In GNSS outage holdover analysis, which quantity primarily governs how quickly the position error grows?

• A. The GNSS satellite geometry
• B. The INS process noise and bias instability primarily govern error growth during holdover.
• C. The vehicle speed
• D. The barometric altitude accuracy

Answer: (B)

When GNSS is unavailable, the way position errors grow is governed by how the inertial sensors themselves behave. The accelerometers and gyros introduce two main imperfections: process noise (random fluctuations in the sensor measurements) and biases that drift over time (bias instability). Because the INS integrates acceleration to velocity and then to position, those small, persistent errors accumulate. A constant or drifting bias in acceleration can cause position error to grow roughly with the square of time, and random noise adds further growth, so the holdover error becomes significant quickly if the biases drift and the process noise is non-negligible.

Satellite geometry only affects the quality of GNSS fixes when signals are available, not how errors accumulate during holdover. Vehicle speed doesn’t set the rate of inertial error growth in the absence of aiding, and barometric altitude accuracy mainly impacts vertical measurements rather than the primary horizontal position error growth during GNSS outage.

Thus, the dominant factor in how quickly the position error grows during holdover is the INS process noise and bias instability.