Polarization in Bistatic Radar Probing of Planetary Surfaces: Application to Mars Express Data

Spacecraft-to-ground bistatic radar provides a straightforward method for surveying planetary surfaces on scales of importance to landers and rovers. Centimeter wavelengths, currently in use for deep-space telecommunications, interact with surface structure of similar to somewhat larger scales. For the quasi-specular component of scattering and for surfaces uniformly illuminated by monochromatic signals from an orbiting or flyby vehicle, the echo Doppler dispersion is proportional to the root mean square (rms) surface slope. When the specular condition occurs within 10°-20° of the Brewster angle, the surface dielectric constant can be derived from relative echo power measured simultaneously in orthogonal polarizations and the Fresnel reflection laws. Cross spectra, computed from outputs of the orthogonally polarized receivers, may be used to calculate a complete description of the polarization properties of the scattered fields. Application to planetary studies requires accurate amplitude and phase calibration of the polarization channels, including correction for any leakage between the two receiving paths, such as from imperfectly isolated antenna feeds. We illustrate these techniques using Mars Express results from “Stealth” (Medusae Fossae), a region on Mars that has not previously been detected by Earth-based radar, and from a long profile including Acidalia Planitia. Single-location Stealth observations support previous conclusions that the surface is rough and porous (dielectric constant ≈ 1.4). But the longest experiment (in which the specular point was followed for an hour) yields relatively high dielectric constants (2.8), suggesting that the model is incomplete. The surface of Acidalia Planitia has low dielectric constants ( ≈2.6) over 60-90 W at 50 N and higher values (≈3.6) as the specular point moves south and crosses the equator.