computers, others are in the program itself. Hopefully, some or all of these problems, those I can reproduce at least, will be repaired in due course. Some problems are caused by other software interfering with the program's operation. Still other problems are the result of incompatible "IBM-compatible" computers for which there is no remedy. One "problem" which I frequently hear about is that the user's CGA color monitor only displays STSPLUS in monochrome. THIS IS NOT A BUG! CGA systems display "high resolution" 640 x 200 graphics in monochrome ONLY. Compared to the EGA or VGA, that resolution is barely acceptable. The color graphics modes for the CGA are 320 x 200 which is inadequate for STSPLUS. 1. As quite a few users have reported, the FAST modes do NOT operate correctly with automatic map generation enabled. This problem is "on the list" to be worked on ... any day now! For now, use the FAST modes only with automatic map generation OFF (press the TAB key so that the letter "A" is not present at the upper right of the display), preferably with the WORLD map. However, I have used the FAST modes successfully in conjunction with the PAUSE to move a satellite back and forth on the Location Map (while the map is displayed select PAUSE with F6, then set the desired time increment using F4, and then use the "+"/"=" and "-" keys to move the satellite) to determine the altitudes of a pass. Once the PAUSE is released, the time will return to normal. Another alternative is to determine the approximate time of interest and then use simulated time (F8+F3 on the Main Menu) to display that time. Depending upon the speed of your computer, it's pretty simple to select another time if you guess wrong. Return the program to real time with F8+F1 from the Main Menu. 2. Several users have reported occasional "BASIC Error 11" problems; BASIC reports this error as "Division by Zero". The only way I have been able to reproduce this error is to restart the program using the command line "STSPLUS /R" after I have deleted the 2-line file used on the prior run; to avoid this error, do not use the "/R" command line option and select a new 2-line file using F2. This error may also be the result of a memory limitation (with multitasking enabled?) or it may be a BIOS problem associated with a specific computer. 3. STSORBIT PLUS has been run extensively on systems using Microsoft DOS 3.3 and 5.0 and there are no known problems with those operating systems. However, users report memory allocation problems with Digital Research DRDOS 6.0 and certain third party memory allocation programs. A typical symptom of this kind of problem is that you cannot return to DOS without either an error message or the computer freezing. 4. Some users report problems with certain Terminate and Stay Resident (TSR) programs for which the only remedy is to remove the offending TSR. This usually requires a "trial and error" approach to pinpoint the TSR causing the problem. The best method is to remove ALL such programs from your AUTOEXEC.BAT file, including the DOS 5 "DOSSHELL", to make sure the program will work with your computer. Similarly, delete all special memory and device drivers from your CONFIG.SYS file. One user reports a problem when his mouse is active (but I regularly execute STSPLUS on different computers with my mouse active!). Program STSORBIT PLUS Satellite Orbit Simulation Page 16 5. All VGA adapter cards are not equal, in case any of you had some illusions left. In at least one case, the aspect ratio of the display is incorrect when the display is operated in the EGA mode! The vertical scale is compressed by about 20% as compared to either a true EGA display or other (correct) VGA displays. So far as I know, there is no remedy. Early VGA cards (the 449 card from Zenith is an example) are not always recognized as VGA; the card is not register compatible with the IBM standard and is recognized as EGA instead. 6. There appears to be a subtle problem when changing to or from Daylight Savings Time (which recently happened and "announced" the bug). The display appears to get caught in a loop, endlessly redrawing the screen. To avoid the problem, change the Daylight Flag then EXIT THE PROGRAM AND RESTART. 7. Finally, as noted elsewhere, all computers are NOT equal. There are some computers which will not execute these programs under any circumstances. Tandy is the most common offender followed by Leading Edge. Some models from these manufacturers have BIOS problems or errors which prevent programs compiled with the Microsoft PDS BASIC compiler from operating (sometimes only in graphics modes). There is no remedy. Other computers, Ergo for example, exibit "strange" behavior in some graphics and text modes. There is sometimes a workaround for these problems; check with the manufacturer. Program STSORBIT PLUS Satellite Orbit Simulation Page 17 PROGRAM OPERATION ----------------- The program automatically checks for the presence of a VGA or EGA and will execute in color if one is found UNLESS the /M command line option is used to force monochrome operation. However, if you wish to operate STSPLUS in the EGA mode when you have a VGA monitor, use the /EGA option. In cases where a monochrome monitor is connected to an adapter which simulates color with gray scale, the /M command line option may be omitted but the various portions of the display may or may not be visible. STSPLUS depends upon the Microsoft BASIC Compiler to determine whether or not a particular monitor type is available. Some video adapter boards which claim to be VGA are not recognized as such by BASIC and therefore cannot be used in the higher resolution VGA display mode. Similarly, "clone" Hercules Graphics Cards do not always perform correctly. The /M option is not required for HGC and CGA graphics operation, since those adapters always render their "high resolution" graphics in monochrome. Although color CGA systems do have a 3-color mode, the limited number of colors and coarse resolution of 320x200 is not suitable for STSPLUS. Naturally, the appearance of the program is enhanced by the use of color. The vertical resolution is also adjusted depending upon the type of adapter which has been detected. Once STSPLUS has started, the display type may not be changed without halting the program at the Main Menu with the ESC key, then restarting the program with the desired command line options. The program checks for the presence of a math coprocessor and will use it if found. Since the calculations required to determine orbital data are very complex, the use of a math coprocessor will improve the speed of operation by a very substantial amount. Use the SLOW option on the Set Program Options Menu (F10+F7) if the program has difficulty operating on your system. STSPLUS selects the icon or symbol used to graphically represent the satellite based upon the mission name. Names which start with the letters "STS" will use the space shuttle icon and all other missions will use an icon resembling the Hubble Space Telescope. STSPLUS first reads in the map coordinates from file STSPLUS.MPT, which must be in the current drive and directory. These coordinates are converted to screen coordinates for the type of monitor detected and stored internally so that they need only be read once. After map coordinates have been loaded, the initial title screen is shown on the display. The title screen is displayed for 15 seconds for normal program operation or for 3 seconds if the /R command line option has been used. The Main Menu, described in the following section is then displayed. Press any key to proceed to the Main Menu immediately. Program STSORBIT PLUS Satellite Orbit Simulation Page 18 Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 Last Mission = STS-45 [STS45J13] (C) Copyright David H. Ransom, Jr., 1989-1992 All rights reserved. Program STSORBIT PLUS Satellite Orbit Simulation Page 19 THE STSORBIT PLUS GROUND TRACK DISPLAY -------------------------------------- The principal objective of STSPLUS is to graphically display the position of the space shuttle or satellite relative to a map of the world or some relevant portion of the world. Six different map displays are available: World, Quadrant, Zoom, Location, Tracking Station, and Satellite Motion. Each is discussed below. In addition to the map itself, a number of other items of interest are displayed. Some features are available only with higher resolution displays (EGA and VGA) in order to avoid cluttering the display screen. Other features may be enabled or disabled according to the user's preference. The sections which follow the map types discuss these various features. World Maps ---------- The STSPLUS ground track display defaults to a map of the world centered on the Prime Meridian (0 degrees) and extending from approximately +85 degrees North latitude to -85 degrees South latitude using a linear cylindrical projection. Omitting the two 5 degree bands at the poles permits better detail in the mid latitudes where all space shuttle orbits and many other satellite orbits are concentrated. Ground track details very near the poles are therefore sacrificed for a better display in the main portion of typical orbits. The vertical resolution of the display is automatically adjusted for the type of display system in use from 200 lines (CGA) to 480 lines (VGA). Two World Maps are available: one centered on the Prime Meridian at Greenwich, England (0 degrees longitude); and, one centered on the International Date Line (180 degrees longitude). All screen maps drawn by STSPLUS use a vector database derived from the Micro World Database II. The full map of the world as used here includes approximately 30,000 vector pair coordinates describing the world's coastlines, islands, and major rivers. (The rivers are not displayed on the world maps to avoid "cluttering" the display.) Pressing the "W" or "0" key will switch the display to the World map. If automatic map generation is enabled, STSPLUS will select the map which most nearly centers the satellite on the map. If automatic map generation is disabled, pressing "W" or "0" will toggle between the two maps. When automatic map generation is enabled, the letter "A" appears in the upper right of the screen. Quadrant Maps ------------- The original STSORBIT program used a digitized pixel map of the world derived from an EGA display. As a consequence of the EGA source, boundaries were sometimes discontinuous on VGA displays and the display on a CGA was sometimes difficult to read. STSPLUS uses a vector map drawing method which automatically adjusts to the display type and the scale of the map. A full world map is available which is quite similar in appearance to the original display. I happen to think it is a big improvement, but that's a matter of personal opinion. However, some geographic details are still difficult to distinguish, Program STSORBIT PLUS Satellite Orbit Simulation Page 20 even on a VGA display. STSPLUS includes twelve quadrant maps, each overing 1/4 of the Earth's surface. These are numbered 1 to 12 and are centered on the world map roughly according to the following illustrations: 0 180 +---------------------------+ +---------------------------+ | | | | | | | 1 4 7 | | 7 10 1 | | | | | | | | | | | | | | | | | | | | 2 5 8 | | 8 11 2 | | | | | | | | | | | | | | | | | | | | 3 6 9 | | 9 12 3 | | | | | | | +---------------------------+ +---------------------------+ The center vertical quadrants in the left illustration, 4 through 6, are centered on the Prime Meridian at zero degrees longitude (Greenwich, England). The center vertical quadrants in the right illustration, 10 through 12, are centered on the International Date Line at 180/-180 degrees. The center horizontal quadrants, 2, 5, 8, and 11, are centered on the Equator. Each individual quadrant map may be selected by pressing the corresponding number key, "1" through "9"; use keys "!", "@", and "#" to select quadrants 10, 11, and 12 respectively. Pressing any of these keys for individual quadrant maps will disable automatic map generation if it is enabled (indicated by the letter "A" at the upper right of the screen). Pressing "Q" will allow STSPLUS to select the quadrant most appropriate for the satellite's current position. Zoom Maps --------- Although I was pleased with the enhanced maps using the Quadrant Mode, the vector map file contains far more information than can be effectively displayed in that mode. The next obvious step was to add the ZOOM feature, maps which yielded greater detail and which spanned as little as 45 degrees across the screen, six times better than the 180 degree quadrant maps. This approaches the practical limit for the vector database. Because of the smaller area covered, a different approach was used for map selection. There would simply be too many different possibilities for manual selection so a fully automatic Zoom Mode was implemented. Press the "Z" key to enable Zoom Mode. STSPLUS will use the current position of the satellite to select the center of the map. The initial map width is 75 degrees; use PgUp to widen the map width (up to 180 degrees) or PgDn to narrow the map width (down to 45 degrees, roughly the limit for acceptable maps given the vector data being used). The Home key will always select 75 degrees width and the End key will return to the prior field of view. The width of the map is shown at the upper left of the map display. Press the TAB key to enable or disable automatic map generation (the map will always be redrawn). When automatic map generation is enabled, the Program STSORBIT PLUS Satellite Orbit Simulation Page 21 letter "A" appears in the upper right corner of the screen next to the map width or field of view. Zoom field of view is limited to 45, 60, 75, 90, 120, and 