Mars Express astrometric observations of Phobos

The measurements have been made using a newly developed technique which involves positional measurements of surface control points and verification of camera pointing by background stars. The astrometric positions are in excellent agreement with currently available Phobos orbit models.

However, we find remaining systematic offsets of 1.5-2.6km, with Phobos ahead of the predicted position along its track. Our observations will be a basis for further improvements in the Phobos ephemeris. The methods we have developed will be useful for astrometric tracking of planetary or asteroidal targets and spacecraft optical navigation in future planetary missions.