The most prominent feature of Simulators is phenomenon isolation. This technique allows engineers to separate variables while testing their system for potential issues. What if the message sent by a satellite has an error? What if a GPS satellite stops working? What if I use a different antenna? What about multipath signals? What if the power levels drop (which happens because GNSS is already a very weak signal)?
Sum-up of the concepts behind GNSS Simulation
1. Phenomenon isolation
With phenomenon isolation, you can set all the constant variables and repeatedly change the control variable. This allows engineers to pinpoint flaws in their system and then, create a more robust solution that can deal with many potential issues. With real or recorded signals, you have no choice but to deal with all variables simultaneously in a complex environment.
The typical phenomenon we may want to detect, isolate in order to mitigate them are ionospheric and tropospheric disruptions, and also jamming and spoofing.
2. Ionosphere effects
Ionosphere scintillations can have a strong disruptive effect on a GNSS receiver (especially on the L5 frequency where its effect is stronger). This phenomenon will disturb GNSS signals seasonally in some places in the world such as Brazil. It is possible and very valuable to simulate them to mitigate their impact while developing prototypes in the lab.
Multipath signals are a major problem for vehicles in urban canyons, and also for airplanes during take-off and landing. They create positioning errors and are often paired with masking.
To help implement a multipath mitigation algorithm, a simulator is the right solution as a first step, the final validation being done with a real-life test, for example in an urban area. In order to be able to reproduce such a test, it is necessary to use replay/record equipment, allowing to evaluate the improvement with 2 comparable scenes.
4. User motion, antenna, and augmentation system
Other phenomena which can be simulated are user motions (impact of jerk and acceleration could be high for handheld devices), antenna imperfections (imperfect antenna could disrupt signal during motion) and combined augmentation signals such as SBAS, inertial systems (IMU like Mems or Fogs), position correction with RTK/PPP or military GNSS signals (Y Code, PRS, M-Code)...
5. Testing anywhere in the world
Simulators have the advantage of testing many world locations. Some constellations (IRNSS NavIC, QZSS, SBAS, EGNOS, WAAS) only cover a part of the globe, so testing them requires sending a team to either take recordings or to use a simulator. While sending a team to record data is possible, using a simulator is far more cost-effective.
In addition, using real data doesn’t provide access to many transmitting satellites simultaneously.
For example, testing EGNOS, IRNSS NavIC, QZSS is not possible from America without a simulator, as their emission targets other parts of the globe.
6. Testing important but rare events
Another benefit of simulators is that they can recreate events from the past and generate future events. Receivers can be tested on GNSS system's major failures or constellations that are not yet deployed. They also allow the simulation of signals on a specific date in the future (e.g. date of launch), or specific events like week-number roll-over.
Moreover, simulation can generate non-nominal functioning of GNSS satellites & constellations (maintenance, power degradation, clock drifts, etc.) and really put to the test receivers’ resilience.
Limitations of simulated data
Simulation is based on models, and even complex models can be representative of the real world only to a certain extent. Hence, it remains an approximation of real-world phenomenon.
There is a level of granularity that you can’t reach for high-precision positioning in urban canyons (e.g. with a lot of multipaths). Simulated signals can get close to real ones, even for complex trajectories, but recorded signals are essentially the “real world” technique that can faithfully reproduce all the propagation phenomena that altered the GNSS signal between the satellite antenna and that of the receiver.
Beyond the limitations
Leveraging the complementarity of recorded signals
Using recorded GNSS signals solves the limitations of simulated signals, to learn more by reading the following article: