MARSIS data as a New Constraint for Phobos’ orbit

MARSIS radar onboard MEX did numerous observations of Phobos the past 20 years, trying to highlight geological structures in the Phobos subsurface. Up to now, these measurements never allowed us to identify subsurface features while the onboard software upgrade in 2023 promises a significant sensitivity improvement. On the other hand, a deeper analysis demonstrated a deviation between the orbit prediction and the ranging measurements done by MARSIS and then allowed us to derive a new constraint for Phobos ephemeris calculation.

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 (Desage, 2024). 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 of Phobos along its track that would 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. The systematic analysis of MARSIS data set from 2007 to 2021 is available in Desage, 2024 and was taken into account in the last Phobos ephemerid (V. Lainey, 2024, NOE-4-2024-MMX-D3). The possibility of a joint Radio-Science and MARSIS observation is also under evaluation to better constrain Phobos distances during Radio Science measurement and then improve gravity field measurement.