Creating a Space Simulation - SatGen 4
Most GNSS receivers on the market are restricted to 1000 knots and/or 18000 m but the SatGen Software has no such limitations and can simulate any speed and any altitude (by standard up to the geostationary orbit, but higher altitudes are available on request). This makes SatGen 4 suitable for simulating space scenarios, such as rocket launches, CubeSat deployments, satellites, space probes, etc.
To run a space simulation, you must first create an NMEA file containing the relevant data and trajectory for your simulation. See guidelines below. You can then load this NMEA file into SatGen 4 to create a scenario file for the simulation. The software will use the True horizon elevation mask to continuously calculate where the horizon will appear when observed from each trajectory point and increase the number of visible satellites accordingly.
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.
For space simulations, the trajectory must be precise and smooth. Some popular implementations of orbit algorithms, such as SGP4, have numerical instabilities that will result in a jerky trajectory. Jerky trajectories will get rejected by receivers when simulated.
We recommend that your NMEA trajectory file includes the following data:
- A sample rate of 1 to 10 Hz.
- Latitude and Longitude values with a minimum of 6 decimal places, for example, 1234.123456.
- An altitude value with a minimum of 3 decimal places, for example, 123456.123.
You can use higher sample rates for the NMEA trajectory files if required. However, you must state the Latitude, Longitude and Altitude values with a higher number of decimal places to avoid sudden jumps due to low resolution of latitude/longitude.
Note: Sudden jumps in the position, which are unrealistic for a GNSS receiver's real usage, can result in the receiver being unable to maintain the satellite lock throughout the simulated scenario.
Example of NMEA lines that can be simulated with SatGen 4:
$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
To simplify the creation process of the NMEA files, SatGen 4 does not enforce the 79-character NMEA line length limit and does not verify the checksum.
Getting Started
- Make sure that you are connected to the Internet.
- Connect the security dongle to your computer. Wait for the dongle to activate and the light on it to become red.
- Open the SatGen 4 software.
Import the NMEA File
- Click on the Import button.
- Select NMEA File in the panel on the left-hand side.
- Adjust the import settings if necessary.
- Click Import.
SatGen 4 will import your NMEA file and plot the trajectory/route on the map.
Note: SatGen 4 was originally designed for local driving scenarios and will only give a 2D representation that is not accurate for trajectories such as orbits. When SatGen 4 creates a space simulation, the speed and position graphs in the software will not correctly represent the trajectory loaded or produced by the simulation engine.
Generate the Simulation
- Go to the Create tab
- Set the format to correspond with the LabSat unit you want to use to replay the scenario.
- Choose a file save location and file name.
- You can also select available constellations and frequencies in the scenario, as well as the scenario quantisation and hardware attenuation.
- When you are happy with the configuration of the output format, click the Create file button to make SatGen generate the GNSS scenario file. If you need to create multiple scenarios and simulations, we recommend that you use the queue. Click on the Add to queue button to make SatGen send the file to the generation queue. (The file generation will be paused until you are finished configuring all your scenarios and manually start the file generation.)