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Program STSORBIT Space Shuttle and Satellite Orbit Simulation (C) Copyright David H. Ransom, Jr., 1989-1990 All rights reserved. Version 9122A May 27, 1991 by David H. Ransom, Jr. Rancho Palos Verdes, CA BBS: (213) 541-7299 Program STSORBIT Space Shuttle Orbit Simulation Page i TABLE OF CONTENTS ----------------- INTRODUCTION ......................................................1 PROGRAM DESCRIPTION ...............................................4 STSORBIT FILES ....................................................6 STARTING PROGRAM STSORBIT .........................................7 STSORBIT MAIN MENU ................................................10 F1 Program STSORBIT Demonstration ..............................10 F2 Read NASA/NORAD 2-Line Elements .............................11 F3 Read Prior Mission Information from *.INI File ..............12 F4 Enter New Orbital Information ...............................13 F5 Adjust Orbital Parameters ...................................14 2-Line Elements Model .......................................14 Simple Orbital Model ........................................15 F6 Set Elapsed Time Option (2-line elements only)...............16 F8 Set Program TIME and DATE....................................17 F9 DOS Shell ...................................................20 F10 Set STSORBIT Program Options and Features ...................20 ENTER Resume Mission ...........................................21 ESC Quit STSORBIT and Save Current Mission ...................21 PROGRAM OPTIONS AND FEATURES MENU .................................22 F1 Program STSORBIT Information ................................22 F2 Set New Local Coordinates ...................................22 F3 Set Display Features ........................................24 F6 Set Map Center (USA/Europe) .................................24 F7 Set for SLOW COMPUTER or NO COPROCESSOR .....................25 F9 Set UTC Time Offset and Daylight Flag .......................25 F10 Enable/Disable Printer Logging ..............................26 SET DISPLAY FEATURES ..............................................28 F2 Display TDRS Coverage .......................................28 F3 Enable/Disable Additional Map Grid Lines ....................28 F4 Display NASA Tracking Stations ..............................29 F5 Show Ascending & Descending Node Data .......................29 F6 Display Spacecraft Circle of Visibility .....................29 F7 Display South Atlantic Anomaly Zone .........................29 F8 Display Terminator, Sun and Spacecraft Lighting .............29 F9 Select Distance Units: NM or KM .............................30 F10 Select Satellite Coordinates: RA/DEC, AltAz or XYZ ..........30 USING GROUND TRACK DISPLAY OPTIONS ................................31 THE STSORBIT GROUND TRACK DISPLAY .................................33 On-line Help (F1 Key) ...........................................33 World Map .......................................................33 Spacecraft or Satellite and Orbit Projections ...................34 User's Circle of Visibility .....................................34 Spacecraft Circle of Visibility .................................35 TDRS Satellite Features .........................................35 NASA Ground Tracking Stations ...................................36 STSORBIT'S ORBITAL MODELS .........................................37 ADJUSTING ORBITAL PARAMETERS (Simple Orbital Model) ...............39 STSORBIT REVISION HISTORY .........................................41 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 1 INTRODUCTION ------------ Program STSORBIT is an orbital tracking and display program for use during Space Shuttle missions, for simulating a Space Shuttle mission, or for general satellite tracking using NASA/NORAD 2-Line Elements. STSORBIT is copyrighted software; you are granted a non-exclusive license for non- commercial use only. Use it if you like it, discard it if you don't. There are no warranties of any kind. If you wish to use STSORBIT 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. Program STSORBIT is intended to display the orbital position and ground track of the Space Shuttle on a global map during a typical NASA mission. When used with NASA/NORAD 2-Line Elements, the position and ground track of other satellites, such as the Hubble Space Telescope, Gamma Ray Observatory, or the Soviet MIR Space Station, may also be displayed. Data for over 100 different satellites is included. An IBM-compatible computer is required. When used with an EGA or VGA video adapter, the display is in color; CGA and HGC systems can only present graphics in monochrome. In the past, NASA has not published or made available very precise information as to the orbital parameters either before or during a "normal" mission; recently, however, more accurate data has been published in magazines such as Aviation Week and in various NASA publications. NASA now also releases the "2-Line Elements", classical orbital elements which enable much more precise calculation of orbital positions. Military missions, of course, are almost completely blacked out. It is something of a challenge, therefore, to devise a program which will at least come close and which, as more or different information becomes available, will permit the program's data to be updated "on the fly". The primary objective is to present an interesting and attractive real time graphics display of a Space Shuttle mission or satellite ground track, rather than to generate mathematically precise positional information. The initial premise of 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 attempted to do that same task, but each fell short of my objectives for one reason or another; since then other programs have appeared which produce similar information, most notably Paul Traufler's excellent TRAKSAT. 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 +80 degrees to -80 degrees in order to achieve reasonable proportions and vertical resolution while at the same time showing recognizable land features. As a consequence of the basic inaccuracies relative to the orbital parameters of a Space Shuttle mission while it is in progress, I did not originally try to be especially precise with respect to the orbital mathematics. Additionally, mathematical complexity must be held to a reasonable minimum if older computers not equipped with a math coprocessor are to be able to maintain the presentation in real time. The higher STSORBIT Space Shuttle and Satellite Orbit Simulation Page 2 resolution and color graphics available with more advanced displays such as the EGA and VGA greatly enhance the program, but my Zenith laptop computer emulates the older CGA graphics display in (blue) monochrome. Therefore, in order to use my program on all of my own computers, STSORBIT maintains compatibility with all three display standards. Recently, compatibility with HGC (Hercules Graphics Card) has been improved. 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. 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 orbital model. The accuracy of that model degrades rapidly after five or ten orbits and, although it may be the only method available at the start of a mission, more accurate data are now readily available within a day or so of launch. The NASA SpaceLink BBS in Huntsville, Alabama also began posting 2-line orbital elements for the Space Shuttle in early 1991 due in part to my continuing suggestions. Beginning with Version 9022, therefore, STSORBIT was modified to also read orbital data from these NASA 2-line elements and thereby maintain significantly improved accuracy over longer periods of time. As an incidental benefit, the ground tracks of other satellites (such as the Russian space station MIR) may now 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) for best accuracy; STSORBIT calculates data and displays a ground track for deep space objects but the accuracy of these data has not been carefully validated; it is believed to be "reasonably" accurate. In mid-1990, STSORBIT 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 orbital data and permit following the mission timeline using MET. Although the launch time and date must be entered independently of the 2-line elements, the MET capability has now been added. Another suggestion from Ron and others was to include the sun terminator. 