180 degrees. During Mission STS-48, I found the Zoom Mode extremely effective in helping me to distinguish various coastlines and rivers, even on my poor old Zenith CGA/LCD laptop! The only problem with all of this is that I am now approaching the limit of an 80C88 with 8087 math coprocessor (8 MHz at that!) in terms of its ability to get the calculations done in the time allowed. Location Maps with Isocontours ------------------------------ By popular request, especially from the amateur radio community, I have added the Location Map with Isocontours. (Isocontours is a description coined by Rob Matson for his SkyMap program and for which he supplied sample code.) Press the "L" key when the map display is present to select this display. The map will be drawn with the current zoom factor and centered on the user's location. The usual circle of visibility will be drawn and within that "circle" are seven isocontours representing viewing angles of 10 through 70 degrees in ten degree increments. The balance of the Location Map includes the usual features. If you have entered a second location (using F10+F2 from the Main Menu), pressing the "L" key when the Location Map is already displayed will toggle between your primary location and the second location. The data displayed at the lower right of the screen (Location, Altitude, Azimuth, etc.) is calculated with respect to the indicated location. If no second location has been entered, pressing the "L" key while the Location Map is displayed will have no effect. The principal advantage of the Location Map is, of course, the isocontours -- lines of equal viewing altitude (line of sight not taking into account any refraction near the horizon) from the user's own location. The user can immediately tell by inspection whether current or upcoming passes will be "good" and what approximate maximum satellite viewing altitude can be expected. Amateur radio buffs who need to know if a pass will appear above some altitude threshold, say 20 degrees, now have that information available visually. Since this map mode uses the ZOOM map algorithms, the usual zoom map features (PgUp, PgDn, Home, End) are active. Tracking Station Maps with Isocontours -------------------------------------- The Tracking Station Maps with Isocontours are similar to the Location Maps except that they use the current TRACKING STATION file locations rather than the user's location(s). This feature was implemented at the request of folks working on the STS-49 Intelsat Reboost Mission. Pressing the "T" key will select this map mode. STSPLUS calculates which of the available tracking stations is nearest to the current satellite position and centers that tracking station on the screen. This is calculated by determining the angular difference between the sub-satellite point and each tracking station. However, this means that depending upon the Zoom factor in effect, the satellite may or may not be visible on the screen. For Program STSORBIT PLUS Satellite Orbit Simulation Page 22 example, if the sub-satellite point is in South America and the only tracking station in the Western Hemisphere is in the United States, the satellite cannot be seen at narrower fields of view. The "tracking stations" may be any locations the user chooses and included in the current TRACKING STATION file. Several different tracking station files are included with the normal STSPLUS distribution as described in the section "NASA Ground Tracking Stations" below. Use Function Key F7 from the Main Menu to select the desired file. Since this map mode uses the ZOOM map algorithms, the usual zoom map features (PgUp, PgDn, Home, End) are active. Satellite Motion Maps --------------------- It is sometimes instructive and interesting to see the ground track from the satellite point of view. The Satellite Motion Map, available ONLY with EGA and VGA displays, centers the satellite in the display and draws the ground map accordingly. Unlike all other map displays, this mode takes advantage of the dual-page capability of the EGA display and the VGA display (operating in the EGA mode); the current map is always displayed and the new map is drawn "off screen" and updated every ten seconds (or as frequently as the capability of the processor will permit). Users with slow computers or computers without math coprocessors may prefer this display because, once the map has been drawn the first time, a complete map is always displayed. For VGA users, the vertical map resolution in this mode is reduced from 400 lines to 280 lines. The Satellite Motion Map is enabled by pressing the "M" key when the map is displayed. When switching to this map mode, the message "Switching to EGA Dual-Page Mode ..." is displayed on the screen; all EGA and VGA systems display the Satellite Motion Map in EGA resolution: 640 x 350 pixels. The complete map is then drawn off-screen and will require the "usual" time during which the screen will be unchanged. (My 386 systems update every 10 seconds but my 286 systems can only manage every 20 or 30 seconds -- and they all have math coprocessors!) As with other zoom modes, the zoom magnification may be adjusted by pressing the Hone, End, PgUp, or PgDn keys. Note that ONLY the "M", Home, End, PgUp, PgDn, and ENTER keys are active while the Satellite Motion Map is displayed; press the "M" key or ENTER to return to normal operation. On-line Help ------------ An on-line Help Screen is available during the ground track display to remind the user of the available functions and which keys to press to trigger those functions. When the ground track display is active, press Function Key F1 to display the Help Screen in the lower portion of the display: F1=Resume Data F6=Pause (+,-) TAB=Auto Maps On/Off L=Location Maps F2=Sat Blink F7=Circle of Vis W=World Maps T=Tracking Maps F3=Printer Log F8=SUN and Term Q=Quadrant Maps M=Motion Map F4=Time Step F9=Units (nm/km) Z=Zoom Maps: Home, F5=MET/T+Epoch F10=Sat Coordinates PgUp,PgDn STSPLUS Version 9218 Program STSORBIT PLUS Satellite Orbit Simulation Page 23 The ground track display will continue to be updated in real time while the help screen is displayed. If only the graphical display of the ground track is of interest, the help screen may be kept on the display continuously. Press Function Key F1 again to return to the normal data display in the lower portion of the screen. Note that the Help Screen is disabled when the Motion Map is displayed. Satellite Position and Orbit Projections ---------------------------------------- The focus of the display, and the reason for program STSORBIT, is to show the position of the spacecraft or satellite. For the space shuttle (and provided the mission name begins with the letters "STS"), a symbol has been chosen which resembles that spacecraft. For all other satellites, a symbol has been chosen which resembles the Hubble Space Telescope. In either case, the symbol is shown in the following colors (EGA and VGA displays only): Satellite is sunlit Bright White Satellite is in penumbra Yellow Satellite is in umbra Dim White In addition, an asterisk ("*") is shown to the right of "Orbit #" when the satellite is sunlit or in penumbra; this will assist users of monochrome monitors where colors cannot be distinguished. For better visibility, the satellite symbol will normally "blink" on CGA monitors; the symbol may be made to blink on any system if desired by pressing Function Key F2 while the ground track is displayed. Note however that for very slow computers, the blink may appear erratic if most of the time is spent performing calculations. Just seeing the spacecraft or satellite on the map display yields information as to its present position. However, for satellite viewing and planning purposes, STSPLUS calculates the predicted ground track for approximately three hours in the future and the past ground track for approximately one and a half hours in the past. The predicted ground track is shown as a series of light green dots, and the past ground track is shown as light red dots. These dots are plotted at one minute intervals and "move" as the spacecraft or satellite changes position; one new dot is added to the predicted track each minute and one dot is removed from the past track at the same time. The ground track may optionally be omitted, be shown as a light green line, or be shown with both dots and line. User's Circle of Visibility --------------------------- Centered around the user's geographic location, and marked with a small cross on EGA and VGA systems, is a magenta "circle" of the approximate line of sight visibility for the mission in progress. The "circle" appears on the display approximately as a circle near the Equator and as a rounded triangle in higher latitudes. These strange shapes are entirely an artifact of the map projection; if projected on the surface of Program STSORBIT PLUS Satellite Orbit Simulation Page 24 a sphere, they would be true circles. The radius of this circle of visibility is calculated for each satellite based upon its altitude at the instant the map is first drawn as well as the user's elevation above mean sea level and corresponds to "line of sight" visibility for that satellite. When a satellite is within the circle, direct visual, radio or radar communications with the satellite should be practical. Actual visibility, of course, depends upon more than simply whether or not the satellite is above the viewer's horizon. Most important is the sun to satellite to viewer geometry; the satellite must be in sunlight and the viewer in darkness for reasonable visibility. Almost as important is the size and geometry of the satellite itself; a large, bright-metal satellite with huge solar arrays reflects far more sunlight than a small dark satellite. In addition to the satellites themselves, many booster rockets and other "spare parts" are orbiting the Earth. Since they are not attitude stabilized, booster rockets usually are tumbling and may therefore appear to flash on and off as they pass over. For the Space Shuttle as well as most other satellites with near circular orbits, the circle of visibility calculations are reasonably accurate; however, the position of the sun and the effect of atmospheric refraction are not taken into account, only whether or not the satellite is in line of sight view from the observing location. For highly eliptical orbits, however, the accuracy is substantially degraded since the radius of the circle of visibility changes dramatically depending upon whether the satellite is nearer apogee or perigee at the time the calculation is made, and the period of the orbit. In the course of a single orbit, the altitude of such a satellite may change by thousands of miles. Satellite "DE 1", usually included in the NASAnnn.TXT 2-line elements file, is in a highly elliptical orbit with long period and illustrates the problem. Spacecraft Circle of Visibility ------------------------------- The spacecraft circle of visibility is calculated using the same alrorithm as for the user's circle of visibility. Like the user's circle, the spacecraft circle appears on the display as an ellipse because of the scaling factors used by the map projection. The difference is that the spacecraft circle moves with the spacecraft and illustrates the approximate area visible from the spacecraft at any given moment. The circle is updated at the same rate as the spacecraft symbol, normally every ten seconds. Comparisons with a VGA display system during the STS-35/ASTRO-1 mission in December, 1990 confirmed that the circle shown is quite close to that shown by one of the special graphics displays occasionally shown on NASA Select Television. TDRS Satellite Features ----------------------- TDRS Satellite features are shown only on HGC, EGA and VGA monitors. The three TDRS satellites (Tracking and Data Relay Satellites), used for most communications to and from the Space Shuttle and the Hubble Space Telescope, are each shown as a dot inside a small yellow circle on the Equator. There are three TDRS satellites in geosynchronous orbit, TDRS East at approximately 72 degrees West longitude and the TDRS West Cluster Program STSORBIT PLUS Satellite Orbit Simulation Page 25 consisting of two satellites at approximately 170 degrees West longitude. Both of the TDRS West Cluster satellites have partially failed but between them they can provide the facilities of a fully operational satellite. Each TDRS location provides communications coverage for almost half of the Earth. However, since the ground station is at White Sands, New Mexico, the coverage overlaps to provide good communications. This, in turn, means that there is a narrow band, known by NASA as the Zone of Exclusion, off the East coast of Africa which is not covered by either TDRS. Four slightly curved vertical red lines on the display show the limits of coverage for each TDRS location. With the map centered on the United States and scanning from left to right, the red coverage lines are acquisition of TDRS East, loss of TDRS West, loss of TDRS East, and acquisition of TDRS West. During most of an orbit, the satellite being used for communications will be shown in the data block opposite the title "Com:"; the messages "TDRS-West" or "TDRS-East" will appear. Approximately five minutes before the spacecraft reaches loss TDRS East, the message will change to "LOS 5:07" with the numbers indicating the minutes:seconds until anticipated signal loss. Once the signal is lost, the message will change to "AOS 7:35" to show the time remaining until acquisition by TDRS West. The algorithm used for the calculation of the TDRS coverage is very approximate but is usually accurate to within approximately 30 seconds. Ground Tracking Stations and .TRK files --------------------------------------- NASA