OrbSim2W ORBIT SIMULATION PROGRAM INFORMATION

Brief Summary of Program
OrbSim2W is a powerful yet user friendly satellite orbit program for the simulation and analysis satellite motion using NASA and NORAD two line element files (tle's) readily available for existing satellites. The sub-satellite position is projected onto a map of the earth to provide tracking information for ground based observation locations. OrbSim2w is a versatile, technical tool that is completely self contained and does not require additional modules to carry out the various simulation functions as do some other simulation programs. The OrbSim2W program allows the user to add, remove, or create satellites, switch from real time to simulation time, start and stop the file writing operation, change the simulation time step, change start and stop times, change the ground observer location, change the color traces of the satellites and change or modify almost all other program functions while in simulation. The default program constants are in a text file that can be modified by the user, some on a temporary basis when initiating a simulation run, or most of them on a permanent basis by modifying the text file that contains the default constants.

Overview
The OrbSim2W is designed to run on a personal computer (IBM type PC). It is capable of simulating the motion of all types of earth satellite orbits. The program can be used to generate orbital elements of conceptual satellites for engineering and experimental analysis and also simulate various types of launch injection conditions to create conceptual orbits. In addition, OrbSim2 provides information relating to sunlight or shadow (eclipse) conditions, ground coverage of satellite sensors, and many other features, including:

• Orbit prediction/ephemeris for up to ten satellites at any one time (eleven when in CSAT mode). Orbital elements are obtained from element data files that can include hundreds of element sets. Any element set can be added or removed at any time during the simulation.
• Sub-satellite ground track plots.
• Data relative to an earth based observer (azimuth, elevation, range, range-rate).
• Satellite ground coverage plot (sensor view capability).
• Satellite-to-satellite range, range-rate, line-of-sight direction cosines in satellite coordinates, and line-of-sight visibility for communication analyses including whether the background view is earth or space (also for ASAT analyses).
• Sunlight relationships (eclipse data).
• Day/night line (including sub-solar point and geosynchronous shadow plot).

Most of the program features can be turned on or off at any time during a run and as many times as desired. All relevant orbital, observational, and dynamical data can be stored in disk files for examination at some later time.

Simulation Modes
Orbsim2 has two basic operating modes:
Normal: This mode provides observational and communication information for all satellites in simulation relative to a surface based observer (land, sea or air).
Satellite-to-Satellite: This mode, also referred to as the CSAT mode, provides observational data for all satellites in simulation relative to a prime communication satellite (CSAT) and to a surface based observer.

Computer System Requirements
OrbSim2W requires the Windows operating system version 95 or higher or NT and a color monitor

Screen Presentation Data
During a simulation run, the data that can be viewed on the monitor screen include:

• Ground track of all satellites in display, plotted on an earth map/grid.
• Ground observation station data and a "bird's eye" plot of all satellites in display relative to the ground site, when above the observer's topocentric horizon.
• Azimuth, elevation, range, and range-rate numerical data for all satellites in display.
• A graphical plot of range between a prime communication satellite (CSAT) and all other satellites in display.
• Numerical values for range and range-rate for satellites being observed by the prime satellite for inter-satellite communication (or ASAT operations); data for range, range-rate, and LOS direction cosines for all satellites can be saved to data files.
• A plot of the sub-satellite communication coverage areas for either ground station control or for continuous coverage.
• Sunlight/shadow indicators on satellite ground traces, day/night terminator and sub-solar plot.
• All relevant time and other display control data.

Ground Observation Point
The user can select any observation point from a data file of over 80 current NASA, NORAD, and other sites in the file. Or, the user can specify any observation point by either modifying the data file, or by entering the longitude, latitude and altitude of an observer during the simulation. The observation point, whether ground, shipboard, or airborne, can be changed as often as desired during a simulation run.

Satellite Orbital Elements
The satellites to be displayed are selected from satellite element files. Elements in the data files can be added to the simulation at any time during a run. Satellite element sets can be easily created by the user, in an interactive mode, to experiment with potential scenarios, to create and examine satellite constellations, or for creating general "what if" type satellite orbits. Element sets for satellites currently in orbit can be obtained from various sources. Specific instructions for obtaining elements from open sources are provided in the instruction manual. A sample element data file, is provided as part of the OrbSim program package.

Satellite Selection
Satellites are selected by using their North American Aerospace Defense (NORAD) identification number which is supplied for all satellites in earth orbit. Satellites can be added, removed, swapped, injected, or created at any time during a simulation run. User friendly prompts are presented to simplify all data entries or selections.

Satellite Creation Feature
Satellites element sets can be created by specifying the starting sub-satellite point (either the selected observation point or any longitude and latitude) and then entering data by following a series of prompts that request data entry. (Perigee or apogee altitude above 1st point, then either altitude at opposite point, or period, or semi-major axis, then azimuth or inclination. Other elements are defined by geometry etc.) Prompt limits are presented to preclude creating physically impossible orbits. The element set can then be assigned a name and number and saved to a disk file, if so desired, or simply used in the simulation.