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, 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. If STSORBIT 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. In addition to individual users all over the world, STSORBIT is also being used in an educational setting. 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 3 widely distributed at a recent National Association of Science Teachers convention and by radio amateurs at regional "ham fests". 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. NASA SpaceLink, supervised by Bill Anderson of 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 (including STSORBIT), 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, available 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 bulletin board system (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 Elements. My thanks also to Brian K. Jones and his program SUNMAP (available on my BBS), an interesting program in its own right, for the initial map coordinate data file used in that program. SUNMAP served as a demonstration that a reasonable world map display WAS possible, particularly on EGA and VGA monitors, and encouraged me to start this STSORBIT project. Finally, my thanks to all those individuals who have taken the time to write to me or leave a message on my BBS with comments and suggestions. While I haven't been able to implement 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 bulletin board system (BBS) at (213) 541-7299. If the BBS has not answered after the fourth 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 now has over 850 users and is often busy, so be patient. If you do not have access to a modem, you may send US$10.00 to cover materials, postage and handling for a copy of the latest version; please specify 5-1/4" 360K or 3-1/2" 720K disks. David H. Ransom, Jr. 7130 Avenida Altisima Rancho Palos Verdes, CA 90274 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 4 PROGRAM DESCRIPTION ------------------- A typical Space Shuttle orbit is nearly, but almost never exactly, circular with an altitude of approximately one hundred and sixty nautical miles plus or minus thirty nautical miles and an inclination of from 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, Mission Control Center does occasionally announce the orbital altitude, which is usually in the neighborhood of about one hundred and sixty nautical miles; 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. Whenever practical, I generate the information required by STSORBIT as an initialization file, for example HST.INI for the Hubble Space Telescope, which may be read directly by the program and which uses the most current data. These .INI files are normally posted on my BBS during a mission. When orbital parameters change during a mission, as was the case with STS-31, new .INI files are posted as soon as the new data are determined. Within a day or two 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, NASA710.TXT) and the name of the desired satellite (HST for Hubble Space Telescope). STSORBIT takes care of all the rest. The 2-line element set available at the time of this release of STSORBIT 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" STSORBIT Space Shuttle and Satellite Orbit Simulation Page 5 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-character/line format. Particularly with the relatively low resolution (640x200) of a CGA monitor, the display errors alone are significant; using an EGA or VGA monitor improves the vertical resolution by about a factor of two (640x350 or 640x480, respectively), but does nothing to improve the horizontal resolution. The HGC (Hercules Graphics Card) has also been included; HGC users will note that of the 720 horizontal pixels available, only the left 640 pixels are used in the present code. Higher resolution monitors are available (at a price!) but are not in general use and so are not suitable for a program such as STSORBIT which is intended for public distribution. STSORBIT displays a portion of the Earth using a cylindrical projection of the surface area almost from one pole to the other; a small area near each pole (approximately ten degrees) is omitted to maintain optimum map proportions. The map shows most of the Earth's land boundaries, and continental areas and major oceans are easily recognizable. At any time, the display shows the Space Shuttle or satellite as a small symbol, the projected orbit for approximately the next three orbits (or four and a half hours), and the orbit track for the past orbit (or one and a half hours). The program may be operated in real time or in "fast time", which is ten or sixty times normal time. For "simple" orbits, 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 6 STSORBIT FILES -------------- STSORBIT is normally distributed in archived format using either the PAK or ZIP format. The following files are usually included: STSORBIT.EXE Main program (required) STSORBIT.DOC Documentation (not required) STSORBIT.MPT World map data (required) STSORBIT.CTY City coordinates (optional) STSORBIT.INI Initialization data (optional) MSHERC.COM Hercules driver (required for HGC) NASAnnn.TXT 2-Line Elements (optional) HST.TXT HST 2-Line Elements (optional) README STSORBIT 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 STSORBIT is operated but provide additional features or information if present. If file STSORBIT.INI is not present it will be created. Program MSHERC must be executed before running STSORBIT for systems equipped with Hercules Graphics Cards (HGC) or Hercules Graphics Plus Cards. STSORBIT will otherwise report an error and remind the user to run MSHERC. File NASAnnn.TXT (where "nnn" will be a number such as "847") is a set of NASA/NORAD 2-line elements as of the program release date. 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 always available on my bulletin board system. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 7 STARTING PROGRAM STSORBIT ------------------------- Before starting program STSORBIT, delete the file STSORBIT.INI if it has been created by a previous version of STSORBIT. The format of the .INI file has changed with Version 9046 and, although the program will read prior versions of .INI files, it is safer to "start from scratch". To start program STSORBIT, enter one of the following commands: STSORBIT (any monitor, CGA/HGC/EGA/VGA) STSORBIT /EGA (force EGA or lower monitor) STSORBIT /CGA (force CGA monitor) STSORBIT /M (force monochrome operation, EGA/VGA) STSORBIT /EUR (center map on Prime Meridian, EUROPE) STSORBIT /R (resume last mission automatically) STSORBIT /S (force shuttle icon for speed) 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, force the shuttle icon, and force EGA mode: STSORBIT /R/S/EGA * * * * * Hercules Graphics Card USERS NOTE: ---------------------------------- Run the program MSHERC prior to running STSORBIT. This Microsoft program works with QuickBASIC 4.5 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 STSORBIT. At least one HGC "clone" user reported improper operation. * * * * * If you have already run STSORBIT (or if you have file STSORBIT.INI) and simply wish to resume viewing that same mission, use the /R command line option. STSORBIT 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 STSORBIT.INI, is used to initialize the program. Once started in this manner, pressing the ENTER (or RETURN) key after plotting has started will return to the Main Menu. European users, or those users accustomed to viewing a map of the STSORBIT Space Shuttle and Satellite Orbit Simulation Page 8 world centered on the Prime Meridian (zero degrees longitude at Greenwich, England), may include the /EUR option by itself or in addition to other options. 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 STSORBIT 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. STSORBIT depends upon Microsoft QuickBASIC 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 QuickBASIC 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. Naturally, the operation of the program is enhanced by the use of a color monitor. The vertical resolution is also adjusted depending upon the type of adapter which has been detected. Once STSORBIT 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 new option selection(s). 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. STSORBIT normally 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. However, the "other satellite" symbol has approximately three times as many pixels (dots) as does the space shuttle symbol and therefore takes longer to draw and erase. If you are using a slower computer or one not equipped with a math coprocessor, using the "/S" option may improve performance by forcing the use of the space shuttle symbol. The /S option may be used by itself or in combination with other options. STSORBIT first reads in the map coordinates from file STSORBIT.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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 9 Program STSORBIT Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:34:24 PDT 23:34:24 UTC Current date: 05/21/1991 05/21/1991 Last Mission = STS-40 (est) [STS40D01] (C) Copyright David H. Ransom, Jr., 1989-1991 All rights reserved. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 10 STSORBIT MAIN MENU ------------------ Once the map coordinates have been stored internally, STSORBIT presents its Main Menu: Program STSORBIT Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:34:56 PDT 23:34:56 UTC Current date: 05/21/1991 05/21/1991 F1 Program STSORBIT 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) F8 Set program TIME and/or DATE F9 DOS Shell (CAUTION: DOS Version 3.x+ ONLY!) F10 Set STSORBIT Program Options and Features ENTER Resume Mission (STS-40 (est) [STS40D01]) ESC Quit STSORBIT and Save Current Mission Select desired function: F1 Program STSORBIT Demonstration ------------------------------------- The F1 command may be used to demonstrate the operation of STSORBIT 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 several minutes the shuttle reaches orbital altitude and proceeds along the ground track normally, passing a dot every minute. Press F (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 STSORBIT in standard formats: 4/05/1990 Date in month/day/year STSORBIT Space Shuttle and Satellite Orbit Simulation Page 11 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. Typical files from my BBS system have names such as "NASA769.TXT" where the "769" corresponds to the particular NASA data set and may change several times per week. A sample file is included with the normal distribution of STSORBIT. Entering the F2 command will display 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 [NASA769.TXT]: Enter desired Satellite Name [HST]: In each case, pressing ENTER will select the default choice shown inside the square brackets. For the example shown, the defaults are the file NASA710.TXT and the satellite HST. If you wish to change the information (or if no default is shown), enter the required information followed by ENTER. For the filename, if no filetype is included, STSORBIT will automatically add ".TXT". For the satellite, only sufficient letters to unambiguously identify the desired satellite, upper or lower case, are required. For example, "Alou" would select "Alouette 1". Once this information has been entered, STSORBIT will attempt to locate the specified file and the data for the requested satellite. Once a satellite matching the requested name is found, the data is displayed: Satellite Name: HST Elements File: NASA769.TXT Elements Record #: 91 Elements Epoch: 90312.61189795 8 NOV 1990 @ 14:41:08 UTC Orbit # at Epoch: 2946 Satellite Number: 20580 Launch Year: 1990 Launch Number: 37 Launch Piece: B Element Number: 327 Inclination: 28.4715 RA of Ascend Node: 50.186 Eccentricity: .0006172 Arg of Perigee: 184.0423 Mean Anomaly: 176.0412 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 12 Mean Motion: 14.85441208 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 STSORBIT. If, on the other hand, a different satellite is desired, press any other key (such as the SPACE BAR) and STSORBIT 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, STSORBIT 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 90 37 B 90312.61189795 .00004882 00000-0 53771-3 0 3278 2 20580 28.4715 50.1860 0006172 184.0423 176.0412 14.85441208 29461 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 STSORBIT.INI is the file which has the data from the last time the program was run; to use file STSORBIT.INI, press ENTER. STSORBIT will automatically supply the filetype .INI if you do not include a filetype. If the requested file is not present, an error will be reported. To create new .INI files, enter the desired data then exit STSORBIT and use the DOS COPY command to copy STSORBIT.INI into a file with a the desired name. Additional .INI files may be included with the distribution files for satellites of interest at the time of release. These files will have names such as STS28.INI and STS30.INI. Since Space Shuttle missions normally last only five to ten days, using these files at later dates will generate "unusual" mission elapsed times and orbit numbers! Note that the launch time in these files use Coordinated Universal Time (UTC); prior releases (Version 9019 and earlier) of these files included the launch time in PACIFIC standard or daylight time and had to be converted to your local time. STSORBIT 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 may "fool" STSORBIT by setting your computer time and date 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 fictitious time, use STSORBIT's internal time setting functions (F7 on the Main Menu) to set a desired simulation time. This method does not STSORBIT Space Shuttle and Satellite Orbit Simulation Page 13 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 Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:34:56 PDT 23:34:56 UTC Current date: 05/21/1991 05/21/1991 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): 11/13/1990 11/13/1990 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-30 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. Enter orbit inclination: STSORBIT Space Shuttle and Satellite Orbit Simulation Page 14 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 is required and be sure to use the SLASH "/" rather than the MINUS "-" as the separator; STSORBIT'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 using NASA 2-line elements; all other orbital parameters are included in the 2-line elements and may not be altered within STSORBIT. 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 NASA 2-line elements are usually available within about 24 hours after a space shuttle launch and are far more accurate than the manually entered "simple orbital 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 15 and launch time independent of the 2-line elements and manually enter that data into STSORBIT. 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 is required and be sure to use the SLASH "/" rather than the MINUS "-" as the separator; STSORBIT'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 STSORBIT.INI. However, use caution when changing from one satellite to another since 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 brackets [...]