Simulation Coherence: Generating Positioning Signals with Real-World Fidelity


In the physical world, GPS signals and radio frequency (RF) interference are naturally coherent. In simulations, every effect at every instance of simulated time must be calculated separately. Synchronizing and combining these calculations to generate coherent GPS and interference RF wavefronts is not trivial. Without coherence, your simulation will not match real-world conditions.

GPS reception is never perfect

To understand the challenges of simulation coherence, we need to look at how antennas detect signals in the field. Controlled Reception Pattern Antennas (CRPAs) combine GPS signals received by multiple antenna elements. The resulting signal lets GNSS or EGI systems calculate the vehicle’s position while mitigating jamming.

Outside the lab, CRPA systems rarely receive a pure GPS signal. Whether on land, at sea, or in the air, vehicles travel through an RF environment that interferes with GPS signals.

Ionospheric interference

GPS and other positioning systems compensate for the typical effects of Earth’s ionosphere. Even so, geomagnetic storms and scintillation can disrupt the ionosphere and alter GPS signals. In extreme conditions, receivers may lose signal lock entirely.

Terrain effects

Surface vehicles and low-flying aircraft are subject to interference as the surrounding terrain reflects or obscures signals from different satellites at different times.

Synthetic interference, jamming, and spoofing

Vehicles operating in urban environments may experience interference from industrial sources. In contested space, adversaries’ jammers can weaken or overwhelm GPS signals while spoofing can introduce deceptive positioning data.

GPS reception in motion

Interference effects can change dramatically over the course of a given scenario. Not only does the vehicle move, but so do the GPS satellites and jammers. With each shift in relative position between RF sources and the receiver, the combined GPS and interference wavefront changes.

Coherently simulating reality

In the real world, physics combines GPS and interference signals “for free”. But in simulation, nothing comes without a price. The more complexity and fidelity a scenario requires, the more calculations a simulator must make at each instance of simulation time.

  • Multi-vehicle position and 6-DOF motion.
  • Multi-satellite position and orbital motion.
  • Multiple interference sources (and motion if applicable).
  • Multiple jamming and spoofing sources (and motion if applicable).

The simulation must also account for the vehicle’s specific CRPA configuration. While some CRPA designs use as few as two antenna elements, the most robust CRPA designs can use seven or more antenna elements. Furthermore, the CRPA chassis is mounted within each vehicle type at a different location and orientation. This means every antenna element in every vehicle moves through the simulated RF environment in a unique way.

Simulations only work by perfectly synchronizing every calculation. The signals from each simulated satellite, all sources of natural interference, and all jamming sources must be exactly in phase. If not, then the simulation output becomes non-coherent.

What happens when simulators are non-coherent?

Non-coherence can cause a wavefront simulation to fail. The simulated output will be so bad that the rest of the test cannot work. A CRPA system won’t be able to combine the simulated element output. A GNSS or EGI system will generate unusable positioning results. Complete failure is the best-case scenario because null results are better than bad results.

Simulator non-coherence can be subtle and may go unnoticed. In the lab, experiments will generate inconsistent or misleading results. Bad simulation output during CRPA tuning could result in poor operational performance. When non-coherence introduces errors like this, projects waste time and resources searching for root causes to problems that do not exist.

CAST Navigation ensures coherent simulations

CAST Navigation has a well-earned track record of delivering consistent, repeatable simulations. Our close relationship with industry-leading manufacturers lets us accurately simulate CRPA, GNSS, INS, and jamming systems. Forty years of subject expertise and technology development ensures the precision and accuracy of our simulator systems.

Whether you are tuning CRPA systems or simulating squadron maneuvers, CAST Navigation’s proprietary timing distribution technology ensures every simulated antenna output produces coherent GPS and interference RF wavefronts.

CAST Navigation simulators deliver the right results the first time every time. Contact CAST Navigation, LLC at to learn how consistent, coherent simulations can improve your testing requirements.

September 30, 2021