Photo: CAST Navigation

Photo: CAST Navigation

GNSS/INS simulators can model dynamic motion far beyond what any real vehicle will experience. They can produce accelerations that would crush airframes, much less pilots, and simulate speeds that no missile or spacecraft has ever achieved. So, do these motion specifications matter?

They are certainly indicative of a simulator’s overall capabilities and its fit for your project’s test envelope. But unless you are modeling an artillery shell’s acceleration in the barrel or the velocity of a sample return mission from Jupiter’s cloud banks — neither of which need GPS — you will never push the simulated physics to its limits.

Much more important is the simulator’s accuracy and repeatability. Do GNSS and INS simulations stay in lockstep throughout the test? Does the simulation address the effect of vehicle configuration on GNSS and INS output? And will you get the same results with each run of a particular scenario? Thanks to CAST Navigation’s experience the answer to these questions is “Yes”.

Are Your Simulations in Sync?

Photo: CAST Navigation

Photo: CAST Navigation

A map is not the territory, and a simulation is not reality. Simulators may generate results that match actual experience. However, the underlying system does not work the same way physical systems do.

In the physical world, a GNSS system and an INS system respond to the changes in an aircraft’s attitude or position simultaneously. The INS detects a bank, turn, and climb at the same time that the GNSS receiver detects the apparent change in the GPS constellation’s position. These systems output their response in concert, providing commensurate inputs to the navigation system.

Vehicle motion in the simulated world is discrete, not continuous. Each tick of the system clock moves the vehicle through simulated airspace in discrete steps. Separate clocks drive the GNSS simulator and the INS simulator as they respond to the modeled vehicle’s motion. If the separate responses to the same change in vehicle state register at different clock times, their output will not be commensurate. These errors will build as the simulation progresses, steadily feeding the navigation system’s diverging inputs.

Photo: CAST Navigation

Photo: CAST Navigation

How Faithful is the Model to Reality?

Differences between simulation and reality can occur on a larger scale. In the real world, vehicles and their internal systems occupy physical space. GPS antennas, receivers, and INS units are placed in different locations. The locations and distances between these components in ships versus transport aircraft or an F-15EX versus an F-35A are significantly different.

Essentially, these components move in formation with the vehicle’s center of gravity (CG), but the motion they experience at any point in time is different. An antenna mounted to a ship’s mast will move considerably more than an inertial unit close to the ship’s CG.

Basing a model on the assumption that a vehicle’s GNSS/INS components are collocated at the CG produces idealized results that will not accurately simulate the actual vehicle motion. Again, the navigation system’s output will become less accurate as these errors compound.

What Happens When Simulations Go Wrong?

GNSS/INS simulators that rely on simplistic assumptions or that cannot maintain commensurate output may cause your test to fail outright. However, the effect of these errors could be more subtle, generating unsatisfactory output from the navigation system. The impact of these kinds of errors is also sensitive to initial conditions. Each run of a scenario will yield different unsatisfactory results.

When this happens, you must spend your project’s schedule budget finding the root cause of the navigation system’s inconsistent output — which has nothing to do with the navigation system itself.

Photo: CAST Navigation

Photo: CAST Navigation

Get GNSS/INS Simulations Right with CAST Navigation

CAST Navigation’s reputation for producing commensurate, high-fidelity GNSS/INS simulations is built on more than forty years of experience serving aerospace and defense customers. Our proprietary synchronization technology produces simulations with precise, accurate, and repeatable results. Intuitive graphical interfaces let you model specific vehicles and navigation units with real-time 6-DOF trajectories.

With performance that exceeds any realistic test envelope, CAST Navigation’s consistently accurate and precise GNSS/INS simulators are ready to deliver results. Visit to learn more.

Header image: CAST Navigation

This page was produced by North Coast Media’s content marketing staff in collaboration with CAST Navigation. NCM Content Marketing connects marketers to audiences and delivers industry trends, business tips and product information. The GPS World editorial staff did not create this content.