. Press ENTER to leave an item unchanged. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 16 Program STSORBIT Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:34:56 PDT 23:34:56 UTC Current date: 05/21/1991 05/21/1991 Enter MISSION TITLE [STS Mission Simulation]: STS Mission Simulation Enter LAUNCH TIME (HH:MM:SS) [23:58:12 UTC]: 15:58:12 PST 23:58:12 UTC Enter LAUNCH DATE (MM/DD/YYYY) [11/13/1990]: 11/13/1990 11/13/1990 Enter ORBIT INCLINATION (degrees) [28.4500x]: 28.4500x Enter ORBITAL ALTITUDE (nm) [ 160.00]: 160.00 nm ( 296.52 km) Enter LONGITUDE adjust (deg) [ 0.000x]: 0.000x 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. In the case of STS-30, the orbital time at Orbit 34 changed to 90.85 minutes and the longitude of the descending node was approximately -36 degrees, crossing at 2/2:18:00 MET. See the section ADJUSTING ORBITAL PARAMETERS below for more information. F6 Set Elapsed Time Option ------------------------------ When using NASA 2-line elements, STSORBIT normally 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 STSORBIT. Once entered, STSORBIT saves the information in file STSORBIT.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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 17 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 the "T" key. The F6 command checks that you have already entered the launch time and launch date or that it has been read from a previous STSORBIT session from file STSORBIT.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. F8 Set Program TIME and DATE -------------------------------- It is often convenient to set the TIME and DATE within STSORBIT 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 Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:34:56 PDT 23:34:56 UTC Current date: 05/21/1991 05/21/1991 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 18 effect). Both your local date and time, "PST" or Pacific Standard 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 STSORBIT 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 "Universal Time" or "UT" (and sometimes, depending upon the application, as "UT1" or "UT2"). STSORBIT 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. STSORBIT 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 "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), STSORBIT 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 19 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 Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:34:56 PDT 23:34:56 UTC Current date: 05/21/1991 05/21/1991 CAUTION: This function will change the computer's SYSTEM CLOCK! Press ENTER to leave an item unchanged Enter TIME (HH:MM:SS): 12:37:38 PST Enter DATE (MM/DD/YYYY): 11-11-1990 Press ENTER to accept, SPACE BAR to repeat: _ 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 Space Shuttle and Satellite Orbit Simulation Version 9122 Simulated time: 13:00:08 PST 21:00:08 UTC Simulated date: 11/08/1990 11/08/1990 Press ENTER to leave an item unchanged Enter SIMULATED TIME [12:05:06]: 13:00:00 PST 21:00:00 UTC Enter SIMULATED DATE [11/11/1990]: 11/09/1990 11/09/1990 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 20 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. Program STSORBIT Space Shuttle and Satellite Orbit Simulation Version 9118 Simulated time: 00:17:18 PST 08:17:18 UTC Simulated date: 10/09/1990 10/09/1990 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 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 QuickBASIC'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, STSORBIT 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 QuickBASIC 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 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 21 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). ESC Quit STSORBIT and Save Current Mission --------------------------------------------- Press ESC (the key marked "ESC" or "Esc", not the letters E+S+C) to quit program STSORBIT. If you press ESC to quit the program and have manually entered orbital data, STSORBIT will save all required mission data in file STSORBIT.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 STSORBIT, 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 STSORBIT.INI, but the map data is lost and will be re-read when you next use program STSORBIT. NOTE: If you have neither entered data (with the E command) nor read a previously saved .INI file (with the F command), no data will be saved when you exit STSORBIT. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 22 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 Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:35:25 PDT 23:35:25 UTC Current date: 05/21/1991 05/21/1991 F1 Program STSORBIT Information F2 Set New Local Coordinates (Calaveras County, CA) F3 Select Display Features F6 USA Set Map Center (USA/Europe) F7 OFF Set for SLOW COMPUTER or NO COPROCESSOR F9 -7.00 Set UTC Time Offset and Daylight Flag F10 OFF Enable/Disable Printer Logging ENTER Return to MAIN MENU Select desired function: F1 Program STSORBIT Information ----------------------------------- Function Key F1 displays information about program STSORBIT 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 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 2400 and 1200 baud. F2 Set New Local Coordinates -------------------------------- In order to perform the calculations related to satellite visibility and altitude/azimuth, STSORBIT must know the geographic coordinates for the user's location. The name of the current location is shown in parentheses. When STSORBIT 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 23 city names and coordinates (STSORBIT.CTY); or manually entering the information. Pressing F2 will display the following reminder: STSORBIT 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, STSORBIT 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 STSORBIT and subsequently saved in file STSORBIT.INI. If you wish to search further in the file, press the SPACE BAR. NOTE: The elevations contained in file STSORBIT.CTY are almost all zero except for Rancho Palos Verdes, CA and Calaveras County, CA, locations for which I have accurate elevations above mean sea level. If you know the correct elevation, edit the file using any ASCII text editor and change the last number on the line. STSORBIT.CTY contains over 800 cities and uses a different format than the .CTY files for program ASTROCLK (the elevation has been added to each entry). If users send me their correct elevations (or additional cities they wish added), I will incorporate that data into subsequent versions of file STSORBIT.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): STSORBIT Space Shuttle and Satellite Orbit Simulation Page 24 DDD.DDDDD Degrees and decimal fraction DD,MM.MMM Degrees, minutes and decimal fraction 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 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, STSORBIT 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 STSORBIT.CTY (Y/n): _ Press "Y" or ENTER to append the data to the file, or press "N" to not modify the STSORBIT.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 (USA/Europe) ---------------------------------- The F6 command changes the center of the displayed world map. By default, the map is centered on the United States at 90 degrees West longitude. Most Americans are accustomed to viewing the map in that orientation, but it is sometimes convenient to shift the map center when the Space Shuttle is near the edge of the map. If you prefer to have the map centered at the prime meridian or zero degrees (Greenwich, England), use the F6 command or use the /EUR command line option when starting STSORBIT. Each time you use the F6 command the map center is changed to the alternate meridian and the map data is re-read from disk. