I. Introduction
Radar sounders (RS) are low-frequency instruments for profiling the subsurface of the Earth or of other planetary bodies. By measuring the echo returns from interfaces of significant dielectric contrast, RS produces radargrams indicating compositional and structural discontinuities that can help to understand and characterize planetary shallow subsurfaces. At present, there are RS orbiting around Mars (i.e., SHAllow RADar (SHARAD) [1] and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) [2]), the Moon (i.e., Lunar Radar Sounder (LRS) [3]) and a number of airborne RS missions over the polar ice caps of the Earth. Furthermore, two Jovian missions that will carry RS are under development. These are the Radar for Icy Moon Exploration (RIME) [4], which is aimed at studying the icy moons Ganymede, Callisto and Europa, and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) [5], which is specifically targeted at Europa.