maintains a number of ground tracking stations around the world. Some of these tracking stations are essential for the ascent or landing phases of a space shuttle flight; others are used for in-flight communications. Files STSPLUS.TRK and NASA.TRK contain the information for these ground tracking stations. Other files with filetype .TRK contain the information for other launch and/or tracking stations. Each ground tracking station is shown as a small symbol surrounded by a brown or light yellow "circle of visibility" which gives the approximate area of antenna coverage and shows how small a proportion of each orbit can be monitored without the TDRS system. When for some reason the TDRS system is down (as has occurred during infrequent computer failures at White Sands, New Mexico, the TDRS Ground Station), these ground tracking stations become the only means of communication with the Space Shuttle. Because of budget constraints, many of these ground tracking stations may be (or have already been) shut down. Some, such as MIL and BDA (see list below) will be retained because they are required for the ascent phase of a space shuttle mission. Others, such as HAW, CTS and GWM, are operated jointly with, or independently by, the U. S. Air Force. The shape of the antenna range "circle of visibility" varies as a function of the latitude and is an artifact of the map projection; if projected on a sphere, they would be true circles. In order to avoid cluttering the display with countless meaningless lines, tracking station circles of visibility are shown only if that circle has an angular diameter of 90 degrees or less. For all systems except CGA, the locations of the three TDRS (Tracking and Data Relay Satellite) are shown as small circles on the Equator at -41 degrees and at -171 degrees; on CGA displays the TDRS coverage is NOT shown in order to avoid confusing the display. The following table lists the internal ground tracking stations as of early 1989 which are shown along with their abbreviations and approximate map coordinates (longitude, latitude): Program STSORBIT PLUS Satellite Orbit Simulation Page 26 MIL -81,28 Merritt Island, FL BDA -64,32 Bermuda DKR -17,14 Dakar, Senegal ACN -14,-8 Ascension Island MAD -5,41 Madrid, Spain IOS 56,-5 Indian Ocean HAW -156,20 Hawaii GWM 143.33,14 Gwam VAN -120.57,34.73 Vandenberg, CA YAR 115,-29 Yargidy, Australia CAN 149,-36 Canberra, Australia GDS -116.88,35.93 Goldstone, CA CTS -105,38 Colorado Springs, CO AGO -71,-34 Santiago, Chile NGT -106,33 White Sands, NM STSPLUS first checks for the presence of file STSPLUS.TRK for its ground tracking station information. This is the default TRACKING STATION filename used if no other selection has been made using F7 from the Main Menu. If that file is present, its data is used instead of the internal data above. The supplied STSPLUS.TRK has the following format: "Maui, Hawaii",-156.7,20.9,0,"HAW" "Vandenberg, CA",-120.5667,34.7333,112,"VAN" "White Sands, NM",-106,33,0,"NGT" "Colorado Springs, CO",-105,38,0,"CTS" "Merritt Island, FL",-81,28,0,"MIL" "Santiago, Chile",-71,-34,0,"AGO" "Bermuda",-64,32,0,"BDA" "Dakar, Senegal",-17,14,0,"DKR" "Ascension Island",-14,-8,0,"ACN" "Madrid, Spain",-5,41,0,"MAD" "Indian Ocean Stn",56,-5,0,"IOS" "Yargidy, Australia",115,-29,0,"YAR" "Guam",143.3333,14,0,"GWM" "Canberra, Australia",149,-36,0,"CAN" These files are ASCII and may be prepared or edited with any standard ASCII editor; if using a word processor, select the "non-document" mode. Five items are required for each location; the longitude and latatude are expressed in degrees and fraction of a degree, elevations above Mean Sea Level are expressed in meters, and names or abbreviations are included in double quotation marks. The following example illustrates the .TRK file format: "Merritt Island, FL",-81.0,28.0,0,"MIL" --------+----------- --+-- --+- + --+-- | | | | | | | | | +--- 3-Letter Abbreviation | | | +------- Elevation (meters) | | +---------- Latitude (degrees) | +---------------- Longitude (degrees) +------------------------------- Location Name Program STSORBIT PLUS Satellite Orbit Simulation Page 27 For those interested in the Russian space program, a list of Russian ground tracking stations is provided in file USSR.TRK (data courtesy Ellwood Marshall). With the breakup of the Soviet Union, some of these installations may no longer be active. "Baikonur Cosmodrome",63.3392,45.9235,0,"BAI "Dzhusaly USSR",63.85,46.15,0,"DZH" "Kalingrad Control Ctr",37.816,55.916,0,"KAL" "Plesetsk Cosmodrome",40.7,45.9235,0,"PLS" "Petropavlovsk USSR",158.933,53.216,0,"PTR" "Tbilisi USSR",44.75,41.66,0,"TBL" "Ulan Ude USSR",107.683,51.983,0,"ULN" "Ussuriysk USSR",132.15,43.8,0,"USS" "Yevpatoria USSR",33.3666,45.2166,0,"YEV" Other nations also have facilities for satellite launches. As of early 1992, file SPACENTR.TRK includes the following locations: "Alcantara LC Brazil",-44.3999,-2.3999,0,"ALC" "Esrange,Kiruna Sweden",21.067,67.883,0,"ESR" "Jiuquan Space LC China",100.033,40.83,0,"JIU" "Kagoshima Center Japan",131.083,31.25,0,"KAG" "Kourou Space Ctr Fr.Gu",-52.7669,5.23,0,"KOU" "San Marco Platform",40.2,-2.9329,0,"SMP" "Sriharikota Ctr India",80.25,13.78,0,"SRI" "Tanegashima SC Japan",130.967,30.4,0,"TAN" "Xichang Space LC China",102.217,27.967,0,"XUC" The first mission of Endeavour, STS-49, is scheduled for May, 1992. This mission is planned to reboost the INTELSAT VI (F3) satellite, stranded in a useless orbit by its booster rocket failure, and will involve a complex rendezvous of the orbiter and the communications satellite. INTELSAT will use its own ground communications stations for communications with INTELSAT VI (F3); the ground stations which plan to participate in the mission are listed in file INTELSAT.TRK (information courtesy Dee Smith). "Paumalu, Hawaii",-158.0342,21.6711,157.86,"PAU" "Tangua, Brazil",-42.7845,-22.7442,35.38,"TAN" "Jatiluhur, Indonesia",107,-6.5213,161.49,"JAT" "Perth, Australia",115.25,-31.8,0,"PER" "Gandoul, Senegal",-17.4745,14.43,0,"GAN" These TRK files are standard ASCII files and may be edited with any editor; word processor users be sure to use the ASCII or non-document mode. The files use a standard comma-delimited format as shown; positions are given in longitude (degrees) and latitude (degrees), rounded to the nearest degree. A maximum of 25 ground stations is permitted. Note that although the two quoted strings at the beginning and end of each line are not presently used, they are required for proper operation. The use of TRK files is not restricted to tracking stations, of course. So long as the correct data format is observed, any desired location may be included in the tracking station file up to the maximum of 25 locations. Program STSORBIT PLUS Satellite Orbit Simulation Page 28 STSORBIT PLUS MAIN MENU ----------------------- Once the map coordinates have been stored internally, STSORBIT PLUS presents its Main Menu: Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 F1 Program STSORBIT PLUS Demonstration F2 Read NASA/NORAD 2-Line Elements (from *.TXT File) F3 Read Prior Mission Information (from *.INI File) F4 Enter New Orbital Information (Simple Orbital Model) F5 Adjust Launch Time and/or Launch Date F6 Display MET & Launch Times (now 2-Line Epoch Times) F7 Set FILENAMES and Paths F8 Set program TIME and/or DATE F9 DOS Shell (CAUTION: DOS Version 3.x+ ONLY!) F10 Set STSORBIT PLUS Program Options and Features ENTER Resume Mission (STS-45 [STS45J13]) ESC Quit STSORBIT PLUS and Save Current Mission Select desired function: F1 Program STSORBIT PLUS Demonstration ------------------------------------------ The F1 command may be used to demonstrate the operation of STSPLUS and to verify that the program will operate correctly. No other information is required. The demonstration automatically sets up the data to simulate a mission and clears the screen. A slight delay may follow while the program calculates the initial parameters of the orbit. Then the world map is drawn, the ground track for the first three orbits (approximately) is drawn as a series of dots (green for color monitors), and mission data is displayed at the bottom. Mission Elapsed Time (MET) is set to shortly before launch time and counts down to zero. When MET reaches zero, the shuttle symbol appears over Cape Canaveral. For CGA and HGC systems, the shuttle will blink; for EGA and VGA systems, the shuttle will be steady. At first the shuttle will move along the ground track very slowly, simulating the ascent phase of the mission. After about ten minutes the shuttle reaches orbital altitude and proceeds along the ground track normally, passing a dot every minute. Press F4 (for Fast time) once or twice to speed up the progress of the simulation. Press ENTER to return to the Main Menu. Data are displayed by STSPLUS in several standard formats: Program STSORBIT PLUS Satellite Orbit Simulation Page 29 4/05/1990 Date in month/day/year 14:33:00 Time in hours:minutes:seconds 3/09:23:15 MET in days/hours:minutes:seconds -69.34 Longitude and Latitude in degrees WEST longitude and SOUTH latitude are negative 320.50 nm Altitude in nautical miles F2 Read NASA/NORAD 2-Line Elements from *.TXT File ------------------------------------------------------ In order to read the NASA/NORAD 2-line elements, you must have a file with that data. A current file is included in the standard distribution of STSPLUS. These files have names such as "NASA971.TXT" where the "971" corresponds to the particular NASA data set and may change several times per week. Pressing F2 will display a list of all files with filetype .TXT and will then offer the following prompts: Read NASA/NORAD 2-Line Element File Press ENTER to enter the current default name as shown in square brackets [...]. Enter 2-Line Filename [NASA971.TXT]: Enter desired Satellite Name [HST...]: In each case, pressing ENTER will select the default choice shown inside the square brackets. If no default is shown, you MUST type an entry. For the example shown, the defaults are the file NASA971.TXT and any satellite whose name begins with the letters "HST". If no prior satellite has been selected, the satellite name will default to "STS..." for space shuttle missions (provided the filename begins with "STS") and to "HST..." for all other satellites; otherwise, it will be the first three letters of the currently selected satellite. If you wish to change the information (or if no default is shown), enter the required information followed by ENTER. If no filetype is included in the filename, STSPLUS will automatically add ".TXT". For the satellite name, only sufficient letters to unambiguously identify the desired satellite, upper or lower case, are required. For example, "Alou" would select "Alouette 1". Alternatively, you may enter the NORAD number for the desired satellite; leading zeroes may be omitted. Once the information has been entered, STSPLUS will attempt to locate the specified file and the data for the requested satellite. If a satellite matching the requested name is found, the data for that satellite are displayed. Certain non-essential data are not always included in the 2-line elements and may be replaced by spaces; this is indicated in the example below by "(n/a)". Satellite Name: HST Satellite NORAD Number: 20580 Elements File: NASA971.TXT Elements File Record#: 113 Program STSORBIT PLUS Satellite Orbit Simulation Page 30 Elements Epoch: 92096.00331828 05 APR 1992 @ 00:04:47 UTC Orbit # at Epoch: 10601 Launch Year: (n/a) Launch Number: (n/a) Launch Piece: (n/a) Element Number: 692 Inclination: 28.471 RA of Ascend Node: 5.0575 Eccentricity: .0004889 Arg of Perigee: 131.6384 Mean Anomaly: 228.4514 Mean Motion: 14.91160614 BSTAR Drag: .00050873 Press ENTER to ACCEPT this satellite, OR Press any other key to REJECT and continue searching: If this is the satellite you wish, press ENTER and the data will be entered into STSPLUS. If, on the other hand, a different satellite is desired, press any other key (such as the SPACE BAR) and STSPLUS will search for another name matching the requested satellite. For example, there are a number of NAVSTAR Global Positioning Satellites usually included in the file with official names such as "GPS-0001", "GPS-0002", "GPS BII-01" and so forth; requesting "GPS" will allow you to cycle through all the available choices. The file NASAnnn.TXT is an ASCII file; it may be helpful to print the file to show the available satellite names. Once the satellite has been selected, STSPLUS will require a brief time to calculate certain required orbital parameters, then will proceed directly to the display of the ground track. As a point of interest, the NASA 2-line elements for the Hubble Space Telescope used in the example here are as follows: HST 1 20580U 92096.00331828 .00005316 00000-0 50873-3 0 6927 2 20580 28.4710 5.0575 0004889 131.6384 228.4514 14.91160614106013 F3 Read Prior Mission Information from *.INI File ----------------------------------------------------- The F3 command displays all files in the current directory with a filetype of .INI and then prompts the user to enter the name of the desired data file. File STSPLUS.INI is the file which has the data from the last time the program was run; to use file STSPLUS.INI, press ENTER. STSPLUS will automatically supply the filetype .INI if you do not include it. If the requested file is not present, an error will be reported. To create new command to copy STSPLUS.INI into a file with a the desired name. STSPLUS also expects your computer clock to be set to the correct local time and date; use the DOS TIME and DATE commands. See the F10+F10 command below for information on setting the UTC Zone Offset. Of course, you could "fool" STSPLUS by setting your computer to a desired time and date using the DOS TIME and DATE commands, but be aware that for most computers using MS-DOS/PC-DOS 3.3 or greater the hardware clock is also set with these commands. Instead of setting your computer clock to some Program STSORBIT PLUS Satellite Orbit Simulation Page 31 fictitious time, use STSPLUS's internal time setting functions (F8 on the Main Menu) to set a desired simulation time. This method does not affect your computer's internal hardware or software clocks. F4 Enter New Orbital Information (Simple Orbital Model) ----------------------------------------------------------- The F4 command uses the "simple" orbital model and allows the user to enter the orbital altitude, inclination, launch time, and launch date using your local time or UTC (Coordinated Universal Time) or GMT (Greenwich Mean Time, essentially identical to UTC for the purposes of this program) to determine the orbital parameters. If you enter the time and date using local time, it will internally be converted to UTC so that the .INI file will be valid in any time zone. To manually enter orbital data, you will be prompted for the required information. Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 Enter mission title: STS Mission Simulation Enter orbit altitude (nm): 160.00 nm ( 296.52 km) Enter orbit inclination (deg): 28.450 degrees Enter Launch Time (HH:MM:SS): 15:58:12 PST 23:58:12 UTC Enter Launch Date (MM/DD/YYYY): 4/08/1992 4/08/1992 Press ENTER to accept, SPACE BAR to repeat: _ The sample above illustrates the data required to generate the simple orbital model. A brief discussion follows for each item. Enter mission title: Enter the title of the mission, such as "STS-45 ATLANTIS" without the quotation marks. If you press ENTER, the title "STS Mission Simulation" will be used. Enter orbit altitude (nm): [Add 'km' for altitude in kilometers.] Enter the orbit altitude (or average orbit altitude for non-circular orbits), in nautical miles. If you wish to use kilometers, add the letters "KM" (upper or lower case) at the end of the number. Program STSORBIT PLUS Satellite Orbit Simulation Page 32 Enter orbit inclination: Enter the inclination of the orbit in degrees; a decimal fraction is allowed. For example, STS-30 was flown at an inclination of 28.85 degrees and STS-28 used an inclination of 57 degrees. Press ENTER to use the default value of 28.45 degrees. Enter Launch Time (HH:MM:SS): [Press ENTER for ASAP; add 'U'or 'G' for UTC/GMT] Enter the time in the format shown using 24-hour notation and local time. Add the letter "U" to signify UTC (Coordinated Universal Time) or the letter "G" to signify GMT (Greenwich Mean Time, essentially identical to UTC for the purposes of this program). If you enter "U" or "G", the abbreviation used in the menus will be set to "UTC" or "GMT" respectively. One or two digit numbers may be used as required. [The comma is also acceptable as a separator in place of the colon.] You may omit seconds [or minutes and seconds] if desired. For example, an entry of "16" will be entered as 16:00:00 or 4:00 PM. Press ENTER for "launch" ASAP, as soon as possible; depending upon the system, this will be in from 30 to 60 seconds from the current time. Enter Launch Date (MM/DD/YYYY): [or press ENTER for TODAY] Enter the date in the format shown. Note that if you requested UTC or GMT when entering the time, the date is interpreted as the UTC/GMT date. The full four digit year may be used OR you may use two digits as in "92". Be sure to use the SLASH "/" rather than the MINUS "-" as the separator; STSPLUS's internal date algorithms will interpret the minus sign as just that and some rather strange dates can result! You may also use relative dates: -1 will use the prior day, +2 will use two days hence, and so forth. Press ENTER to use today's date. Press ENTER to accept, SPACE BAR to repeat: When all data have been entered, the program pauses for your approval. If all data are correct, press ENTER. Press the SPACE BAR to start over. F5 Adjust Orbital Parameters -------------------------------- ADD/CHANGE LAUNCH DATE AND TIME (2-line elements) The full F5 command assumes the "simple" orbital model and is abbreviated when using NASA 2-line elements; only the LAUNCH DATE and LAUNCH TIME may be set when NASA 2-line elements have been selected; all other orbital parameters are included in the 2-line elements and may not be altered within STSPLUS. A 2-line element set must have been previously loaded using the F2 command on the Main Menu before the launch date and time may be set or changed. Since estimated NASA 2-line elements are usually available prior to a launch, and actual 2-line elements within about 24 hours after a launch (and are far more accurate than the manually entered "simple orbital Program STSORBIT PLUS Satellite Orbit Simulation Page 33 data"), those data should be used when available. However, since most events occuring during a space shuttle mission follow the MET (Mission Elapsed Time) timeline, the time elapsed since launch, it is convenient to show MET. This requires that you obtain the launch date and launch time independent of the 2-line elements and manually enter that data into STSPLUS. Note that STSPLUS retains the launch date and time information from one set of 2-line elements to another. Be sure to reset these data when you change from one mission to another! Enter Launch Time (HH:MM:SS): [Press ENTER for ASAP; add 'U'or 'G' for UTC/GMT] Enter the time in the format shown using 24-hour notation and local time. Add the letter "U" to signify UTC (Coordinated Universal Time) or the letter "G" to signify GMT (Greenwich Mean Time, essentially identical to UTC for the purposes of this program). If you enter "U" or "G", the abbreviation used in the menus will be set to "UTC" or "GMT" respectively. One or two digit numbers may be used as required. [The comma is also acceptable as a separator in place of the colon.] You may omit seconds [or minutes and seconds] if desired. For example, an entry of "16" will be entered as 16:00:00 or 4:00 PM. Press ENTER for "launch" ASAP, as soon as possible; depending upon the system, this will be in from 30 to 60 seconds from the current time. Enter Launch Date (MM/DD/YYYY): [or press ENTER for TODAY] Enter the date in the format shown. Note that if you requested UTC or GMT when entering the time, the date is interpreted as the UTC/GMT date. The full four digit year may be used OR two digits as in "92". Be sure to use the SLASH "/" rather than the MINUS "-" as the separator; STSPLUS's internal date algorithms will interpret the minus sign as just that and some rather strange dates can result! You may also use relative dates: -1 will use the prior day, +2 will use two days hence, and so forth. Press ENTER to use today's date. Press ENTER to accept, SPACE BAR to repeat: When all data have been entered, the program pauses for your approval. If all data are correct, press ENTER. Press the SPACE BAR to start over. Once the launch date and time have been set, this information is saved in file STSPLUS.INI. Remember, when changing from one satellite to another this information is NOT cleared automatically. ADJUST ORBITAL PARAMETERS (Simple Orbital Model) When the orbital parameters change in mid-mission for the "simple" orbital model, use this command to adjust the orbital period/altitude, orbital longitude, and time along orbital track. This technique allows the "launch time" to remain correct, which in turn assures that Mission Elapsed Time is correct, while adjusting the orbital parameters so that the position is displayed correctly in later orbits. The following items may be adjusted; in each case, the required units or input format is shown in parentheses (...) and the current value of the item is shown within square Program STSORBIT PLUS Satellite Orbit Simulation Page 34 brackets [...]. Press ENTER to leave an item unchanged. Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 Enter MISSION TITLE [STS Mission Simulation]: STS Mission Simulation Enter LAUNCH TIME (HH:MM:SS) [23:58:12 UTC]: 16:58:12 PDT 23:58:12 UTC Enter LAUNCH DATE (MM/DD/YYYY) [04/08/1992]: 4/08/1992 4/08/1992 Enter ORBIT INCLINATION (degrees) [28.4500x]: 28.4500 Enter ORBITAL ALTITUDE (nm) [ 160.00]: 160.00 nm ( 296.52 km) Enter LONGITUDE adjust (deg) [ 0.000x]: 0.000 Enter TIME adjust (min) [ 0.000]: 0.000 Press ENTER to accept, SPACE BAR to repeat: In order to make these adjustments, the current orbital altitude is required, along with the longitude and time of one or more equator crossings (obtained from the NASA Television wall map). NASA is not too good about making sure that the map is on the screen when the Shuttle makes an equator crossing (Northbound is Ascending Node, Southbound is Descending Node); they have a distressing tendency to cut away just before the crossing. With sufficient patience, however, the data can be interpolated from the television screen. See the section ADJUSTING ORBITAL PARAMETERS below for more information. F6 Set Elapsed Time Option ------------------------------ When using NASA 2-line elements, STSPLUS by default displays the time elapsed since the epoch date of the elements in the upper right portion of the data block. This is marked on the display as "T+Epoch". While this information is not of particular value for satellite viewing purposes, it does indicate the relative age of the orbital data. As a general rule, especially for lower Earth orbits, the effects of orbit decay make position predictions less accurate as time passes. Data which are more than 10 or 20 days old may produce less accurate positions. For a space shuttle mission, however, all mission events are scheduled against the mission timeline and are reckoned in Mission Elapsed Time (MET), the time elapsed since launch. It is therefore useful to be able to display MET during the course of a mission or to review the flight post- mission. Unfortunately, the standard NASA/NORAD 2-line element format does not include the launch time and launch date and therefore this information must be secured independently and manually entered into STSPLUS. Once entered, STSPLUS saves the information in file STPLUS.INI. The F6 command may be used to switch the display between Time Since Epoch ("T+Epoch") and Mission Elapsed Time ("MET"). The command description Program STSORBIT PLUS Satellite Orbit Simulation Page 35 on the Main Menu indicates which mode will be selected if Function Key F6 is pressed: F6 Display MET & Launch Times (now 2-Line Epoch Times) F6 Display 2-Line Epoch Times (now MET & Launch Times) The first example (the default) indicates that MET will be displayed if the function key is pressed; the second example indicates that the display will return to Time Since Epoch if the function key is pressed. (The elapsed time option may also be switched at any time during the tracking display by pressing Function Key F5.) The F6 command checks that you have already entered the launch time and launch date or that it has been read from a previous STSPLUS session from file STSPLUS.INI. However, since the launch time and date are not included in the 2-line orbital elements, no validity check can be made; if you change satellites, be sure to either clear or change the launch time and date. If no launch time and date are present, you will be prompted for the information as if you had pressed function key F5 (see above). The F6 command is inactive unless you have read in 2-line elements using function key F2. F6 Set FILENAMES and Paths ------------------------------ Function Key F7 allows the user to select the filenames (and drive and path, if included) for the files that STSPLUS uses to select satellites and tracking stations. Select filename to set or press ENTER when done: F1 Set 2-LINE ELEMENTS filename [NASA975.TXT] F2 Set TRACKING STATION filename [STSPLUS.TRK] Enter selection or ENTER: The current filename is shown in square brackets for each selection. Press F1 to set the 2-LINE ELEMENTS filename or F2 to select the TRACKING STATION filename. Press ENTER to leave a filename unchanged. The following is a typical prompt for filename: Enter TRACKING STATION filename: _ (Press ENTER to leave unchanged) If no filetype is entered, STSPLUS will automatically supply ".TXT" for 2-line elements files and ".TRK" for tracking station files. If the desired file has no filetype, include the period in the filename entered (for example: "STATION.") to prevent the automatic addition of a filetype. After each entry, the Filenames Menu is again displayed with the current selections. Press ENTER when done to return to the STSPLUS Main Menu. Program STSORBIT PLUS Satellite Orbit Simulation Page 36 F8 Set Program TIME and DATE -------------------------------- It is often convenient to set the TIME and DATE within STSPLUS to something other than the current system time and date, or to return to the current system time and date if the program time and date have been changed. Press F4 to go to the TIME and DATE Menu: Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 ACTUAL SYSTEM DATE AND TIME SHOWN ABOVE F1 Restore SYSTEM date and time (use "real time") F2 Set DOS SYSTEM CLOCK using calendar date and time F3 Set SIMULATED date and time using calendar date and time F4 Set SIMULATED date and time using Mission Elapsed Time ENTER Return to MAIN MENU Select desired function: The Date and Time Menu, shown above, displays the available time setting functions along with the actual system date and time as determined by the DOS software clock in your computer (even if simulated time is in effect). Both your local date and time, "PDT" or Pacific Daylight Time in the example, and "UTC" (Coordinated Universal Time) date and time are displayed. If times have been set using the letter "G", the abbreviation at the right will be "GMT" (Greenwich Mean