A novel method for automatic clutter detection in radargrams acquired by orbiting radar sounders

Orbiting radar sounders are very important instruments that can probe the subsurface of planetary bodies from orbit. In particular, they produce vertical profiles revealing the geological structure of the subsurface. As a consequence of their nadir-looking acquisition geometry, radargrams may be affected by spurious off-nadir echoes due to surface clutter. These returns represent a relevant problem, as they may be detected as (or mask) actual subsurface targets. Clutter is usually detected by means of manual visual comparison between radargrams and surface clutter simulations obtained using available digital elevation models (DEM). This procedure is inherently subjective and time-consuming, thus leading to a possible reduction of the scientific return of the data. This paper addresses this problem by proposing a novel technique for the automatic detection of surface clutter returns in radar sounder data which can both greatly support the scientific community in manual analysis of radar sounder data and drive the development of reliable automatic methods. The proposed method is made up of three steps: i) simulation of surface clutter returns using available digital elevation models, ii) automatic coregistration between radargrams and simulations, and iii) extraction of surface clutter returns from the coregistered radargrams. The technique has been tested on two large datasets acquired on different areas of Mars by the Shallow Radar (SHARAD) instrument. The results point out that the method is robust to radargram deformations (e.g., due to ionosphere effects) and that the coregistration process can achieve a very high accuracy.