The term global navigation satellite system (GNSS) is the standard for all types of satellite navigation systems that have global coverage. These include GLONASS, GPS, Beidou, Compass, Galileo, and other regional satellite navigation systems. GNSS uses satellite constellations for the broadcasting of signals, which are then processed by a GNSS receiver. These processed signals will then determine the time, speed, and location of the device’s user.
Though there are multiple GNSS receiver manufacturers, no receiver is perfect. This is why manufacturers need to recreate the common inherent GNSS errors and test them using a single or multi-element GNSS simulator.
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A single element simulator will replicate one satellite’s signal and is generally for R&D and production testing.
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A multi-element simulator will test multiple satellite signals. It is often used for manufacturing, post-launch, and R&D tests. This simulator — unlike the single element one — allows repeatability of the test signal’s generation and completes constellations during runtime.
The following are the standard errors tested using GNSS simulators:
Space Segment Errors
The two common space segment errors of GNSS are those arising from the clocks used by the system and from the satellites’ positioning. Satellite orbits are affected by the gravitational influence of the moon and sun, solar radiation pressure, and the elliptical geometrical nature of the earth. These and the inaccuracy of the clock used by GNSS receivers could cause an error in the device’s output.
The common types of errors recreated using a GNSS simulator in space segment errors include satellite clock errors, orbital perturbations, intentional satellite clock noise, and satellite geometry anomalies.
User Segment Errors
These generally stem from a fault in the GNSS receiver’s characterization and design. The typical user segment error is incorrect compensation for tropospheric and ionospheric delays. Fundamental design flaws, including integration issues and inter-channel bias, might also contribute to user segment errors. The GNSS simulator used for testing will use different software to recreate these errors
Control Segment Errors
These will be manifested in the navigation message a GNSS device will transmit. There are three kinds of control segment errors including navigation data, ionospheric prediction, and ephemeris prediction errors. Ephemeris prediction errors prevent a receiver’s tracking of a satellite since it is not where a receiver calculates it to be.
Navigation data errors arise from the replicated uploading of navigation message to each satellite in the device’s space segment. Ionospheric prediction errors are based on the type of ionospheric model a device uses.
Multipath Errors
GNSS signals work much like light rays and will fall on a reflective surface at a lesser angle compared to the internal reflection angle. If your receiver receives a direct and reflected ray from the satellite, rather than only the direct one, it will use both of these signals. This results in an error in your receiver’s position estimate.
It is essential for manufacturers and designers of GNSS devices to understand the errors that might cause an inaccuracy with their devices’ outputs. This is only possible if they can simulate the above errors as much as possible and get the right data on what is affecting their systems’ accuracies. The right GNSS simulator is, therefore, an essential component in your device’s production.