Time). If you wish to execute STSPLUS in "real time", cancelling any simulated time that may be in effect, use the F1 command. This will restore the time and date used for the tracking display to that shown at the top of the menu. If the actual system date or time displayed is incorrect, use the F2 command to correctly set your system clock. If the zone abbreviation or UTC date or UTC time is incorrect, use the Set Program Options command from the Main Menu (F10+F9) to correctly set the UTC Offset and the Daylight Flag. Some organizations, NASA for example, continue to use the wording "Greenwich Mean Time" or "GMT" for what is now usually referred to as "Coordinated Universal Time" or "UTC" (and sometimes, depending upon the application, as "UT", "UT1" or "UT2"). STSPLUS uses Coordinated Universal Time or "UTC", the time used for civil timekeeping and broadcast by radio stations such as WWV and the BBC. Although technically these different time standards are not exactly the same, the difference is only a maximum of 0.9 seconds and the program treats them all as identical. STSPLUS defaults to the abbreviation "UTC" but if you prefer to use "GMT", enter any simulated time using F3 and include the letter "G" (upper or lower case) at the end. The time abbreviation at the top of the screen will change from "UTC" to Program STSORBIT PLUS Satellite Orbit Simulation Page 37 "GMT" and will continue using that abbreviation until a time is entered suffixed with "U". Times are always entered as "HH:MM:SS" where HH is HOURS, MM is MINUTES, and SS is SECONDS. The time entry format is very flexible. Leading zeroes are not required. The comma (",") may be used in place of the colon (":") as a separator if desired. SECONDS or MINUTES and SECONDS may be omitted if desired. Time entries are assumed to be local time; to enter UTC or GMT times, add the letter "U" or "G" (upper or lower case) respectively following the entry. For example, the following are valid time entries: Entry Interpreted as ------ -------------- 12 12:00:00 13,1 13:01:00 4:1:15 04:01:15 1,1,1 01:01:01 13,45U 13:45:00 UTC 1:20g 01:20:00 GMT Dates are always entered as "DD/MM/YYYY" where DD is DAYS, MM is MONTHS, and YYYY is the full four-digit year. The full date must always be entered; leading zeroes are not required. The date entered is assumed to be for the same time zone as the time entered. If local time is entered, the date will be treated as the local date; if UTC (or GMT) time is entered, the date will be treated as the UTC/GMT date. After a time or date entry has been read (after you press the ENTER key), STSPLUS reformats the entry to its standard format, clears the characters you entered, and replaces them by the standard format. This provides a double check that the program has interpreted your entry as you wished. Press F1 to restore the program date and time to the system date and time. This command reads the DOS clock and restores the program to "real time" operation. If the program date and time have not been changed with the F3 or F4 commands, this command will have no effect. Press F2 to set the DOS system clock. Use this command if you wish to change the actual date and time on your system. Note that on many systems using DOS 3.3 or higher, this command will set BOTH the software clock AND the hardware clock. Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 CAUTION: This function will change the computer's SYSTEM CLOCK! Press ENTER to leave an item unchanged Enter TIME (HH:MM:SS): 16:34:24 PST Enter DATE (MM/DD/YYYY): 4/08/1992 Press ENTER to accept, SPACE BAR to repeat: _ Program STSORBIT PLUS Satellite Orbit Simulation Page 38 The sample above shows the screen after the time and date entries have been completed. The current ACTUAL system date and time are displayed for approval. Press ENTER to accept the time and date displayed, or press the SPACE BAR to repeat the entries. Press F3 to set a simulated date and time. The date and time may be either in the past or in the future. This command does NOT affect the DOS clock in your system! Use the F2 command above to restore the date and time to "real time". Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Simulated time: 13:00:08 PST 21:00:08 UTC Simulated date: 11/08/1991 11/08/1991 Press ENTER to leave an item unchanged Enter SIMULATED TIME [12:05:06]: 13:00:00 PST 21:00:00 UTC Enter SIMULATED DATE [11/11/1991]: 11/09/1991 11/09/1991 Press ENTER to accept, SPACE BAR to repeat: _ The sample above shows the screen after the time and date entries have been completed. The new SIMULATED date and time are displayed (and counting) for approval. Press ENTER to accept the time and date displayed, or press the SPACE BAR to repeat the entries. Press F4 to set a simulated date and time using MET (Mission Elapsed Time). Note that this command will appear ONLY if the mission name begins with the letters "STS", signifying a Space Transportation System (Space Shuttle) mission AND if a launch time and date have previously been entered. Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Simulated time: 00:17:18 PST 08:17:18 UTC Simulated date: 10/09/1991 10/09/1991 Enter desired Mission Elapsed Time (MET) Enter MET DAY (NN): 3 day(s) Enter MET TIME (HH:MM:SS): 04:30:00 MET Press ENTER to accept, SPACE BAR to repeat: _ The sample above shows the screen after the day and time entries have been Program STSORBIT PLUS Satellite Orbit Simulation Page 39 completed. The Mission Elapsed Time is immediately converted to actual date and time and the current SIMULATED date and time, based upon the MET just entered, are then displayed (and counting) for approval. Press ENTER to accept the time and date displayed, or press the SPACE BAR to repeat the entries. Press ENTER to return to the Main Menu with the date and time as displayed on the screen (Current or Simulated). F9 DOS Shell (CAUTION: DOS Version 3.x+ ONLY!) ------------------------------------------------- If a system function is desired at the Main Menu, press F9 to execute BASIC's DOS SHELL function. This will return you to a DOS prompt and most DOS commands may be executed immediately. When the Shell is executed, STSPLUS remains in memory and the map data will not be re-read when you return. However, this means that a substantial amount of memory is in use and not available to DOS during the shell operations. Enter "EXIT" (without the quotation marks and followed by ENTER) at the DOS prompt when you wish to return to STSORBIT. CAUTION: The BASIC SHELL function is only reliable for versions of DOS of 3.0 or higher! Systems with less than 640K memory may fail to execute the shell and applications requiring large amounts of memory may also fail. F10 Set STSORBIT PLUS Program Options and Features ----------------------------------------------------- A number of program features and display options are set using the F10 Program Options Menu. These selections are further described in the section "Program Options Menu" below. ENTER Resume Mission --------------------- Pressing ENTER resumes the current mission shown in parentheses to the right of the command on the Main Menu. The 2-line elements file from which the data was read is shown in square brackets. ENTER Resume Mission (STS-41 [STS41F]) Any manually entered data is retained. "ENTER" means the key marked ENTER, RETURN, or with a left pointing arrow -- but not the backspace or cursor position keys which may also be marked with arrows! (I am afflicted with too long a memory; once upon a time this function was known as Carriage Return and was often shortened to RETURN or even CR. With the advent of electronic typewriters, video terminals, dot matrix printers and all the rest, "carriages" have long since disappeared but old habits die hard! Most PC keyboards are now marked with "ENTER".) Use ENTER to resume plotting a mission in progress after returning to the Main Menu to perform some change (such as enabling the node display, enabling the NASA tracking stations, or adjusting the time or date). Program STSORBIT PLUS Satellite Orbit Simulation Page 40 ESC Quit STSORBIT PLUS and Save Current Mission -------------------------------------------------- Press ESC (the key marked "ESC" or "Esc", not the letters E+S+C) to quit program STSORBIT PLUS. If you press ESC to quit the program and have manually entered orbital data, STSPLUS will save all required mission data in file STSPLUS.INI prior to terminating. This will be the data available with the ENTER key the next time you execute the program. The demonstration data will not be saved, preserving any previously saved mission data. When you have finished with STSPLUS, press ESC at the Main Menu to return to DOS. The data (and any adjustments you have made) for the current mission are saved in file STSPLUS.INI, but the map data is lost and will be re-read when you next use program STSPLUS. Program STSORBIT PLUS Satellite Orbit Simulation Page 41 PROGRAM OPTIONS AND FEATURES MENU --------------------------------- A number of program features and display options are set using the F10 Program Options Menu. When used with CGA displays, the features shown below as selected by function keys F3 and F4 are not available because of the low resolution of the CGA display. The following Options Menu is displayed when the F10 command is entered from the Main Menu: Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 04/08/1992 04/08/1992 F1 Program STSORBIT PLUS Information F2 Set New Local Coordinates (Rancho Palos Verdes, CA) F3 Select Display Features F5 OFF Show Ascending & Descending Node Data F6 ZOOM Set Map Center and Size F7 OFF Set for SLOW COMPUTER or NO COPROCESSOR F9 -8.00 Set UTC Time Offset and Daylight Flag F10 OFF Enable/Disable Printer Logging ENTER Return to MAIN MENU Select desired function: F1 Program STSORBIT PLUS Information ---------------------------------------- Function Key F1 displays information about program STSORBIT PLUS including the copyright notice, version number, my name and address, and the telephone number of my RPV ASTRONOMY BBS (Bulletin Board System). The current version of STSORBIT PLUS is always posted on the BBS. The BBS has a power controller; if it hasn't answered after the THIRD RING, hang up and call back in two minutes. The BBS is available 24 hours per day at 9600, 2400 and 1200 baud. F2 Set New Local Coordinates -------------------------------- In order to perform the calculations related to satellite visibility and altitude/azimuth, STSPLUS must know the geographic coordinates for the user's location. The name of the current location is shown in parentheses. When STSPLUS is first started, the default coordinates are set to Rancho Palos Verdes, California, near Los Angeles. The current location is indicated by the name in parentheses on the Options Menu. The program provides two methods for setting your own coordinates: reading a file of Program STSORBIT PLUS Satellite Orbit Simulation Page 42 city names and coordinates (STSPLUS.CTY); or manually entering the information. Pressing F2 will display the following reminder: STSPLUS will search for the CITYNAME you enter anywhere in the city names in the file. For example, 'SAN' appears in 'SAN FRANCISCO', 'SAN DIEGO', 'SANTA ANA', 'THOUSAND OAKS', etc. Press ENTER to cancel the search or enter '*' to enter a new city name and coordinates. In other words, when you enter a name or partial name, STSPLUS will attempt to match that group of characters anywhere in the names which appear in the city file. For example, 'SAN' matches 'SAN diego' as well as 'SAN jose' and 'thouSANd oaks'. To get 'SAN FRANCISCO' on the first try, enter 'SAN F' with a space between the 'N' and 'F'. Case is not significant; upper and lower case letters are treated identically. If you change your mind and wish to cancel the operation, simply press ENTER by itself. Use BACKSPACE to make corrections. To begin the search, enter the desired name after the prompt. In the example which follows, the name 'ran' was entered for the search. Enter CITYNAME to search for: ran Processing record 1 City Name: Rancho Palos Verdes CA Latitude: 33.7675 Longitude: -118.4033 Elevation: 186 meters Press ENTER to ACCEPT this city as your location, OR Press TAB to also display this city's location, OR Press SPACE to search for next city: _ If the city displayed is the one you wish to use as your local coordinates, press ENTER. If you wish to display this city as a second location on the display, press TAB. The data will be entered into STSPLUS and subsequently saved in file STSPLUS.INI. If you wish to search further in the file, press the SPACE BAR. NOTE: The elevations contained in file STSPLUS.CTY are almost all zero except for Rancho Palos Verdes, CA for which I have accurate elevation above mean sea level. If you know the correct elevation for your location, edit the file using any ASCII text editor and change the last number on the line. STSPLUS.CTY contains over 800 cities. If users send me their correct elevations (or additional cities they wish added), I will incorporate that data into subsequent versions of file STSPLUS.CTY. To enter location data manually, press "*" (without the quotation marks) followed by ENTER. You will be prompted for the city name, latitude, longitude, and elevation. Latitude and longitude may be entered using three different formats for convenience (note the use of comma and decimal point): DDD.DDDDD Degrees and decimal fraction DD,MM.MMM Degrees, minutes and decimal fraction Program STSORBIT PLUS Satellite Orbit Simulation Page 43 DD,MM,SS.SS Degrees, minutes, seconds and fraction Note that SOUTH latitude and WEST longitude must be entered as NEGATIVE numbers as measured south of the Equator or west of the Prime Meridian at Greenwich, respectively. The default unit for elevations is