Calibration of Mars Reconnaissance Orbiter Shallow Radar (SHARAD) data for subsurface probing and surface reflectivity studies

Abstract

Abstract The Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter (MRO) has been operating since 2006, revealing the detailed layered structure of the polar caps, mid-latitude glacial deposits, and a range of volcanic features. Here we address the major sources of change in the effective gain and signal-to-noise ratio of data collected by SHARAD: (1) the altitude and background noise level, (2) the configuration of the solar arrays (SA) and high-gain antenna (HGA), (3) ionospheric attenuation, and (4) the roll angle of MRO. Background noise fluctuations have a range of ~5\xa0dB, and electromagnetic interference may significantly affect comparisons among limited bands of the full spectrum. We use the dense spatial coverage of sounder data to define a model for gain as a function of SA and HGA orientations, and demonstrate its predictive capability over a 4-dB range with radargrams collected under different configurations. Ionospheric attenuation as a function of the phase distortion correction used in radargram processing is refined through a larger dataset than available in earlier studies. Gain improvements due to MRO rolls up to 28o are also a function of the SA-HGA configuration, such that the maximum span of combined SHARAD gain contributions is about 7\xa0dB. After calibration, Planum Boreum in the north polar layered deposits is ~3\xa0dB less reflective than Planum Australe, and multi-band analysis suggests destructive interference occurs in the more densely layered shallow structure of the northern cap. The new calibration model allows targeting of future observations during optimum conditions over features of interest.