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 25 F7 Set for SLOW COMPUTER or NO COPROCESSOR ---------------------------------------------- Particularly since the addition of the calculations related to 2-line element sets, 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 being performed by STSORBIT. For example, my 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. While perhaps not the "ideal solution" as compared to having a math coprocessor, setting STSORBIT 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 five 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 STSORBIT is operating in the Slow Mode, the phrase "SLO" appears at the lower right of the tracking display. F9 Set UTC Time Offset and Daylight Flag -------------------------------------------- STSORBIT uses UTC or Coordinated Universal Time, an adjusted version of Universal Time (which STSORBIT 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 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. STSORBIT 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 STSORBIT. The following shows the display when using the F9 command: Set UTC TIME ZONE OFFSET and DAYLIGHT FLAG STSORBIT 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, STSORBIT can STSORBIT Space Shuttle and Satellite Orbit Simulation Page 26 automatically adjust launch or Epoch times and dates for your local time zone. In addition, STSORBIT 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 STSORBIT.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 STSORBIT 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. STSORBIT: Space Shuttle Tracking Program, Version 9122 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 27 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 STSORBIT 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 28 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 Space Shuttle and Satellite Orbit Simulation Version 9122 Current time: 16:35:52 PDT 23:35:52 UTC Current date: 05/21/1991 05/21/1991 F2 ON Display TDRS Coverage F3 OFF Display Additional Map Grid Lines F4 OFF Display NASA Tracking Stations F5 OFF Show Ascending & Descending Node Data F6 ON Display Spacecraft Circle of Visibility F7 ON 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 or XYZ ENTER Return to MAIN MENU Select desired function: 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. STSORBIT assumes, as is often the case, that TDRS East will be selected when it is available. 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 29 F4 Display NASA Tracking Stations ------------------------------------- The F4 command enables/disables the display of NASA's 14 ground tracking stations; this command is not available for CGA monitors. For additional discussion, see the section Ground Track Display. 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 STSORBIT's data against other sources such as the wall map in Mission Control. F5 Display Spacecraft Circle of Visibility ---------------------------------------------- STSORBIT can calculate the approximate circle of visibility from the orbiter, the area of the Earth's surface which is visible from the cockpit windows and television cameras. This feature may be enable for space shuttle missions and for the Hubble Space Telescope (HST), Gamma Ray Observatory (GRO), and the Soviet MIR Space Station. 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 and varies somewhat with spacecraft altitude. F8 Display Terminator, Sun, and SpaceCraft Lighting ------------------------------------------------------- Many types of observations, especially Earth observations, often require that the targeet 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. These solar display features are 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: STSORBIT Space Shuttle and Satellite Orbit Simulation Page 30 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 use 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, 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. STSORBIT calculates whether the spacecraft is in full sun, penumbra/refracted sunlight, or umbra (full shadow) and adjusts the color of the spacecraft icon accordingly: white, yellow, and bright white respectively. This feature is not available on CGA and HGC monitors. 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. You may also use Function Key F9 while the ground track display is in progress to switch between units of distance measure. F10 Select Satellite Coordinates: RA/DEC, AltAz or XYZ --------------------------------------------------------- This feature selects between topocentric (referenced to the Earth's surface and in this case the current local geographical coordinates) horizon, 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 siaplay 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 31 USING GROUND TRACK DISPLAY OPTIONS ---------------------------------- Once the orbital display is in progress, a number of keys are active: 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 space shuttle or 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 for space shuttle missions, Hubble Space Telescope, Gamma Ray Observatory, or the Soviet MIR Space Station. 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.) STSORBIT Space Shuttle and Satellite Orbit Simulation Page 32 - During PAUSE mode only, moves the satellite to the PREVIOUS 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. R Resynchronize MET or T+Epoch with Local Time. Resets plot to real time. NOTE: The Resynchronization was 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. The simulated time, taking into account any fast time that may have been in effect up to that point, is shown at the lower left of the screen. For reference, the launch or epoch time and the actual local time (as determined by the computer's internal DOS clock) are also displayed. The MET (Mission Elapsed Time) or T+Epoch display at the lower right of the screen shows the current elapsed time in the mission and will include an additional message, "(x10)" or "(x60)", when fast time is in effect. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 33 THE STSORBIT GROUND TRACK DISPLAY --------------------------------- One of the principal objectives of STSORBIT is to graphically display the position of the space shuttle or satellite relative to a map of the world. 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 following sections discuss these various features. On-line Help ------------ Thanks to numerous suggestions, an on-line help screen may be displayed 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 following help information at the lower portion of the screen. F1 Resume Data F6 Pause Display ---------+ During PAUSE: F2 Satellite Blink F7 Circle of Visibility | + Move Forward F3 Printer Logging F8 SUN and Terminator | - Move Backward F4 Time Step F9 Distance Units F5 MET/T+Epoch F10 Satelitte Coordinates STSORBIT 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. Versions prior to 9122 used various letter keys to select the various functions during the ground track display. While many of these keys are still active, I have switched to the function keys in an attempt to better organize these operations. World Map --------- The STSORBIT ground track display includes a map of the world centered on either the United States or Europe and extending from approximately +80 degrees North latitude to -80 degrees South latitude using a linear cylindrical projection. Omitting the two 10 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). The world map consists of a series of some 7500 coordinate pairs. The equator and lines of equal longitude at 0, 90, and -90 are also shown. Additional lines of longitude and latitude may be added on EGA and VGA monitors by enabling that feature using F10+F3+F3. When used with an EGA or STSORBIT Space Shuttle and Satellite Orbit Simulation Page 34 VGA color monitor, the coastal outlines are in cyan, the equator and longitude grid are in blue with the equator and Prime Meridian highlighted in bright blue. Spacecraft or Satellite 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 Light White Satellite is in penumbra Yellow Satellite is in umbra White For better visibility, the symbol will normally "blink" on CGA monitors; the symbol may be made to blink on any system if desired by pressing the "B" key while the ground track is displayed. Just seeing the spacecraft or satellite on the map display yields information as to its present position. However, for satellite viewing and planning purposes, STSORBIT calculates the predicted orbital ground track for approximately three hours in the future and displays the past orbital ground track for approximately one and a half hours in the past. The future ground track is shown in light green, and the past orbital ground track is shown in light red. The orbital ground track consists of a series of dots plotted at one minute intervals. These dots "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. 