MARSIS data as a New Constraint for Phobos’ orbit

Phobos’ orbit is currently known down to a precision of 300m, mostly directed along its track. It has mainly been determined with imagery, and more recently with the Super Resolution Channel of the HRSC camera onboard Mars Express (MEX). This method is associated with an error mainly normal to the plane of imagery. By dynamical constraints, Phobos’ trajectory determination error is mainly spread along its orbit. In order to refine the orbitography and reduce the range error of the measurements, we propose to use data from the MARSIS radar onboard MEX. To do so, we perform a SAR synthesis on the MARSIS data in order to locate the radar echoes in a range/along-track plane. For every one of the 35 datasets at our disposal measured between 2008 and 2021, we also perform a coherent simulation using a Phobos shape model by Willner et al. (2014), and apply the SAR synthesis the same way we did for the MARSIS datasets. Given the geometry of our simulations and the SAR synthesis, the simulated radargrams are not sensitive to a range error of a few km in MEX’s trajectory, they can therefore be taken as reference points. We measure range errors between simulations and MARSIS data, distributed around +1km, with a standard deviation of 350m. The measurements being spread all around Phobos, the most probable cause for the non-zero average of the offsets measurements is an instrumental delay. After subtracting this average from the measurements, we estimate the offset that Phobos would have along its track to create this standard deviation. We find that this offset is of about 100m before 2017, and that the estimated value is rising linearly after this date to reach about 1.3km in 2021, date of our last observation. Since 2017 is the date of the last control point of the NOE-4-2020 ephemeris used for this study, our measurements exhibit a significant drift after this time period suggesting that there may exist a source of inaccuracy in the Phobos ephemeris. This study shows that radar data can be used as control points for the orbitography.