Estimation of Mars geophysical information through Same Beam Interferometry
Abstract
A mission deploying two or more landers on the surface of Mars would enable very accurate geodesy and geophysics experiments, by means of a tracking technique called Same Beam Interferometry (SBI). A microwave carrier is sent by a single Earth antenna towards two or more widely separated Mars landers, equipped with identical digital transponders with high long-term frequency stability, that retransmit coherently the signal back to Earth. The observable quantity is the difference between the phases of the two signals received simultaneously by the same ground antenna. The path delays due to interplanetary plasma, Earth's troposphere and ionosphere cancel out in the differential phase, as they act in nearly identical ways in both paths, contributing for less than 0.2 mm in the error budget. The cancellation does not occur for the Mars atmospheric and ionospheric delays, which however may be kept at a level respectively below 0.2 and 0.6 mm by suitable calibrations and adopting Ka-band links. Adopting Code Division Multiple Access (CDMA) technique allows to use the same frequency band for all links, thus maximizing the commonality of the delays. The envisaged overall error budget for SBI observations on Mars is better than 0.7 mm at Ka-band, so it would provide measurements of Mars tidal deformation and rotation with an unprecedented accuracy. The determination of tides and rotational state with such an accuracy will allow to focus on interior processes and provide essential constraints for models of the thermal, geochemical, and geologic evolution of Mars. Using X-band links is also an option, nevertheless a degradation of the measurement accuracy is foreseen, up to about 10 mm (mostly due to Martian ionosphere contribution). In this work we show the expected performance of SBI technique at Ka-band, in a mission scenario involving a network of three landers on Mars.
- Publication:
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European Planetary Science Congress
- Pub Date:
- April 2014
- Bibcode:
- 2014EPSC....9..395G