A novel approach to the automatic detection of subsurface features in planetary radar sounder signals

Nowadays, planetary radar sounding missions operating at Mars are providing a huge amount of data which are mostly analyzed by means of manual investigations. The development of techniques for the automatic extraction of information from radar sounder data is thus a crucial issue for a proper data exploitation from the scientific community. As an important step to the definition of automatic methods that can be implemented in a more general framework for the analysis of radar sounder data, in this paper we present a study of the statistical properties of radar sounder signals. The statistical distributions of the amplitude of the samples related to different subsurface targets are analyzed in order to define a suitable fitting model. Exploiting the results of such a study, we also propose a novel automatic technique capable to extract the deepest scattering area visible in the radargrams. The basal scattering area is composed using an advanced region-growing approach properly defined for the analysis of radargrams. The obtained regions are kept or discarded according to the statistical distribution of their samples. Experimental results obtained on data acquired by the Shallow Radar (SHARAD) on Mars confirm the effectiveness of the proposed approach.

1. INTRODUCTION

Planetary radar sounders are orbiting ground penetrating radars (GPR) which operate at very low frequency (1–20 MHz) with a nadir looking geometry. Thanks to their characteristics, radar sounder are able to investigate the subsurface of planetary bodies by measuring the backscattering from buried structures. Nowadays, two radar sounders are operating at Mars: the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on the Mars Express orbiter of ESA, and the Shallow Radar (SHARAD) on-board the NASA's Mars Reconnaissance Orbiter [1]. These instruments are providing a new insight on the subsurface of Mars and their success lead to the inclusion of radar sounder instruments in future planetary missions [2]. MARSIS and SHARAD are providing a huge amount of data. Such data have been mostly analyzed by means of manual investigations, thus leading to subjective interpretations of the data and limiting their scientific return. In fact, manual analyses are non-objective and time consuming tasks. This calls for the development of techniques for the automatic extraction of information from sounder signals. Indeed, the use of reliable techniques allows an objective and fast extraction of information from each radargram. Moreover, such techniques allows the joint analysis of many acquisitions, resulting in the possibility to analyze large areas and possibly highlighting structures that are not visible from single tracks. Automatic methods can also play a significant role in the integration of the radargrams with measurements obtained from other instruments. Finally, these methods can result very important for supporting the analysis of radar sounder data acquired on the Earth's subsurface by airborne or future possible spaceborne missions.