Distortion of radar pulses by the Martian ionosphere

In spacecraft borne radar investigations of planets such as Venus and Mars the waves must penetrate an ionospheric layer which causes absorption and dispersive phase delay if the waves reach the surface at all. If the purpose of the radar system is to explore the planetary subsurface, the radar frequency should be as low as possible for maximum skin depth, yet high enough to reach the surface. In order to resolve subsurface discontinuities the radar must use short pulses and because of the low frequency the dispersive pulse distortion in the ionosphere becomes a problem. In this paper we discuss ways to avoid the pulse distortion and to recover the original pulse shape. As the ionosphere is unknown and changes with time and position, the necessary ionospheric data must be derived from the radar observations themselves. The scheme described is designed to do this. We also discuss the effect of additive noise on the accuracy of this observation scheme. The investigation was motivated by the MARSIS experiment on Mars Express and by a similar experiment on the Russian spacecraft Mars96. The implementation of the scheme in MARSIS will depend on the computational resources which can be allocated to the task. In future space missions with the proper planning it is thought that the scheme presented will be even more attractive.