Skip to content

ADR-0022: A multipath signature on the GNSS innovations

Context

ADR-0021 chose to ship at most a multipath detector. A rule of two numbers on a window of GNSS innovations (NIS and lag-1 autocorrelation) was tested in two versions, with the shipping criteria written before the fresh runs.

Options

  1. The sensitive rule (NIS at least 2). It sees multipath from 1 m and calls small steps multipath too.
  2. The specific rule (NIS at least 6). It sees multipath from about 3 m and called nothing else on the fresh runs.
  3. Wire a detector into the filter. The filter would report multipath as a cause on the FDIR event.

Decision

Option 2, as a function and not as filter behaviour. navkit.fdir.multipath.signature and is_multipath use the specific rule by default. The sensitive rule stays available as an argument, so the trade-off can be seen. Nothing in the estimator or the FDIR layer calls it, and no FDIR event changes. Wiring it in (option 3) is a separate step with its own ADR, because it changes what a user of the filter is told.

The criteria were met on fresh runs by the specific rule only: 100% of the multipath runs of 3 m and above, and none of the other runs. The floor is about 3 m.

Consequences

The claim is limited to injected white noise on simulated GNSS, two datasets and 20 s windows. Real multipath is correlated and may exceed the autocorrelation limit. A real-receiver check is not done. The Track A exit test is still not met: the multipath call alone does not give 95% detection per type at zero false alarms for spoofing and degradation.