SHARAD Signal Attenuation and Delay Offsets Due to the Martian Ionosphere

We estimate and correct variable time-delay offsets among Mars Shallow Radar (SHARAD) sounding observations of the north polar layered deposits of Mars under a broad range of ionospheric conditions. We find that these delay offsets are linearly related to the magnitude of the phase distortion, which is determined by optimization of the surface and subsurface echo response by autofocusing. We use this relationship to determine an empirical scaling between the phase parameter and the total electron content (TEC) of the Martian ionosphere. This method allows simultaneous correction of the image blurring and time-delay errors induced by the ionosphere, corrections that are crucial for 3-D imaging of subsurface features. We also characterize the attenuation of SHARAD echoes as a function of TEC to facilitate detection of seasonal changes in surface physical properties. During solar flare events, SHARAD signals experience phase distortion and delay consistent with a 50%-100% increase in TEC relative to typical dayside conditions, but up to tenfold greater attenuation than predicted. The increased signal absorption is likely due to enhanced electron content at lower altitudes where the neutral particle density is higher.