Why is bounding GNSS errors under rare or anomalous conditions important, and what makes it difficult?

摘要

Estimating and quantifying errors in GN5S and other navigation user equipment is a common task and is important for understanding what roles or missions a given device is suitable for. Most users are familiar with the concept of accuracy, which is usually expressed as a bound on navigation error (relative to unknown truth) at a probability level between 0.5 and 0.95- This article expands on the concept of accuracy to estimate error bounds that are only violated with far smaller probabilities. These bounds are required to calculate protection level equations that are used to verify navigation integrity in real-time for applications with very strict safety requirements. Protection levels are compared to limits on the maximum safe errors ("alert limits") to confirm that safety is maintained for the current operation. Because much less is known about GNSS errors at the very low probabilities that apply to protection levels (e.g., 10~(-7) to 10~(-9)), it is very difficult to extrapolate accuracy numbers to these low probabilities.