meters above mean sea level; add "F" (upper or lower case without the quotation marks) if you wish to use feet. After the elevation has been entered, the data will be displayed for approval. All data are converted to degrees or meters as appropriate, regardless of the units used on input. City Name: Rancho Palos Verdes CA Latitude: 33.7675 Longitude: -118.4033 Elevation: 186 meters Press ENTER to ACCEPT this city, OR Press SPACE to cancel this data: _ Press ENTER to accept the data as shown or SPACE to cancel the data and return to the Options Menu. If the data is accepted, STSPLUS will ask if you wish to append (add) this city/location to the existing file STSORBIT.CTY so that it will be automatically available thereafter. Do you with to append this data to file STSPLUS.CTY (Y/n): _ Press "Y" or ENTER to append the data to the file, or press "N" to not modify the STSPLUS.CTY file. F3 Set Display Features --------------------------- A number of display features may be enabled or disabled using a separate sub-menu. See complete description below. F6 Set Map Center and Size ------------------------------ The F6 command selects the size and field of view of the displayed map. By default, the map is of the full world and is centered on the Prime Meridian at Greenwich, England at 0 degrees longitude. (Most Americans are accustomed to viewing the map centered somewhere in the Western Hemisphere, and this orientation may be selected by pressing the "W" or "0" keys while the map is displayed.) It is sometimes convenient to shift the map center when the Space Shuttle is near the edge of the map. Pressing F6 will select between WORLD, QUAD, and ZOOM maps. Selecting QUAD will select one of twelve Quadrant Maps showing a field of view (horizontal size) of 180 degrees. Selecting ZOOM will select a Zoom Map with field of view adjustable from 45 degrees to 180 degrees; the default field of view is 75 degrees. The Zoom Map is approximately centered on the current ground track position of the satellite. See the sections QUADRANT MAPS, ZOOM MAPS, and AUTOMATIC MAP GENERATION for additional information. Program STSORBIT PLUS Satellite Orbit Simulation Page 44 F7 Set for SLOW COMPUTER or NO COPROCESSOR ---------------------------------------------- IMPORTANT NOTE: ALWAYS select the Slow Mode if your computer is not equipped with a math coprocessor. When operating in real time, if the seconds portion of the times shown as "Local:" and "Simulation:" do not agree to within 1 or 2 seconds, your computer is NOT able to maintain real time operation without the SLOW MODE! Press F7 to switch the SLOW MODE between "ON" and "OFF". "ON" means that the SLOW MODE is active; "OFF" means normal operation. As features have been added to STSPLUS, it has become increasingly difficult for very slow computers or computers not equipped with a math coprocessor to keep up with real time orbital calculations. For example, my old vanilla Zenith Z-148, operating at 4.77 MHz and no math coprocessor, takes approximately 4.5 seconds to make a full set of orbital position calculations; when switched to 8.0 MHz, the same calculations require only 2.5 seconds. In either case, the program gets hopelessly behind itself and can never catch up when operated in the normal program mode. While perhaps not the "ideal solution" as compared to having a math coprocessor, setting STSPLUS for SLOW MODE can help in all but the most severe cases. Instead of attempting to update its position calculations every second, the update is slowed to every ten seconds in this mode. This allows the computer to more or less keep up at a cost of not having data quite in real time on a second-by-second basis. Except for the frequency of updates, all program features are unchanged. When STSPLUS is operating in the SLOW MODE, the phrase "SLO" appears at the lower right of the tracking display. Since STSPLUS makes no test to see if it is keeping up with real time (the program cannot tell the difference between the PAUSE or SIMULATED TIME modes versus REAL TIME) and since most features are available for all computer configurations, it is the responsibility of the user to make sure that the appropriate options have been selected. Even in the SLOW MODE, certain very slow computers (such as an old XT-class computer running at 4.77 MHz) cannot keep up if the solar terminator is enabled when using the 2-line elements orbital data. F9 Set UTC Time Offset and Daylight Flag -------------------------------------------- STSPLUS uses UTC or Coordinated Universal Time, an adjusted version of Universal Time (which STSPLUS considers the same as GMT or Greenwich Mean Time), for certain functions such as launch time. The difference between UT, UT1, UT2 and UTC is never more than 0.9 seconds. UTC is used because it is the standard for civil timekeeping and agrees with standard atomic time, TDB or Terrestrial Barycentric Time, used by astronomers. However, NASA continues to use the GMT designation, a holdover from earlier days before the introduction of UTC. Using UTC permits critical data to be used across many time zones without conversion. However, it also means that STSORBIT must know what number of hours to add to UTC in order to obtain your local time, and whether or not you are currently using daylight Program STSORBIT PLUS Satellite Orbit Simulation Page 45 savings time (summer time in the UK). When prompted, enter the time offset in hours from your local time to Coordinated Universal Time. Examples are shown for most time zones in North America. STSPLUS then asks if you are using daylight savings time; enter "0" if not, and "1" if so. The sum of these two values is shown on the Main Menu; for example, if the computer is set to Pacific Daylight Time (UTC offset is -8.00 hours and Daylight Flag = 1), the sum will be -7.00. For most time zones in North America, the correct zone abbreviation will be shown on the ground track display for Local date and time. When you change your computer from/to daylight savings time, use this command to update STSPLUS. The following shows the display when using the F9 command: Set UTC TIME ZONE OFFSET and DAYLIGHT FLAG STSPLUS must know the difference between your local time zone and Coordinated Universal Time (UTC), also sometimes known as Greenwich Mean Time (GMT). With this information, STSPLUS can automatically adjust launch or Epoch times and dates for your local time zone. In addition, STSPLUS must know if your computer is now set to STANDARD or DAYLIGHT time. First, enter the difference between your STANDARD time zone and UTC in hours. Do NOT include the hour for daylight time if you are now on DAYLIGHT time; it will be entered separately. For most time zones in the United States and Canada, the entries required are: Eastern Standard Time EST -5.0 Central Standard Time CST -6.0 Mountain Standard Time MST -7.0 Pacific Standard Time PST -8.0 Enter UTC Offset (hours): -8 Enter DAYLIGHT Flag (0=OFF, 1=ON): 1 Once this information has been entered, it will be saved in file STSPLUS.INI and will not be requested again. If you change from Standard to Daylight Time or vice versa, use the F10+F9 command to update the Daylight Flag. INI files obtained from other users may be from a prior version of STSPLUS and the time information may have to be corrected. F10 Enable/Disable Printer Logging ------------------------------------- I have found it interesting to log the orbital data and the ascending and descending node information on my printer when analyzing the mission data over long periods of time. The F10 command toggles the printer logging function on and off. The first page of the log includes the current orbital data and subsequent pages contain only node information. In addition to the information presented on the display, the printer log also calculates the current orbital time, the time from one ascending (descending) node to the next. A typical log is shown below. IMPORTANT: BE SURE THE PRINTER IS TURNED ON PRIOR TO ENTERING THE F10 COMMAND. Program STSORBIT PLUS Satellite Orbit Simulation Page 46 STSORBIT: Space Shuttle Tracking Program, Version 9218 Page 1 ORBITAL DATA for STS-31 Discovery/HST Launch Date: 04/24/1990 Launch Time: 05:33:52 Orbit Inclination: 28.4695 Orbit Altitude: 329.50 nm Adjust Longitude: 9.80 Adjust Orbit Time: 8.40 min ORBITAL LOCAL DATE TIME ORBIT LONG MET TIME 04/28/1990 20:32:52 Ascend Node: 70 -69.95 4/14:58:07 1:36:55 04/28/1990 21:20:52 Dscend Node: 70 97.64 4/15:46:35 1:36:55 04/28/1990 22:09:52 Ascend Node: 71 -94.77 4/16:35:02 1:36:55 When printer logging is enabled and the ground track is displayed, the word "LOG" will appear in red at the right of the text area. Enabling printer logging also automatically enagles the display of ascending and descending node information. When a printer log is prepared for 2-line orbits, the Adjust Longitude and Adjust Orbit Time entries will not be shown in the header data. The Launch Date and Launch Time entries are given if that information has been entered indepentently. The Epoch Date and Epoch Time are always shown for 2-line element simulations. Note also that the orbit altitude shown is the altitude at the time the log was started and will not be correct for subsequent entries, especially if the satellite has an elliptical orbit (high eccentricity). A printer log may be prepared in advance of a mission by enabling printer logging from the Set Options Menu (with the F10+F10 command), setting the desired simulation time (F8+F3 command), then starting the ground track display with ENTER; once the ground track has appeared on the screen, pressing the F key twice to set STSPLUS in the X60 fast time mode will generate the date relatively quickly (although the UT TIME printed may be off by as much as one minute in the X60 mode). Allow the simulation to run for the desired length of time, then press ENTER to return to the Main Menu. While the ground track is active Function Key F3 performs the same function as the F10+F10 command to enable or disable printer logging. Program STSORBIT PLUS Satellite Orbit Simulation Page 47 SET DISPLAY FEATURES -------------------- Depending upon the satellte and personal preferences, a variety of display features may be enabled or disabled. Not all features are available with monochrome or CGA monitors. Pressing F3 on the Set Program Options and Features Menu will display the following menu: Program STSORBIT PLUS Space Shuttle and Satellite Orbit Simulation Version 9218 Current time: 09:42:13 PST 17:42:13 UTC Current date: 04/09/1992 04/09/1992 F1 ON Display LOCAL Circles of Visibility F2 OFF Display TDRS Coverage F3 ON Display Additional Map Grid Lines F4 OFF Display Tracking Stations F5 BOTH Display Ground Track: DOTS/LINE F6 ON Display Spacecraft Circle of Visibility F7 OFF Display South Atlantic Anomaly Zone F8 ON Display Terminator, SUN, and Spacecraft Lighting F9 NM Select Distance Units: NM or KM F10 A/A Select Satellite Coordinates: RA/DEC, Alt/Az, XYZ ENTER Return to MAIN MENU Select desired function: F1 Display LOCAL Circles of Visibility ------------------------------------------ Function Key F1 enables and disables the local circles of visibility, centered on your location and a second location if enabled. In some situations (geosynchronous satellites, for example), these circles of visibility cover so large an area that they simply confuse and clutter the display. Use this command to disable the circles. F2 Display TDRS Coverage ---------------------------- This command will display the approximate communications coverage for the Tracking and Data Relay Satellites (TDRS) EAST and WEST. The coverage boundaries overlap between the East and West TDRS satellites and Mission Control may select either satellite during the overlap period. STSPLUS assumes, as is often the case, that TDRS East will be selected when it is available. Program STSORBIT PLUS Satellite Orbit Simulation Page 48 F3 Enable/Disable Additional Map Grid Lines ----------------------------------------------- This command is not available for CGA systems. The basic world map includes the Equator and the meridians at 0 degrees, 90 degrees, and -90 degrees shown in blue on color monitors; the Equator and the prime meridian are shown in bright blue. Turning on the map grid adds lines of longitude every 30 degrees and lines of latitude every 10 degrees. Displaying the additional grid lines on monochrome monitors usually makes the screen too "busy" and is not recommended. In the Quadrant and Zoom Map modes, the spacing of the additional grid lines is adjusted for the map field of view. For EGA and VGA systems, each grid line is labeled at the left or bottom of the display screen. F4 Display Tracking Stations -------------------------------- The F4 command enables/disables the display of the tracking stations included in file STSPLUS.TRK or the current TRACKING STATION filename as set by Function Key F7 on the Main Menu. If that file is not found, internal data are used for NASA's 14 original ground tracking stations plus the NASA Ground Terminal at White Sands, NM. Each tracking station is located with a small red circle. The circle of visibility is also shown if that circle has an angular diameter of 90 degrees or less. For all map modes EXCEPT the World Maps, each tracking station is also labeled with its 3-character abbreviation. This command is not available for CGA monitors. F5 Show Ascending & Descending Node Data -------------------------------------------- The nodes of an Earth orbit are the points on the ground track where the path crosses the Equator. The ascending node crosses from South to North and the descending node crosses from North to South. Orbit numbers normally increment at the ascending node. This command adds two additional lines of data at the lower left of the