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 as an ellipse because of the scaling factors used by the map projection. The radius of this circle of visibility is calculated for each satellite based upon its altitude at the instant the map is 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 STSORBIT Space Shuttle and Satellite Orbit Simulation Page 35 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 TV. 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 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 36 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. NASA Ground Tracking Stations ----------------------------- 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. Each ground tracking station is shown as a small symbol surrounded by a brown or light yellow "circle" 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 antenna range circle is displayed on the screen as an ellipse because of the scaling factors used by the map projection. 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 ground tracking stations as of early 1989 which are shown along with their abbreviations and approximate map coordinates (longitude, latitude): MIL -81,28 Merritt Island, FL BDA -64,32 Bermuda DKR -17,14 Dakar, Senegal ACN -14,-8 Ascension Island MAX -5,41 Central Spain IOS 56,-5 Indian Ocean HAW -156,20 Hawaii GWM 145,14 Gwam VAN -122,35 Vandenberg, CA YAR 115,-29 Yargidy, Australia CAN 149,-36 Canberra, Australia GDX -116,34 Goldstone, CA CTS -105,38 Colorado Springs, CO AGO -71,-34 Santiago, Chile STSORBIT Space Shuttle and Satellite Orbit Simulation Page 37 STSORBIT'S ORBITAL MODELS ------------------------- The initial version of STSORBIT was 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 with an orbital inclination of zero degrees. For orbits with low inclinations, as is typical for launches from Kennedy Space Center, these 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. 2- line element sets for non-military space shuttle missions are typically available by the second or third day of the mission. 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 space, the "Two Body Problem". These six quantities, often referred to as Keplerian orbital elements, are included in the NASA/NORAD 2-line element STSORBIT Space Shuttle and Satellite Orbit Simulation Page 38 sets along with other numerical and statistical data. NORAD, the North American Air Defense Command headquartered in Cheyenne Mountain, Colorado, 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 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. 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 downloaded by Kelso directly from NASA Goddard Space Flight Center by special arrangement. I regularly post a slightly edited version (certain introductory text material is removed so that the file may be used directly by programs such as STSORBIT) of the current element sets on my own RPV ASTRONOMY BBS as file NASAnnn.TXT, where "nnn" is the NASA 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. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 39 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. The original "simple" orbital model used in STSORBIT 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, STSORBIT'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 STSORBIT'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 STSORBIT. 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. 6. Enter the calculated time difference for the node crossing as the time STSORBIT Space Shuttle and Satellite Orbit Simulation Page 40 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. STSORBIT 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. 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. STSORBIT 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 STSORBIT. If all this seems like too much effort, just enjoy the program "as it comes from the box"! 13. During the course of a non-military mission, I normally watch NASA Select TV (if I'm around!) and post updated .INI files from time to time on my BBS. You may call the BBS and download the updated .INI files; these files typically have names such as STS31A.INI, STS31B.INI and so forth. Use the F3 command to read the updated .INI file into program STSORBIT. Also, within a day or two of a Space Shuttle launch, the 2-line elements for the mission will usually be available on my BBS. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 41 STSORBIT REVISION HISTORY ------------------------- Program STSORBIT underwent multiple revisions during the first days of the STS-30 Atlantis/Magellan mission (May 1989) and the process was repeated during the STS-28 Columbia mission (August 1989). At one point, I was releasing new versions every couple of hours! Lacking any equations or data for the first minutes of a typical mission, the first tries were largely guesswork and needed to be refined considerably. After all of that, comparison of STSORBIT's plot with the NASA wall map data (as seen on NASA Select Television) and interpolation of that data shows good agreement through about the first ten orbits. However, after orbital maneuvers, such as occur with the deployment of a satellite or spacecraft, the initial data is much less accurate. Unfortunately, the NASA wall map is not always on the screen and orbital information may otherwise be difficult to obtain during the first day or two of a mission. The launch of the Hubble Space Telescope (May 1990) and the availability of the NASA/NORAD 2-Line Element Sets has prompted a major revision of the program at Version 9022 to permit accurate tracking over longer periods of time using these data. This has required a major rewrite of the program which required many days of effort. As with any major software change, not all problems and bugs are detected in the first releases. User feedback and patience will be appreciated. Each released version of STSORBIT uses a four digit revision code such as 9118. 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. 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 STSORBIT through all of the minor twists and turns that usually accompany the evolution of such a program. It illustrates the tortuous process of maintaining and refining a program as ideas and problems are reported from every quarter. The early versions were rushed to meet the launch schedules, probably too much so. 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. 9122A 05/27/91 ----------------- -This is a minor fix for HERCULES USERS. The persistent bug that caused the program to halt with an error message unless the "/M" command line option was included has, I hope, finally been found and fixed. -There are no other differences from Version 9122. 