On the Observability of Spacecraft Navigation Using Landmarks

Spacecraft navigation using landmarks, a subclass of optical navigation, enables autonomy from external navigation systems, which can reduce the burden on those systems and improve system reliability when faced with outages in external navigation systems. This work investigates the observability of landmark navigation from a spacecraft in a circular orbit about Earth using a directional sensor with its line of sight fixed in the local orbital frame. The conditions for observability are proven for estimating the spacecraft position and velocity as well as for estimating the Earth orientation parameters. Specifically, for Earth satellites using Earth landmarks estimating only position and velocity is always observable, estimating the Earth rotation angle is observable for certain sensor lines of sight in non-polar orbits, and estimating polar motion adds an additional constraint that the orbit cannot be geostationary. The formal proof of conditions for observability when estimating Earth orientation parameters are believed to be novel. While these results are derived for Earth they can be applied to deep space spacecraft in orbit about a suitable central body as well.