Mars Express — ExoMars TGO Mutual Radio Occultation Experiment

Conventional Radio Occultation measurements take place between a spacecraft in orbit around a planet (or during an occasional fly-by), and a receiving ground station on the Earth. When the straight line between the sender and receiver crosses the ionosphere and the atmosphere of the planet the signal is affected by the electrons in the ionosphere and the molecules in the neutral atmosphere. This signal change is observed as a slight frequency shift in the received radio signal with respect to the transmitted signal.

Here we are attempting to do occultation measurements between two spacecraft at Mars, Mars Express and ExoMars Trace Gas Orbiter, using the UHF link normally used for communicating between the orbiters and the landers on the surface of Mars, rather than using the X- or S-band transmitter used for the communication between a spacecraft and the Earth. This was first exercised between NASA’s Odyssey and MRO at three occasions in 2007 (Ao, et al., Radio Sci., 2015), but has not been repeated since then.

Such a Mutual Occultation technique has several significant advantages over the traditional spacecraft to Earth occultation measurements. The perhaps most important being that it allows a much more even distribution of the samples over the surface of the planet and in time, and the observations are independent of Earth occultation seasons.

The first experiment was carried out in November 2020 and demonstrated the feasibility of the technique. Several more experiments were performed between early 2021 and mid 2022. The initial test used a lander (Beagle2) hail signal from Mars Express/Melacom, consisting of a series of 20 seconds of modulated and unmodulated transmissions interspersed with 2 seconds without transmission. In March 2021 a software update was applied to Melacom, which allowed it to generate an uninterrupted, unmodulated carrier-only signal, since. This has improved the performance and the quality of the measurements and has simplified the ground processing significantly. The results are presently being analysed. A complication seems to be the stability of the local oscillator on board TGO, which sometimes causes frequency variations only slightly less than that of the signal itself. Means of mitigating this are under investigation.