9122 05/23/91 ----------------- -This is a MAJOR UPGRADE to the program. Version 9022 incorporates new features tested in Beta Version 9021 (see also below) as well as other new features added as a result of testing that version and user suggestions. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 42 -Added on-line help during the ground track display and changed the keys used during that display to Function Keys. The following keys are active during the ground track display: F1 Resume Data F6 Pause Display During PAUSE: F2 Satellite Blink F7 Circle of Visibility + Move Forward F3 Printer Logging F8 SUN and Terminator - Move Backward F4 Time Step F9 Distance Units F5 MET/T+Epoch F10 Satelitte Coordinates (Although the "old" individual letter keys are still active, they will be discontinued in subsequent versions and only the Function Keys will be used.) Thanks to Mike Gardner and others for their persistent requests for the help screen. -In response to numerous requests, I have added the SUN and the solar terminator to the ground track display (solar terminator only for CGA). The Sun is shown as a yellow circle at the subsolar point and the terminator is shown as a series of yellow dots marking the points at which the center of the sun is at the horizon for observers at mean sea level; both features are updated every 60 seconds. Toggle this display feature using F10+F3+F8 from the Main Menu (which also selects the spacecraft lighting effect for EGA/VGA systems). The additional calculation time is substantial and this feature may not be suitable for slow processors or computers without math coprocessors. This feature is automatically disabled if the SLOW flag is set. Thanks to Rob Matson for providing example code for the terminator. -Changed map file to STSORBIT.MPT and changed method of loading map data to eliminate the prior time consuming method. Map file now loads with a single disk access. If the /R command line option is given, the title screen will then appear for two seconds; otherwise, the title screen will appear for 15 seconds. Press any key to continue immediately. -The spacecraft circle of visibility may be added to or removed from the ground track display using Function Key F7. In prior versions, this could only be accomplished from the menus. This feature is now also available on CGA and HGC systems. -The SUN and terminator may be added to or removed from the ground track display using Function Key F8. In prior versions, this could only be accomplished from the menus. -Distance units may be switched between kilometers (km) and nautical miles (nm) using Function Key F9. -Satellite coordinates may be switched between Altitude and Azimuth, Right Ascension, and Geocentric Rectangular coordinates using Function Key F10. -Added Geocentric Rectangular satellite coordinates: SatX, SatY and SatZ set by F10+F3+F10. Switch between nautical miles (nm) and kilometers (km) using F10+F3+F9. NOTE: XYZ coordinates are available only with 2-line orbital elements. -Corrected a bug in the Hubble Space Telescope icon (used for all non-space shuttle missions) which caused it to be displayed incorrectly; right place but faulty icon. -Spacecraft circle of visibility is now allowed for Hubble Space Telescope (HST), Gamma Ray Observatory (GRO), and the Soviet MIR Space Station (MIR) in addition to space shuttle missions. -Fixed bug that caused minor error after demo. Program now rereads the .INI file after demo to correctly restore all data. -Sun and terminator now displayed for simple orbital model prior to launch time. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 43 9121 05/21/91 ----------------- -This is a non-public BETA RELEASE to test several new features. -Repaired minor bug when switching between MET and T+EPOCH which caused that time data to be incorrect for one calculation cycle. -Repaired bug with HGC displays that required the use of the /M command line option for Version 9118. HGC systems should now operate correctly without the /M command line option. 9118 04/29/91 ----------------- -Moved Display Features to a new sub-menu on Program Options using Function Key F3. All options are now saved in the .INI file. -Shuttle circle of visibility now an option in Display Features menu for EGA/CGA monitors. -Added satellite coordinates in topocentric Right Ascension and Declination (R/D) on Display Features menu. Coordinates may now be displayed as either Altitude/Azimuth (A/A) or RightAscension/Declination (R/D), referenced to the local coordinates as of the epoch of date. -Added distance toggle between nautical miles and kilometers on Display Features menu. -Added feature to display the location and circle of visibility for a second city. Select F10 for Program Options and Features, then F2 for Local Coordinates and proceed as usual. When you have selected or entered the name and coordinate information for the second city, press TAB to ACCEPT. To clear the second city, press F10+F2, then "*" for manual data entry, enter SPACE (press only the SPACE BAR followed by ENTER) as the city name, press ENTER for the coordinates and elevation, then TAB to clear the second city. -When used with a CGA monitor (or when CGA mode is forced with the /CGA command line option), the text screens are now in color. To display all screens in monochrome on the CGA, use the /M command line option. Note that whether or not color is used for text, GRAPHICS (the map and ground track) will ALWAYS be in monochrome for CGA monitors! -Split main source code into two modules for compiler memory problems. 9117 04/27/91 ----------------- -Corrected a serious typographical error introduced in Version 9116. The program would not execute with HGC or CGA monitors. -EGA and VGA monitors now display the SAA (South Atlantic Anomaly) in white between South America and the tip of Africa. This is a rough approximation of the area of the SAA. -Added capability to display the location and circle of visibility of a second location. Use F2 as you would for changing local coordinates except press TAB instead of ENTER for this feature. Enter a city name of one or more spaces, then use TAB to disable this feature. Thanks to several users for this suggestion. 9116 04/18/91 ----------------- -Added code to detect an EGA equipped with a MONOCHROME monitor, QuickBASIC screen mode 10. (Thanks to Brian Kornfeld for reporting the problem.) STSORBIT Space Shuttle and Satellite Orbit Simulation Page 44 9114 04/03/91 ----------------- -Corrected a problem with Version 9112 (and probably 9049 as well) which caused an incorrect date to be entered with Main Menu F4. No matter what date was entered, the date appeared as the year 4713. The following will work around the problem for Version 9112: Accept the "bad" date, then press F5 to adjust the launch date to the desired date. 9112 03/30/91 ----------------- -Added calculations to determine if the satellite is sunlit (2-line element mode ONLY). Special thanks to Rob Matson for his assistance! The satellite color changes depending upon the lighting (EGA and VGA): Light White Fully lighted Yellow Penumbra (including Refracted Sunlight) White Umbra 9049 12/04/90 ----------------- -Added spacecraft circle of visibility for STS missions, EGA/VGA ONLY! -Corrected the logic associated with printer logging so that TDRS Com: data is properly updated when logging is ON. Thanks to Jay Hickman for reporting the bug! -Corrected negative Mission Elapsed Time (MET) in both orbital models. Time BEFORE launch time and date was incorrect. -Changed MET input routine (F8+F4) to allow negative MET dates and times. I don't really know why anyone would want this, but ... -Program now saves presence/absence of additional grid in STSORBIT.INI. -Documentation updated to reflect program changes and additions. The description of the ground track display has been rewritten for improved clarity. 