Orbital Injection Feature
This feature, similar to the creation feature described above, uses three variants of orbit injection data to create the element set. User specifies data for any one of the following injection procedures (within allowed prompt limits):

• Sub-injection point location, injection altitude, velocity magnitude and path angle, azimuth or inclination.
• Launch location, surface range, trajectory plane azimuth, injection altitude, velocity magnitude and path angle, any out of plane velocity error.
• Launch location, six components of a cartesian state vector (3 position and 3 velocity) in a topocentric coordinate system with origin at the launch point.

As with the creation feature, the element sets can be saved to a disk file if so desired.

Data File Feature
Simulation data can be saved to disk files at any time during a simulation run. The user selects the data types to be saved at the initiation of the run or at any time during a run, and the files are created by OrbSim2W if they don't already exist. These files use the day of the year number (1 through 365 or 366) as file extension for archiving purposes. At any time during a run, the user can toggle the data stream on or off with a menu selection. Every time the data stream is toggled, or if a satellite is added, removed, swapped, or created, the data files are annotated to ensure that no ambiguity occurs during data review and analysis at some later time. Partial sample data file data is included with the appended figures. File data can include:

• State vector coordinates (position & velocity) in geocentric inertial system referenced to the equinox of date.
• Earth based observer to satellite tracking data; azimuth, elevation, range, and range-rate.
• Sub-satellite geodetic data; longitude, latitude, and altitude.
• Satellite to satellite data; range, range-rate, LOS direction cosines in satellite coordinates, and LOS background.
• Additional, optional, observation data including right ascension, declination, geodetic and geocentric information relative to both the observer and to the Greenwich reference.

Relay Communication (In Normal Simulation Mode)
When this feature is activated by the user, the program tests for line-of-sight (LOS) observability between the user selected ground station and all of the satellites in display at each time step. When any satellite satisfies the user preset elevation test limit, a "coverage" circle, centered on the sub-satellite point, is drawn on the earth map. This circle illustrates the communication coverage region for "single bounce" communication between the ground station and the region in sight of the satellite. In addition, the LOS between the satellite in view of the ground station, and every other satellite in display is then tested for "double bounce" communication. When the LOS to any other satellite is viable (i.e. above some preset earth limb grazing altitude), the secondary sub-satellite coverage circle at the other satellite is drawn. After all satellites in display are tested, and all of the coverage circles are present, the complete communication coverage via double satellite relay is illustrated.

Relay Communication (In Sat-to-Sat Simulation Mode)
When this feature is activated by the user, the program tests for line-of-sight (LOS) observability between the user selected ground station and the user selected "prime" satellite at each time step. When the prime satellite satisfies the elevation test limit, a "coverage" circle, centered on the sub-satellite point for "single bounce" communication between the ground station and the region in sight of the prime satellite is drawn on the earth map. Also, at the same time, the LOS's from the prime satellite to all other satellites in display are tested for visibility. Whenever the LOS to another satellite is viable (i.e. above some preset earth limb grazing altitude), a "coverage" circle, centered on the "target" sub-satellite point is drawn on the earth map. After all other satellites in display are tested, and when all of the coverage circles are present, the complete communication coverage via double satellite relay is illustrated. This feature's mode can be converted to provide continuous sub-satellite ground coverage at each time step. In addition, the range, range-rate, and LOS vector in the prime satellite coordinates are computed for inclusion in the data files.

Sunlight/Shadow and Day/Night Line Features
When the sunlight/shadow feature is active, the ground trace of a satellite changes color whenever the satellite is the earth's shadow (whether in penumbra or umbra is indicated and this shadow information is added to the data files). When the day/night line feature is activated, the day/night terminator, the sub-solar point, and the earth's shadow at geosynchronous altitude are projected on the earth map. The geosynchronous shadow projection can help visualize the "eclipse" condition of geosynchronous satellites since their "fixed" appearance on the earth map can make any color change seem ambiguous. All of the data files are annotated with S, P, or U at each time step to indicate whether the satellite is in sunlight or in the earth's penumbral or umbral shadow.

Simulation Integrator
The satellite orbits are simulated using a "general perturbation" type integrator that includes earth gravitational zonal terms through degree five. Equations are carried through the 7th power in eccentricity to produce highly accurate orbit data. This type of integrator is independent of step size, thus the user has complete freedom to specify the time span between data points to be plotted on the display. By using the PC's internal clock, the simulation can also run in "real time"; a useful feature for keeping track of manned or other satellites of special interest.

OrbSim2W uses readily available NASA and NORAD two line element sets (tle's) to:

• visually display satellite orbit motion for up to 11 satellites at any time
• display satellite motion in either simulated time or real time
• provide numerical observation data for ground tracking stations
• compute and display the communication coverage areas of satellites (single & double bounce)
• generate satellite-to-satellite data (for inter-satellite communication or other space operations)
• provide eclipse data as satellites move into the earth's penumbra and umbra
• produce well documented archival text files of all numerical information
• select any location on Earth for the ground observer station

In addition, the program provides the capability to:

• create hypothetical satellites for operations research and analysis
• create the satellite orbits by orbit data or launch vehicle data specification
• generate and save the created orbit elements in two line element (tle) format

OrbSim2W for Windows is now ready for shipment. Previous purchasers of any version of OrbSim for DOS can purchase the new version for Windows at a reduced cost.

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