screen giving the time (MET or time since epoch) and longitude of the most recent ascending and descending nodes. This information is useful when comparing STSPLUS's data against other sources such as the wall map in Mission Control. F5 Display Spacecraft Circle of Visibility ---------------------------------------------- STSPLUS can calculate the approximate circle of visibility from the spacecraft, the area of the Earth's surface which is visible from the cockpit windows and television cameras or, for unmanned spacecraft, the direct line of sight visibility from the ground. Note that the shape of the "circle" varies from approximately a circle near the Equator to something like a rounded triangle at higher latitudes. Near the poles, the "circle" spreads out across the map. This is an artifact of the map projection; plotted on the surface of a sphere, it would be a true circle. This feature may be enabled for all spacecraft using 2-line elements. When the program is run in the normal mode, the circle of visibility is Program STSORBIT PLUS Satellite Orbit Simulation Page 49 recalculated every 10 seconds based upon the spacecraft's current altitude. This means that orbits with a high eccentricity (that is, a highly elliptical orbit whose apogee and perigee are very different) will exhibit a constantly changing circle of visibility. When the program is run in the SLOW MODE, the radius of the circle of visibility is constant, as calculated at the moment the ground track display is started. This feature may also be enabled or disabled during the ground track display using Function Key F7. F7 Display South Atlantic Anamoly Zone ------------------------------------------ The South Atlantic Anamoly (SAA) is an area in the southern hemisphere lying between southern tip of Africa and South America which can cause severe electromagnetic disturbances on spacecraft. For example, the semiconductor memory on the Hubble Space Telescope (which regularly passes through the SAA) was being changed by this phenomenon until a patch was uplinked to work around the problem. The area is shown on the ground track as an ellipse for simplicity; its actual outline is more nearly shaped like a kidney bean. Using NASA Mission Charts for various Space Shuttle missions as a reference, the SAA is adjusted for spacecraft altitudes from 160 nm to 350 nm (although it extends out to geosynchronous orbits). F8 Display Terminator, Sun, and SpaceCraft Lighting ------------------------------------------------------- Many types of observations, especially Earth observations, often require that the target or terrain be in sunlight. The solar terminator is a series of yellow points on the display which represent the line at which the center of the Sun is at an observer's horizon for mean sea level. Although a quick glance outside should suffice to determine which side of the terminator line is in sunlight and which in darkness, EGA and VGA systems also display the Sun as a small yellow circle. The terminator is partially updated every 10 seconds (and is fully updated every 60 seconds) and the sun is updated every 60 seconds. Because of the additional calculations and plotting required, these solar features may not be suitable for slow processors or computers without a math coprocessor. NOTE: Only the terminator is available with CGA systems. The terminator is sometimes confused with a line denoting sunrise and sunset. Two factors make the terminator only an approximation: first, the terminator is based upon the center of the Sun, while sunrise and sunset are based upon the upper limb of the Sun; and second, the terminator is calculated for mean sea level rather than a specific local elevation. These two factors combined can result in a difference of ten minutes or more when the times are compared against published values or those calculated by my program ASTROCLK, sunrise being earlier and sunset being later. The performance of solar panels and certain other instruments on a spacecraft is dependent upon whether or not the Sun is in view. Similarly, it is usually impossible to visually see a spacecraft which is not in sunlight. STSPLUS calculates whether the spacecraft is in full sun, penumbra (partial shadow) or refracted sunlight, or umbra (full shadow) and adjusts the color of the spacecraft icon accordingly: bright white, yellow, and dim white respectively. This feature is not available on CGA and HGC Program STSORBIT PLUS Satellite Orbit Simulation Page 50 monitors. Note also that the spacecraft lighting is indicated by an asterisk ("*") next to the "Orbit #"; the asterisk is present when the spacecraft is in full or partial sunlight and is absent otherwise. F9 Select Distance Units: NM or KM -------------------------------------- Spacecraft altitude, range, and satellite geocentric rectangular coordinates (X, Y, and Z) may be expressed in either nautical miles (nm) or kilometers (km). This feature selects which units will be displayed for those data. F10 Select Satellite Coordinates: RA/DEC, AltAz or XYZ --------------------------------------------------------- This feature selects between topocentric (horizon) coordinates (referenced to the Earth's surface and in this case the current local geographical coordinates), equatorial coordinates for the satellite's position (Right Ascension and Declination), or geocentric rectangular coordinates (X, Y, and Z, referenced to the center of the Earth). Press F10 until the desired coordinate system is selected. The abbreviations "R/D", "A/A", and "XYZ" are used on the menu. You may also use Function Key F10 to switch between coordinate systems while the ground track display is in progress. NOTE: The geocentric rectangular coordinates are only available when using 2-line orbital elements. For most purposes, such as aiming an antenna or visually spotting the satellite, the horizon coordinates of Altitude and Azimuth are desired. If you wish to determine the star background, however, the equatorial coordinate system of Right Ascension and Declination (referenced to the epoch of date) is needed. For both of these coordinate systems, the range from the current local coordinates to the satellite is also given, expressed in either nautical miles or kilometers as selected by F9 above. Program STSORBIT PLUS Satellite Orbit Simulation Page 51 ACTIVE KEYS DURING GROUND TRACK DISPLAY --------------------------------------- The following table lists the various keys which are active when the tracking display is shown on the screen. Some of these features are more fully described elsewhere. ENTER Return to Main Menu (cancel this simulation). F1 On-line HELP. Press F1 to display a help screen in the lower portion of the screen. Press F1 again to resume normal data display. F2 Toggle the BLINK mode of the satellite symbol between blinking and steady. F3 Enable or disable printer logging. If logging is enabled, the word LOG appears in the lower right of the screen. Be sure the printer is turned on BEFORE using the L command. The "L" command automatically enables the display of ascending and descending node information. F4 Toggle FAST mode from x1 to x10 to x60 to x1, etc. When either of the fast modes is enabled, "(x10)" or "(x60)" will appear at the upper right of the data block in red. This feature operates in both the normal (real or simulated time) and PAUSE modes. F5 When using 2-line elements, the elapsed time may be switched between "T+Epoch" and "MET" by using the "T" command. If no launch time and date have been entered, the "T" command will have no effect. F6 Enable/Disable PAUSE mode; only the local date and time are updated. The plot is frozen at the current position and the "+" and "-" commands are enabled. F7 Enable or disable the spacecraft circle of visibility. F8 Enable or disable the SUN, solar terminator, and spacecraft lighting feature. NOTE: The SUN and spacecraft lighting are not available on CGA systems. F9 Change units of distance between kilometers (km) and nautical miles (NM). F10 Change satellite coordinates between Altitude and Azimuth, Right Ascension and Declination, and Geocentric Rectangular (XYZ) systems. + During PAUSE mode only, moves the satellite to the NEXT calculated position based upon the FAST mode then in effect: simulated time is advanced 1, 10, or 60 seconds. (NOTE: The "=" key may be used instead of "+" to avoid the SHIFT KEY.) - During PAUSE mode only, moves the satellite to the PREVIOUS Program STSORBIT PLUS Satellite Orbit Simulation Page 52 calculated position based upon the FAST mode then in effect: simulated time is backed up 1, 10, or 60 seconds. NOTE: Since this "-" or reverse feature was implemented primarily for testing and demonstration, the ground track logic does not recognize these reverse movements and will become slightly confused. To restore the ground track and current times properly, press ENTER TWICE to stop the display and then to resume the mission. TAB Enable or disable automatic map generation. When automatic map generation is enabled, the letter "A" will appear in the upper right of the display screen. Pressing the TAB key will always cause the map to be redrawn. PgUp When in one of the zoom modes, increases the field of view up to a maximum of 180 degrees. Press rapidly to execute multiple zoom steps without redrawing the map for each keypress. PgDn When in one of the zoom modes, decreases the field of view down to a minimum of 45 degrees. Press rapidly to execute multiple zoom steps without redrawing the map for each keypress. Home When in one of the zoom modes, returns the field of view to 75 degrees. End When in one of the zoom modes, returns the field of view to the last zoom factor used prior to pressing the HOME key. W,0 Select World Map display, showing the full world from +85 degrees North latitude to -85 degrees South latitude. If automatic map generation is disabled, pressing "W" or "0" will toggle between the two world map displays. Q Select Quadrant Map display, showing 180 degrees field of view and selected so as to approximately center the satellite. 1-9 Select the indicated Quadrant Map. Automatic map generation is !@# disabled when a specific quadrant map is selected. See the chart in the section Quadrant Maps for the map numbers. Z Select Zoom Map display, showing from 180 to 45 degrees field of view and selected so as to approximately center the satellite. The default is 75 degrees. L Select Location Map display, showing concentric isocontours for your location. If a second location has been enabled, press "L" again for that location. T Tracking Station Map display, showing concentric isocontours for the tracking station closest to the current ground track position of the satellite. Uses the data in the current TRACKING STATION file to select the tracking station(s); if the file is not found, STSPLUS defaults to an internal set of tracking stations. Use F7 from the Main Menu to select the TRACKING STATION filename. Program STSORBIT PLUS Satellite Orbit Simulation Page 53 M Satellite Motion Map display, available on EGA and VGA systems only. Displays a zoom map with the satellite centered. Maps are drawn "off screen" and a complete map is always displayed. The map is updated every 10 seconds or as rapidly as the computer processor will permit. R Resynchronize MET or T+Epoch with Local Time. Resets plot to real time. NOTE: The Resynchronization feature was originally intended only for testing and should only be performed when the simulated time and local times are reasonably close to each other. The plot may otherwise have some extraneous pixels left around! This feature has not been extensively tested. To restore the ground track completely, press ENTER to stop the display and return to the Main Menu, then press ENTER again to resume the mission. Program STSORBIT PLUS Satellite Orbit Simulation Page 54 STSORBIT PLUS'S ORBITAL MODELS ------------------------------ The original version of STSORBIT was first prepared without reference materials of any kind and the simplest possible orbital model was therefore selected. The primary objective was to duplicate the NASA wall map at the Mission Control Center in Houston, Texas. This "simple" model assumed that the orbit was perfectly circular at a specified altitude and inclination which never degraded due to other factors such as drag or perturbation. Some simplifying assumptions were incorporated to handle the initial ascent portion of a mission and the launch site was hard coded to Cape Canaveral, Florida. With only minor modifications, the program was essentially unchanged for the next year. The launch of STS-31 and the Hubble Space Telescope highlighted the need for improved accuracy because of public interest and the length of the mission. The orbital calculations were modified (STSORBIT Version 9015) to include the J2 factor, the perturbation of low Earth orbits due to variations in the gravitational field related to the non-spherical shape of the Earth (among other factors); omission of the J2 factor caused errors in longitude of approximately -5 to -7 degrees per day. That is, the orbital track drifted Westward from its true position by that amount. More accurate models of low orbits also include the J3 and J4 perturbation factors, atmospheric drag, and a host of other less significant items. Although by now reasonably accurate for the first day or so of a space shuttle mission, the "simple" model is by no means ideal. In calculating the current orbital longitude, for example, the "simple" model assumes a circular orbit. For orbits with low inclinations, as is typical for launches from Kennedy Space Center, the errors are not particularly significant and are probably overshadowed by the fundamental uncertainties in orbital parameters and by the limitations imposed by display resolution. Orbits with higher inclinations, as would be the case if near-polar launches from Vandenburg AFB in California are ever initiated, would have much larger periodic errors which would be both noticeable and objectionable. More important for longer missions and for general satellite tracking is the fact that due to the method used, errors in the orbital calculations tend to be cumulative. Although it may be the only method available at launch, after about a week the errors can become unacceptably large. The real problem with the simple method, of course, is that the Earth is not a perfect sphere and actual satellite orbits are never perfectly circular. Satellite orbits are significantly perturbed by the non-spherical gravitational field of the Earth, by the Sun and Moon, atmospheric drag, and other factors. Accurate satellite tracking over longer periods of time therefore demands more accurate data and a more rigorous treatment of satellite orbits. The only practical alternative is to use the NASA/NORAD 2-line orbital element sets. Not only are these data readily available publicly, but they are relatively accurate and are updated regularly. Therefore, STSORBIT PLUS, while retaining the capability of using the simple orbital model (primarily for demonstrations), relies on the 2-line element sets for orbit predictions. 