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Documentation for Space Flight Simulator, Version 0.02 Copyright (c) 1989, Ted A. Campbell, Bywater Software 0 Program Description ------------------- 0.1 General Description ------------------- The Space Flight Simulator, version 0.02, allows users to set a number of orbital parameters, then depicts the orbital focus (typically the earth) or the ground track of the orbit in real-time simulation. 0.2 Hardware Requirements --------------------- CPU and Coprocessor: The program will run under MSDOS or PCDOS on any of the 8088 and upwardly-compatible CPUs. Speed of the processor is critical in this program. The presence of a math coprocessor will be automatically detected by the program, and will also greatly enhance the performance of the program. Graphics: The program should work with CGA (very poorly), Hercules (tm), EGA, or VGA graphics. The program automatically detects these graphics systems, except the Hercules-type cards. For systems with Hercules-compatible graphics, you must run the program "msherc.com" first. 1 Running the Program ------------------- To run the program on computers with CGA, EGA, and VGA graphics, simply type sfs followed by a carriage return. To run the program on a computer with Hercules graphics, you must first run the program "msherc." The following commands will run the program with Hercules graphics: msherc<RET> sfs<RET> The program will display the log-in sign for the Bywater ANSI VDI implementation, then the SFS logo (which will remain on the screen for 10 seconds). You will then be given the opportunity to set parameters. Note for advanced users: You can also specify the name of an SFS focal data file (see below) on the SFS command line. For instance, sfs moon will start the program using "moon.fd" for its initial focal data (do not add the ".fd" extension when specifying a focal data file on the command line). 2 Setting the Initial Parameters ------------------------------ The initial parameters screen allows you access to three sub-screens. At this point you can press "O" (lower or upper case) for the Orbital Parameters screen, "I" for the insertion parameter screen, or "S" for the system parameters screen. You can enter these sub-screens repeatedly in order to get the parameters just the way you want them. For each of the three parameters sub-screens, default values are given. You can either enter a new value or simply hit "RETURN" to accept the default. From the main parameters screen you also have the option of pressing "X" to exit the program, or of pressing "RETURN" (or "ENTER") to begin program execution with the currently-set parameters. 2.1 Orbital Parameters ------------------ The "O" option from the main parameters screen brings up the orbital parameters screen. This screen allows you to set your own parameters for the orbit you want the Space Flight Simulator to simulate. When you first enter the Simulator, the file "earth.fd" is taken as the default focal data file (see below for an explanation of this file), and orbital default orbital parameters are calculated. Orbits are traditionally described by six parameters, or "elements," which are: the semimajor axis, the eccentricity, the period, the inclination, the argument of the periapsis, and the longitude of the ascending node. The Space Flight Simulator calculates each of these parameters, but does so on the basis of a file of information on the orbital focus, the orbital periapsis, and the orbital apoapsis. On the basis of the mass of the focus and the radius of the focus (given in the focal data file), together with the apoapsis and the periapsis, the semimajor axis, eccentricity, and period can be calculated. What this means for you is that you only need to know the orbital focus (which planet or other object you want to orbit), and the maximum and minimum altitudes of the orbit. On the basis of these, the first three classical orbital elements will be calculated. 2.1.1 SFS Focal Data File Default: earth.fd The "focus" of the orbit is the object around which the orbit will go. The focus may be a planet (the earth), or a planetary satellite (the moon). A Space Flight Simulator "Focal Data" file is a file that contains some specific information on an orbital focus. Although the default focal data file is "earth.fd" (for selecting a terrestrial orbit), some other focal data files are supplied with SFS version 0.02, and can be supplied at this point: mercury.fd venus.fd earthb.fd moon.fd mars.fd jupiter.fd saturn.fd uranus.fd neptune.fd pluto.fd None of these will be as interesting as "earth.fd," or "moon.fd," though, because surface features have not been mapped for the other foci (in fact, "earthb.fd" is a focal data file for the earth which wll represent it as a blank. This is much faster, but of course very dull. It can be used if you want to see how a given orbit will plot.) Notes for Advanced Users: The focal data file is an ASCII text file which includes the following data, each on a line by itself (all examples are from "earth.fd"): The name of the focus (example: "Earth") An adjective describing the focus (example: "terrestrial") The radius of the focus in kilometers (example: "6378") The mass of the focus in units of the earth's mass (example: "1.0") The period of a sidereal year in seconds (example: "86164") The name of an SFS surface-features data file (example: "earth.sd") If you know these data for other astronomical objects, you can try entering them. The surface features data file is also an ASCII text file, giving latitude and longitude points along with an initial code for each point telling whether it is the beginning of a new line to be drawn, what kind of surface feature the line is to describe, and what level of precision the point describes. There are three surface-features data files supplied with SFS version 0.02: "earth.sd," "moon.ds," and "null.sd." The earth file contains about 2100 points roughly describing continental coastlines and islands, and is based on the "Micro World Database" distributed independently. The "moon.sd" file is a very rough rendering of major near-side maria and a few very large craters. The "null" file simply draws a line along the central meridian and a dashed aline along its opposite meridian. 