Low frequency radar sounding through Martian ionosphere

In radar sounding, it is highly desirable to operate at low frequencies to improve the depth of peneteration. For a spaceborne sounder that is orbiting a planet with an ionosphere, the lowest operating frequency is limited by the plasma frequency of the ionosphere. The ionosphere affects the radar signal through dispersion which changes the shape of the signal and introduces a propagation time delay. The impact on the operation of the sounder is twofold by 1) introducing an unknown delay which makes the surface tracking difficult, and 2) degrading the range compression by changing the chirp shape and duration. In this paper, we first demonstrate the effect of the ionosphere on the radar signal through development of an approximate model for the ionosphere of Mars and then present a scheme to compensate for the ionospheric dispersion by taking advantage of the strong specular surface reflection which is expected at low frequencies on Mars. Although the example which is presented in this paper is limited to Mars, the technique is generic and can be used for low frequency radar sounding in general.