2-line element sets for non-military space shuttle missions are typically available on the same day as the launch. Amateur astronomers and satellite tracking experts often generate "unofficial" 2-line element sets even for military missions. Six quantities are required by classical gravitational theory to completely characterize the orbit of one body about another in time and Program STSORBIT PLUS Satellite Orbit Simulation Page 55 space, the "Two Body Problem". These six quantities, often referred to as Keplerian orbital elements, are included in the NASA/NORAD 2-line element sets along with other numerical and statistical data. NORAD, the North American Air Defense Command headquartered in Cheyenne Mountain, Colorado, (and now known as the U. S. Space Command) developed the 2-line element format many years ago as part of their satellite tracking efforts and NASA subsequently adopted the same format -- more or less. My own analyses of 2- line element sets obtained independently from other NASA centers indicate that NASA and NORAD do not always use the same definition for revolution (orbit) numbers; NASA frequently gives a number one greater than NORAD, calling the first partial orbit number one while NORAD calls that same partial orbit number zero. Except for short duration missions, such as the Space Shuttle, revolution numbers are of no practical importance. Having timely and accurate orbital data is of little help without a computer model or program which can use those data. NORAD has rather arbitrarily divided satellite orbits into two categories: near Earth orbits and deep space orbits. Near Earth orbits are defined as those with orbital periods of 225 minutes or less and deep space orbits are all others. Computer models are described in the literature for each category. STSORBIT PLUS employs the SGP4 Near Earth Model only, using a composite of code of my own combined with translated Fortran and Basic source supplied by Paul Traufler and C source by Paul Hirose. Not only are the near Earth orbits generally of more interest to observers, but the errors associated with deep space orbits processed with the SGP4 model (rather than the correct SDP4 model) are not particularly significant for the purposes of a program such as STSORBIT PLUS. Further, watching a geostationary satellite orbit on the screen is not unlike watching grass grow and is about as exciting. For the past several years, T S Kelso has been making the NASA/NORAD data available as a public service on his Celestial BBS at (513) 427-0674. The 2-line element sets are prepared by Kelso using data received directly from NORAD by special arrangement. I regularly post a slightly edited version of the current element sets on my own RPV ASTRONOMY BBS as file NASAnnn.TXT, where "nnn" is the NASA Prediction Bulletin number. Kelso provides data for several categories of satellites: Amateur Radio, Earth Resources, Manned Spacecraft, Navigation, Weather, and NASA's 30 Day Specials (which contain objects launched within the last 30 days and are often easy to spot visually). More specifically, these include the following satellites or satellite series: OSCAR, Radio Sputnik, UOSAT, Cosmos, LandSat, SeaSat 1, SPOT, Mir, Salyut 7, Soyuz, Space Shuttle, NAVSTAR (GPS), GOES, Meteor, and NOAA. The Canadian Space Society BBS, (416) 458-5907, also regularly posts NORAD 2-line elements. Note that the CSS format is slightly non-standard, having additional information on the first (title) line for each satellite, and may have to be edited for use with some tracking programs. The CSS files also have considerable additional text material (including current satellite news) before and after the actual 2-line elements data. Finally, the NASA SpaceLink BBS in Huntsville, Alabama, (205) 895- 0028, 8 lines @ 300-9600 baud, provides mission information for all space shuttle missions and current 2-line orbital elements both pre-mission and while a mission is in progress. In addition, SpaceLink has a wealth of other NASA information. Program STSORBIT PLUS Satellite Orbit Simulation Page 56 ADJUSTING ORBITAL PARAMETERS (Simple Orbital Model) --------------------------------------------------- NOTE: This section applies ONLY if you are using the simple orbital model for simulation. When using 2-line elements, all orbital parameters are completely defined by the 2-line elements and may not be modified from within STSORBIT PLUS. For accurate satellite tracking, obtain 2-line elements from my BBS or elsewhere. The original "simple" orbital model used in STSPLUS is imperfect, to say the least. The data generally available from NASA prior to launch often does not describe upcoming shuttle orbits at all precisely. In particular, STSPLUS's initial orbital calculations, those related to the time from liftoff to orbital insertion, are more or less guesswork. Once a mission is established in orbit, it may be therefore desirable to adjust the orbital parameters to make STSPLUS's displayed ground track correspond more closely to that displayed on the wall map in Mission Control Center. To make the adjustment, use the following procedure: 1. Start program STSPLUS. After the map data has been read in, select the program options command, F10, then press F5 to enable display of the data for the ascending and descending nodes. Enter the original launch time and orbital data using the F4 command or restore prior data using the ENTER key or F3 command if no adjustments have been entered. The plot will appear on the screen. 2. Observe the crossing time and longitude of the node of interest and calculate the difference from the desired crossing time. Note that NASA performs all orbital calculations referenced to the Ascending Node, the point at which the ground track crosses the Equator in a North-bound direction. In order to view a given node crossing repeatedly, you may find it convenient to reset the DOS clock to a minute or two prior to the expected time. HINT: Use the F9 command to go back to DOS, then use DOS's TIME command to set the clock. BE SURE TO RESET YOUR COMPUTER CLOCK WHEN YOU HAVE COMPLETED ALL ORBITAL ADJUSTMENTS! 3. When adjusting orbital parameters, it is preferable to adjust the TIME before adjusting the LONGITUDE. This is because the longitude of the node is affected by the rotation of the Earth (by 15 degrees per hour) when the time is changed. Calculate the difference between the time of the observed node crossing and the time of crossing plotted by STSORBIT. If the observed time is earlier than the plotted time, the time adjustment must be positive, otherwise it must be negative. The time must be calculated in minutes and decimal fractions of a minute. 4. Press ENTER to return to the Main Menu, then press F5 to enter adjusted data. Enter the new orbital altitude in nautical miles (or altitude in kilometers by adding "km"); usually, the orbital altitude has not changed and you should press ENTER to leave this item unchanged. 5. Press ENTER to leave the longitude adjustment set at zero. Program STSORBIT PLUS Satellite Orbit Simulation Page 57 6. Enter the calculated time difference for the node crossing as the time adjustment (in minutes). 7. Press ENTER to leave the launch time unchanged. 8. Inspect the data displayed to be sure it is correct. If it is not, press the SPACE BAR to re-enter data; if it is correct, press ENTER to accept the data. STSPLUS will immediately begin plotting the new ground track. If necessary, interrupt the plot with the ENTER key, reset your computer's clock, return to STSPLUS, and press ENTER to resume current mission. 9. Again observe the plotted time of crossing and repeat from Step 4 if the time is not correct. If the time is correct, note the longitude of the node and calculate the difference from the observed longitude. If the plotted longitude is less than the observed longitude, the adjustment must be positive, otherwise it must be negative. The longitude adjustment must be entered in degrees and decimal fractions of a degree. 10. Press ENTER to return to the Main Menu, then press F5 to enter adjusted data. Enter the calculated longitude adjustment, then press ENTER twice to leave the time adjustment and launch time unchanged. 11. Inspect the data displayed to be sure it is correct. If it is not, press the SPACE BAR to re-enter data; if it is correct, press ENTER to accept the data. STSPLUS will immediately begin plotting the new ground track. If necessary, interrupt the plot with the ENTER key, reset your computer's clock, return to STSORBIT, and press ENTER to resume current mission. 12. This procedure should enable you to set the orbital parameters with considerable accuracy. However, even minor changes of the orbit in flight can cause a significant cumulative error in the ground track displayed by STSPLUS. If all this seems like too much effort, just enjoy the program "as it comes from the box"! Program STSORBIT PLUS Satellite Orbit Simulation Page 58 STSORBIT PLUS Revision History ------------------------------ Each released version of STSPLUS uses a four digit revision code such as 9218. The first two digits indicate the year and the second two digits indicate the week of the year. In some cases, an additional letter suffix is added to distinguish changes occurring within the same week or to identify special versions. A partial week at the beginning or end of the year is counted as a full week. Using this method, a year will typically have 53 weeks although it is possible to have 54 weeks in a leap year (1972 is an example). The current year-week revision code is shown on the Julian Date display, Display Mode 7, in my program ASTROCLK. This file records the revision history of program STSPLUS through all of the minor twists and turns that usually accompany the evolution of such a complex program. It illustrates the tortuous process of maintaining and refining a program as ideas and problems are reported from every quarter. These notes may also be helpful to users who are upgrading from one version to another to find out what has changed. David H. Ransom, Jr. Version 9218 -- April, 1992 --------------------------- -Version 9218 is the initial formal release of program STSORBIT PLUS and now includes complete program documentation. The beta releases included only update notes to the original STSORBIT 9134 documentation. While the STSORBIT PLUS documentation borrows heavily from that original STSORBIT documentation, many sections are new or have been rewritten and updated. -I had planned to modify STSPLUS for dual satellite operation but this proved more difficult than expected. After many hours of work, I have put that feature aside, at least for the present. The basic structure of the program does not lend itself to multiple satellites; this is particularly true of the multi-layered graphics features. The processor workload was also increased substantially, making things even more difficult for the many users without a math coprocessor chip. Finally, given the other demands on my time, a complete rewrite from scratch would simply require too long. I'm sorry to disappoint those who asked for that feature ... especially the folks working on Mission STS-49. -In order to eliminate problems due to changes from one program version to another, STSPLUS now ignores all data from different versions of file STSPLUS.INI and the program must be initialized as if being run for the first time. Only .INI files written by the same program version will be accepted. Trying to maintain backward compatibility with all prior versions of the .INI file has become both difficult and time consuming -- and has not always been successful. Too many problem reports have been due to corrupt or incompatible .INI files, sometimes from versions dating back several years. -There have been many changes from Beta Version 9206, too many to describe in detail here. Even "seasoned users" of STSPLUS should check this documentation carefully! Beta Versions 9139 thru 9206 ---------------------------- Program STSORBIT PLUS Satellite Orbit Simulation Page 59 -Beta versions released for testing and comment. Version numbers included 9139, 9145, 9146, 9148, 9202, 9203, 9204, and 9206. Significant changes and improvements were incorporated in these releases, often the result of user feedback. Many thanks to those who sent in bug reports and suggestions! Beta Version 9137 -- September, 1991 ------------------------------------ -Initial public beta version.