9048 11/25/90 ----------------- -Corrected a problem with the "simple" model that caused it never to "catch up" at certain times which were a function of the current local time versus UTC. (Yet another time/date problem! Things were simpler before I added all the UTC code. I hope this is the last bug in that department...) -Added fiducial mark at the center of the circle of visibility. -Added 150 selected international cities to STSORBIT.CTY. -Removed time zone changes when changing local coordinates. 9047 11/17/90 ----------------- -Modified the routine that reads 2-line elements to substitute 0.0000001 if the supplied eccentricity is zero. (An eccentricity of zero resulted in division by zero, error #11, and an abort to DOS.) -Blank lines in 2-line element files are now ignored. Lines which begin with a dash ("-") are now treated as a comment and displayed. BULLETIN.TXT files downloaded from Celestial BBS may now be read without editing. -"F" key now changes the fast time mode correctly during PAUSE. -Changed Main Manu F5 display to reflect simple versus 2-line mode. -Changed Main Menu F6 display to show current time/date mode, MET or T+Epoch. STSORBIT Space Shuttle and Satellite Orbit Simulation Page 45 9046C 11/15/90 ----------------- -Further changes and corrections to try and "bullet proof" the time and date entry process. Main Menu commands F4 and F5 modified. 9046B 11/15/90 ----------------- -Corrects a problem with UTC vs. local dates when entering simulated time and date (F8+F3). Thanks to Matt Merrill for reporting the problem. 9046A 11/14/90 ----------------- -Corrected minor bug that caused TDRS communications coverage not to display under some circumstances. 9046 11/13/90 (MAJOR UPGRADE) ---------------------------------- -The current or simulated local time and date and UTC time and date are now shown on all menus. GMT is assumed equal to UTC. -The "catch up" phase when plotting is started has been rewritten to all but eliminate that delay. The past and predicted ground track is still plotted but the satellite immediately begins from the approximate current position. This change will be particularly noticeable on slower computers and has been made in response to user comments. The bell no longer sounds when the system begins tracking in real time. -Function key F8 has been added to the Main Menu to allow the user to set internal time and date functions. The method used does NOT affect the system clock. The program may be set to a simulated date or time using local or UTC/GMT time, Mission Elapsed Time (provided a launch time and date are present), or the program time and date may be returned to real time as determined by the system clock. The system clock may also be set to a new time and date. -Function key F5 has been modified to allow the user to input a Launch Time and Launch Date when using 2-line elements. Note that this information is NOT included in the 2-line elements and must be determined independently. -Function key F6 has been added to allow the user to select between "Time since Epoch" (the time elapsed since the epoch date of the 2-line element set, normally used for satellites other than the space shuttle) and "Mission Elapsed Time" (time since launch). Press F6 to toggle between the two methods. Use caution when changing from one satellite to another since this information is not changed when different 2-line elements are read! -The "T" command has been added during the ground track display to switch between Time Sinc Epoch and Mission Elapsed Time (See F6 addition above). -When changing local coordinates (F10+F2), the program now attempts to calculate the correct time offset from UTC/GMT based upon the longitude of the new location. The program time is automatically adjusted for the change and is marked "simulated" if the time difference is non-zero. -Somehow a conversion factor (from kilometers to nautical miles) has been incorrect for several versions! This has meant that the altitude above the Earth's surface has been incorrect (although the plotted position was correct). NOTE: PRIOR .INI FILES MAY NO LONGER YIELD ACCURATE RESULTS! Since .INI files use spacecraft altitude in nautical miles as one of STSORBIT Space Shuttle and Satellite Orbit Simulation Page 46 the orbital parameters in the "simple orbital model", this means that past .INI files may no longer yield accurate results with Version 9046 and higher. 2-line elements files will continue to operate correctly but the spacecraft altitude will now be correct. Thanks to Ron Parise of the STS-35 crew for finding and reporting the problem! -The program now displays the TDRS satellite being used for communications relay for space shuttle missions (satellite name must begin with "STS") in the upper right corner of the data block. The approximate transfer points are: -138 degrees for TDRS West to TDRS East; 57 degrees for LOS TDRS East; and, 90 degrees for AOS TDRS West. The algorithm used takes into account the curvature of the Earth which changes those positions by up to about 3 degrees toward the poles. The TDRS communications information is shown only in normal display, and not in x10 or x60. -Approximate TDRS coverage is now shown. Curved red lines appear at -138, 57, and 90 degrees. (This is a VERY rough approximation, accurate to only a couple of degrees.) -TDRS positions updated per 2-line elements as of 5 OCT 1990. Added TDRS-1 to the right of TDRS-West, the on-orbit spare TDRS. The spare TDRS was used during mission STS-41 and is still active. -When 2-line elements are displayed for approval (using F2 from Main Menu), the Elements Epoch is shown as conventional date and time in addition to the NASA/NORAD shorthand notation (i.e. 90262.9155368). -The routine to set the UTC Offset (F10+F9) has been corrected so that the current values are displayed. Press ENTER to leave an item unchanged. -The "F" key now changes step time when in PAUSE as well as during normal operation. Step time returns original value upon exit from pause. -Cosmetic corection: Screen now cleared with F10+F6 (change map center). -Corrected location message and color after PAUSE. 9027 7/01/90 ----------------- -Rewrote main menu to use Function Keys and added new secondary menu for program options. Special keys used during map display are unchanged. -Documentation extensively revised to reflect menu changes and additional features. -Added Slow Mode for very slow computers or systems without a math coprocessor. Position calculations performed every five seconds rather than every second when in this mode. -Added circle of visibility to map showing the approximate area about the observing location through which the current satellite may be seen. -Added Range, Altitude and Azimuth to current position calculations. -Added local observing coordinates and ability to read STSORBIT.CTY with approx. 720 city locations. NOTE: The city file includes the local elevation but all elevations are set to zero since I don't have that information. Location data may also be input manually. -Added graphics clipping to restrict graphics to map area for orbits with high inclinations. (Example: Alouette) -Adjusted top edge of map to top of screen. -Revised CGA HST icon for faster drawing and better proportions. -"/R" is now sufficient for RESUME command line option. -Added "/S" command line option to force space shuttle icon. Requires less time to draw than HST icon for slower computers. -Adjusted blink timing for more reliable blinking. If computational delays are too great (especially 2-line orbits which require more than twice the STSORBIT Space Shuttle and Satellite Orbit Simulation Page 47 calculation time compared to simple orbits), blinking is defeated. -Two sample PIF files are included for use with Windows 3.0, one for 386 enhanced mode and one for normal mode. Files must be edited to set correct drive/directory names. [Intermediate revision notes removed to save space. Available on request.] 8916 4/24/89 ----------------- -Initial BETA VERSION Release.