Adaptive compensation of Mars ionosphere dispersion in the MARSIS experiment

MARSIS (Mars Advanced Radar for Subsurface Ionosphere Sounding), selected as scientific payload of the ESA mission Mars Express, is a low-frequency nadir-looking pulse limited radar sounder and altimeter with ground penetration capabilities, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections. In order to maximize the penetration capabilities of the transmitted pulse MARSIS must operate at a frequency as low as possible (few MHz). Moreover the requirement for fine range resolution calls for a relatively large transmitted bandwidth (1 MHz) so that MARSIS will operate with a very high fractional bandwidth and very dose to the expected Martian ionosphere plasm frequency. This will result in a generally large phase distortion across the spectrum of the received pulses (due to either the antenna frequency response or the propagation through the ionosphere) which will cause severe degradation of the matched filter performance in term of SNR, pulse spreading and sidelobe level. This paper deals with the definition and implementation of an adaptive range compression algorithm, which makes use of the contrast maximization technique to estimate the phase dispersion spectrum, in order to perform a matched filtering of radar sounder echoes in the presence of phase distortions across the signal bandwidth.