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RACELOGIC Support Centre

08 - SatGen Space Simulations

Although nearly all GNSS receivers on the market are restricted to 1000 kts and/or 18000 m, SatGen Software has no such limitations and can simulate any speed and any altitude (up to the geostationary orbit, but higher altitudes are available on request). This makes SatGen suitable for simulating space scenarios – rocket launches, cubesat deployments, satellites, space probes, etc.

To run such a simulation, Load an NMEA file with the planned trajectory and SatGen will do the rest. Specially for this purpose, we introduced the 'true horizon' elevation mask, which continuously calculates where the horizon would appear when observed from each trajectory point and increases the number of visible satellites accordingly.
 

SatGen Space Sim.png
 

The trajectory must be very precise and very smooth. A few popular implementations of orbit algorithms such as SGP4 have numerical instabilities resulting in a jerky trajectory, which gets rejected by receivers when simulated.

We recommend creating the NMEA trajectory file with a sample rate of 1 – 10 Hz. The Latitude and Longitudes values within the file should be to a minimum of 6 decimal places, e.g. 1234.123456. The altitude value should be to a minimum of 3 decimal places, e.g. 123456.123.


Higher sample rates can be used for the NMEA trajectory files if required, however the Latitude, Longitude and Altitude values must be stated with a higher number of decimal places to avoid sudden jumps resulting from low resolution of latitude/longitude. Sudden jumps in position that are unrealistic for a GNSS receivers real usage, can result in the receiver being unable to maintain lock throughout the simulated scenario.


Example of NMEA lines which can be simulated with SatGen:

$GPGGA,113000.00,4835.905512038,S,14233.294281653,W,8,16,,416363.7170844,M,,,,*27
$GPGGA,113001.00,4837.279673975,S,14228.068010021,W,8,16,,416374.2386976,M,,,,*2A
$GPGGA,113002.00,4838.649496295,S,14222.836804354,W,8,16,,416384.7328093,M,,,,*24
$GPGGA,113003.00,4840.014971686,S,14217.600671875,W,8,16,,416395.1993741,M,,,,*2B
 

In order to make generating the NMEA files easier, SatGen doesn't enforce the 79 character NMEA line length limit and doesn't verify the checksum.

When creating this type of scenario, the speed and position graphs within the software will not correctly represent the trajectory loaded or produced by the simulation engine and should be ignored. The SatGen UI was designed originally for local driving scenarios and only gives a 2D representation which is not accurate for trajectories such as orbits.

Note: User Defined Script trajectories are defined in a local coordinate system. Due to this, script trajectories aren’t suitable for space and long distance scenario simulations.

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