Autofocus Correction of Phase Distortion Effects on SHARAD Echoes

SHARAD is a frequency-modulated (15-25 MHz) radar sounder that probes the upper few kilometers of the Martian crust and polar layered deposits. At solar zenith angles less than about 100 degrees, the ionosphere of Mars can induce phase distortion in surface and subsurface radar echoes that substantially degrades the signal-to-noise ratio and vertical resolution of the range-compressed data. We present a range-compression autofocus approach that estimates the phase distortion of SHARAD data along ground-track segments of about 100 km, using a power-law image-sharpness metric and an empirically derived scaling between the phase correction and radar frequency. This method is rapid, yields a greatly improved subsurface image, and provides a means to track regional and temporal changes in the Martian ionosphere.