2.1.2 Orbital Periapsis Default: 0.1 x the radius of the focus The "periapsis" (also referred to by the specific term "perigee" for terrestrial orbits) is the lowest point in an orbit. You can specify any positive number of kilometers for the periapsis, although you might remember that orbits below about 250 kilometers begin to run into serious problems with atmospheric drag. (SFS does not account for atmospheric drag.) 2.1.3 Orbital Apoapsis Default: 4 x the radius of the focus The "apoapsis" (also referred to by the specific term "apogee" for terrestrial orbits) is the highest point in an orbit. You can specify any positive number of kilometers greater than or equal to the periapsis. An orbit having an equal apoapsis and periapsis will be perfectly circular; all other orbits will be elliptical. The greater the difference between the two, the greater will be the "eccentricity" of the orbital ellipse. You might remember, as a rule of thumb in setting terrestrial orbital altitudes, that the altitude of the moon is 378,014 kilometers, and this can serve as a practical limit for terrestrial orbits, since orbits beyond this range would be controlled more by the gravitational effect of the sun than by the earth. 2.1.4 Orbital Inclination Default: 25 degrees The "inclination" of an orbit is the amount that the orbit is inclined or "tilted" away from the equator of the orbital focus. The inclination must be specified in degrees between 0 and 180. An orbit having an inclination of 0 is "equatorial," that is, it follows the equator of the orbital focus. An orbit with an inclination of 90 degrees is "polar," that is, it will go over the north and south poles of the orbital focus every orbit. Program limitation: Although you can specify any inclination (between 0 and 180), the SFS visual simulation sometimes has trouble with orbits over polar regions, due to the author's lack of experience in spherical trigonometry. You may see the orbital focus appear to careen wildly when the spacecraft is over the poles. Sorry. Our technical advisors are looking into the matter... 2.1.5 Argument of the Periapsis Default: 0 degrees The next two orbital parameters are somewhat more difficult to understand, but you can learn about them by experimenting with different settings. The "argument of the periapsis" is the point in the orbit where periapsis occurs, measured in degrees (0-360) from the point of the "ascending node," which is the point in the orbit where the ground track crosses the equator, moving in a northward direction. For a non-equatorial orbit, the argument of the periapsis will have the following general effects: 0 Periapsis will occur at the point over the equator where the spacecraft crosses it heading northward, and apoapsis will occur at the point over the equator where the spacecraft crosses it heading southward (0 is the default setting). 90 Periapsis will occur at the northernmost point in the orbit, and apoapsis will occur at the southern- most point. 180 Periapsis will occur at the point over the equator where the spacecraft crosses it heading southward, and apoapsis will occur at the point over the equator where the spacecraft crosses it heading northward. 270 Periapsis will occur at the southernmost point in the orbit, and apoapsis will occur at the northern- most point. 2.1.6 Longitude of the Ascending Node Default: 0 degrees longitude The "longtude of the ascending node" is the point on the focal equator at which ascending node occurs. For SFS, the longitude of the ascending node is specified in degrees (-180 to 180), with west negative and east positive. The longitude of the ascending node changes each orbit, so what you are setting here is the longitude for the first orbit. SFS calculates orbits beginning at periapsis, consequently, you can change either the argument of the periapsis or or the longitude of the ascending node (or the insertion parameter -- see below) to change what portion of the focus you will see when the program begins. Examples: (a) The default settings have the argument of the periapsis at 0 degrees (periapsis over the equator), and the longitude of the ascending node at 0 degrees. For a terrestrial orbit (the default), this means that the orbit will begin at latitude 0, longitude 0, which is just off the coast of West Africa. The orbital direction will be northeastward, so you will first see the African continent, the Mediterranean Sea, and Europe beyond them. (b) By retaining all the default settings except the longitude of the ascending node, and by setting it to -120 degrees, the orbit will begin at periapsis over the equator, latitude 0 and longitude -120, which is just off of the western coast of Central American. Since, again, the orbital direction is northeastward, you will first see Mexico and North America in the visual display. 