A new astrometric measurement technique for Phobos using background stars and surface control points

Abstract

Phobos, the larger natural satellite of Mars is undergoing a complex orbital evolution. Recent analysis on the basis of HRSC/SRC image data showed discrepancies between orbit models and observations of up to 12 km, a fact which renewed the interest in more detailed astrometric analysis of Phobos data to constrain its orbit. We now have carried out new Phobos astrometric measurements in images obtained by the Super Resolution Channel (SRC) on Mars Express (MEX) to verify the accuracy of the new available Phobos orbit models. We have made several improvements over our previous measurement techniques. Background star images before and /or after the Phobos encounter during 35 orbits were used to identify and remove offsets in the nominal pointing data. Also, we use control point measurements rather than limb fits to determine the position of the Phobos COF/COM (center of figure/mass) in the image. Many more Phobos images from 34 flybys were available compared to what we had in our previous studies.