New astrometric observations of Phobos with the SRC on Mars Express

Aims. New astrometric measurements for Phobos are reported on the basis of 69 SRC (Super Resolution Channel) images obtained during 28 Mars Express Phobos flybys executed between 2004 and 2007. Methods: The measurements have been made using a newly developed technique that involves positional measurements of surface control points and verification of camera pointing by background stars. Results: The astrometric positions are in excellent agreement with currently available Phobos orbit models. However, we find remaining systematic offsets of 1.5-2.6 km such that Phobos is ahead of its predicted position along the track. Conclusions: Our observations will be a basis for further improvements in the Phobos ephemeris. The methods that we have developed will be useful for the astrometric tracking of planetary or asteroidal targets and spacecraft optical navigation in future planetary missions