An improved estimation of the Mars rotation and orientation parameters from the Doppler observables of the InSight-RISE and ExoMars-LaRa landers

Abstract

The Planetary Radio Interferometry and Doppler Experiment (PRIDE) technique has supported the exploitation of Doppler observations of planetary landers from Earth-based stations in many radio science experiments. The precise retrieval of the estimates of the Mars rotation and Orientation Parameters (MOPs) is an important application of the PRIDE technique and is the subject of this work. The MOPs that are retrieved can be related to four motions: the polar motion, variations of the rotation rate, precession, and nutation. More precisely, the MOPs analyzed consist of a total of 30 parameters that characterizes
all the motion components, including: the core factor, the free core nutation rate, 8 spin variation parameters and 20 polar motion parameters. Properties related to the internal structure of the Red planet can be deduced from these parameters.

The aim of this work is to reduce the MOPs uncertainty and thus help identify the interplay and signatures of the different parameters, which in turn will provide a better understanding of the state and composition of the Martian inner core. In this context, it is required a thorough assessment of the benefits of combining the Doppler observations from the InSight-RISE and ExoMars-LaRa landers using an extended network of Earth-based stations, including 10 extra receive-only radio telescopes. The analysis has been conducted with Tudat astrodynamical toolkit, where a high-accuracy Mars rotation model can be adopted, in order to simulate the formal errors of the MOPs.

The results obtained can be summarized as follows. The considered two-lander geometry configuration allows a more precise retrieval of the MOPs, with a reduction of more than 92 % overall on the MOPs uncertainty compared to a single-lander configuration. The combined exploitation of the RISE and LaRa Doppler observations allows more efficient retrieval of the full set of the MOPs, and particularly, the polar motion parameters. Furthermore, the accuracy of the MOPs is additionally increased in a configuration with the 10 extra receive-only stations. The MOPs accuracy improvements observed in this case are, on average, a 12 % for the core factor, a 10 % for the free core nutation rate, about 25 % for the polar motion parameters at the Chandler frequency, a 5 % for the spin variations, and a 15 % for the other polar motion amplitudes.