MARSIS data as a new constraint for the orbit of Phobos

Abstract

Context. The orbit of Phobos is currently known with a 1σ precision of 300 m, mostly directed along its track. Most previous determinations were made with the help of the Super-Resolution Channel (SRC) channel of the HRSC camera on board Mars Express (MEX). The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board MEX crosses the orbit of Phobos every 7 months on average and can be used to measure the distance between MEX and Phobos at the time of approach. Aims. We compared data from the MARSIS radar on board MEX to simulations using different shape models in order to provide measurements of the position of Phobos that can be used as control points for the determination of its orbit. Methods. We measured the range offset between SAR syntheses of MARSIS datasets and SAR syntheses of coherent simulations taken as a reference, and link this to the offset of Phobos along its trajectory. Results. We provide a set of measurements of range offsets made with MARSIS alongside the corresponding Phobos along-track offsets that would produce those discrepancies. We also provide measurements of the distance between Phobos and MARSIS. We performed those measurements for two different Phobos shape models and two different Phobos ephemerides, discussing the potential source of error in the range measurements. An estimation of the MARSIS instrumental delay for band III is derived from this work.