2.2 Insertion Parameter ------------------- Default: 9 x the screen update period (in seconds) The "point of insertion" is the moment in the first orbit when the orbit begins (imagine a rocket or spaceplane delivering a spacecraft to a certain altitude where the spacecraft would take up the orbital track -- this would be the point of insertion). The moment of insertion is specified in seconds from periapsis, and is limited by the orbital period (which is given on this screen). Example: If you want to begin by seeing the orbital focus from apoapsis, specify the insertion moment as one half of the orbital period. If the period is 60000 seconds, set the moment of insertion to 30000 seconds and you will be inserted at apoapsis. Program Limitation: SFS has to calculate a certain number of orbital position points before it can begin a visual display. Consequently, the earliest insertion point you can specify is 9 times the screen update period. When the screen update factor is 15 (the default), the earliest insertion moment is 135 seconds (2 minutes, 15 seconds) into the orbit. Note: Setting orbital parameters will affect a recalculation of the orbital period. Moreover, resetting the screen update interval (see below) may force the program to increase the moment of insertion to 9 times the (new) screen update interval. If setting the insertion parameter is critical, set it after the orbital and system parameters have been set, so that the insertion parameter screen will show the correct orbital period. 2.3 System Parameters ----------------- The system parameters screen allows you to set three parameters controlling the manner in which calculations are made and the screen is updated. Note that you can also change the system parameters later, while the program is running, but it is best to specify system parameters from the beginning. 2.3.1 Time Factor Default: 1 x real time The "time factor" is the ratio of flight time to real time. If the time factor is set to one (the default), the program will try to run in real time). If the time factor is set to two, the program will calculate two flight seconds for every real second, and so forth. Increasing the time factor allows you to speed up the orbital progress. The time factor and the screen update interval are closely related (see below). The time factor affects the apparent "roughness" of the visual simulation image. For real-time calculation, this is usually nominal, but for higher factors (especially when the orbital period is relatively low), a too-high time factor can result in serious distortion of the image. If the earth appears as a deflated balloon or a dried prune, you've probably set the time factor too high. 2.3.2 Screen Update Interval Default: 15 seconds The "screen update interval" denotes the number of flight- time (not necessarily real-time) seconds between each update of the screen. This parameter, then, is contingent on the time factor (see above): if the time factor is set to 1 (the default) and the screen update interval is set to 15 seconds (the default), then the program will try to update the screen every fifteen seconds in real time. If, however, the time factor is set to 2 and the screen update interval is still set at 15 seconds, the program will attempt to update the screen every 7 or 8 real-time seconds (corresponding to 15 flight-time seconds). You need to find out, by experimentation, what the best ratio is for your computer system for the relationship between the time factor and the screen update interval. The default 1:15 ratio works nominally on a computer with a 10 mhz 8088 CPU and no math coprocessor. An 80286 machine with a math coprocessor can easily make a 1:5 ratio (e.g., you could set the time interval to 4 and the screem update interval to 20 seconds, and the screen will be updated every 5 real-time seconds). Slower computers may not be able to keep up with the 1:15 ratio. 2.3.3 Trig Precision Level Default: 1 (fast) The "trig precision level" specifies the level of accuracy at which trigonometric functions are calculated by the program. Trig functions take up most of the processing time for the program, so by default SFS utilizes a look-up table for calculating trig functions. By specifying "2 (precise)," however, you can force the program to perform much more accurate (and much slower) trig calculations. The look-up tables work acceptably well, so I would recommend setting the trig precision level to 2 only if you have a math coprocessor. 3 Program Screens and Options --------------------------- 3.1 Initial Calculations -------------------- After you hit <RETURN> from the main parameters screen, the program will begin a series of initalizations and initial calculations. The latitude and longitude grids will be set up in the computer's memory, and the surface data file will be read into memory. Then the computer will figure two iterationss of calculations before it beeps, denoting that orbital insertion is about to occur. It is possible to press <ESCAPE> while the initial calculations are being made. You will be given a menu from which you can (a) toggle between the ground track and vidual simulation modes (neither will be displayed until all the initial calcula- tions have been completed, but this allows you to specify the ground track as the initial display instead of the visual simulation, which is the default), (b) reset the system parameters, or (c) exit from the program. 