Bistatic Radar Occultations of Planetary Surfaces

Assessing the variability of surface roughness on planetary bodies is crucial for understanding the physical mechanisms that have governed their surface evolution. In addition, it provides constraints on surface texture uncertainties associated with landing, anchoring, surface trafficability, and sampling. Herein, we present how orbital bistatic radar (BSR) observations can be successfully conducted during occultations, and thereby used to achieve the above scientific and technical objectives for key planetary bodies that are under investigation by current and future missions. For different planetary bodies without dense atmospheres, we calculate the theoretical differential Doppler shift (δf) between the direct and surface-scattered signals that precede and follow radio occultations during opportunistic BSR experiments. For occultations that last ~10 min, we find that small bodies with diameters ≲100 km will exhibit |δf| ≲5 Hz, and larger bodies with diameters ≳500 km will have |δf| ≳50 Hz. Our results suggest that opportunistic BSR observations of targets ≲100 km in diameter will require an ultrastable oscillator (USO) aboard the spacecraft to retrieve surface echoes and constrain surface roughness.