3.2 The Control Panel Display ------------------------- After orbital insertion occurs, you will be shown the visual simulation, or the ground track display if you have chosen it. In either of these screens, there is a control panel on the left hand side with some specific information in it. This control panel is subdivided into three subpanels. The "Focus" panel at the top has the following fields: (a) "Fo:" gives the name of the orbital focus, and (b) "La:" and (c) "Lo:" give the latitude and longitude of the "sub- satellite point" (SSP), that is, the point on the surface of the orbital focus directly underneath the spacecraft in its current orbital position. The "La:" and "Lo:" fields will change each time the screen is updated (except for a focal-centric orbit). The "Relation" panel in the middle gives information on the relation between the spacecraft and the orbital focus. (a) The "Or:" field shows the number of the current orbit (beginning with 1), and (b) the "Al:" field shows the altitude of the spacecraft above the surface of the focus in its current position. The "Al:" field will change for each screen update for elliptical (non-circular) orbits. The "Timing" panel at the bottom shows three sets of timing information: (a) the "Ti:" field shows Coordinated Universal Time (UTC) for the simulated spaceflight. (UTC is calculated on the basis of your computer's internal clock and the global environmental variable TZ which sets your time zone. See "Note for advanced users" below.) Note that although the program tries to calculate the actual UTC time for orbital insertion, the "Ti:" field is updated from that point with the simulated flight time, not real time (that is, if the time increment is 2, this clock will be incremented 2 seconds for every real-time second). (b) The "Da:" field gives the number of days elapsed in the flight (beginning with 0), and (c) "El:" gives the amount of time (hours, minutes, seconds) elapsed in the flight. This too is simulated flight time. The "Ti:" and "El:" fields should be updated with every screen update, and the "Da:" field should be updated every time the "El:" clock passes 24 hours. Note to Advanced Users: If you want the system to set UTC correctly based on your internal clock, you need to set the environmental variable TZ before running the program. A sample command, which might be included in your autoexec.bat file would be: set TZ=EST5EDT which would set the system for the Eastern time zone of the U.S. Other North American settings would be: set TZ=CST6CDT set TZ=MST7MDT set TZ=PST8PDT for Central, Mountain, and Pacific time zones, respectively. 3.3 The Visual Simulation Display ----------------------------- The visual simulation is the default display for the Space Flight Simulator, and will appear at first unless you specifically request the ground track display by hitting <ESCAPE>-G while the initial calculations are being performed. The entire screen to the right of the control panel will be used to display the image. The latitude and longitude grid will be shown with finely dotted lines (blue, on color systems), and surface features will be shown with complete lines (green, on color systems). If the focal data file specifies "null.sd" as the surface features data file, the only surface features visible will be the central meridian (a connected line), and its opposite meridian (longitude -180; a dashed line). Note that the visual simulation display will grow smaller as the altitude increases. The program attempts to keep the current direction pointed to by the spacecraft at the top of the visual simulation screen. 3.4 The Ground Track Display ------------------------ The ground track display shows the track of the spacecraft laid out on a flat ground map. (On color systems, the ground track will be shown as a red line.) You can switch to the ground track display while the program is running by hitting <ESCAPE>-G. (Return to the visual simulation with <ESCAPE>-V.) If you watch long enough to see more than one orbit, you will probably notice how the second orbit begins at a different longitudinal point than the first (a new longitude of the ascending node). For polar orbits, you may notice strange patterns in the polar regions, due to the distortion of the flat map. For highly eccentric orbits, you may notice some really odd ground tracks (retrograde motion and the like). At the bottom of the ground track display there is a panel showing the six classical orbital elements for the orbit you have chosen. 3.5 Changing System Parameters with Program Running ----------------------------------------------- As mentioned above, it is possible, though not highly desirable, to change the system parameters (time factor, screen update interval, and trig precision) while the program is running. Two matters should be noted about in-flight changes in the system parameters: (a) it may take as many as eight screen updates (one complete calculation iteration) before the changes take effect, and (b) you may notice a time lag (if you have increased the time factor) or a lengthy pause (if you have decreased the time factor) following such a reset in the system parameters. Remember to try to maintain the ratio between the time factor and the screen update interval that is appropriate to the speed of your system. Watch the status line at the top of the screen: a time lag does not necessarily mean that the system cannot keep up, but a consistently increasing time lag does. 4 Use and Copying of the Space Flight Simulator --------------------------------------------- Individuals may freely copy and share the Space Flight Simulator, version 0.02, for their own instructional and recreational use. A minimal fee may be charged for copying the program and for media. Educational and other non-profit organizations may copy and use the program freely, but under no circumstances may it be used by defense agencies or defense contractors in any country. Users and distributors of the program must not use it commercially, and must not remove or alter its copyright notices. Note that these restrictions do not apply to the file "earth.sd," which is based on the "Micro World Database" and whose data is, as I understand it, in the public domain. 5 Communications -------------- Ted A. Campbell Bywater Software 1 August 1989 Genie: T.CAMPBELL1 Usenet: tcamp@dukeac.ac.duke.edu ...!{ethos,ecsgate}!dukeac!numen!tcamp Mail: Ted A. Campbell Bywater Software Box 4023 Duke Station Durham, NC 27706