Radar sounding is the only remote sensing technique allowing the study  
of the subsurface of a planet from orbit. By detecting dielectric  
discontinuities associated with compositional and/or structural  
discontinuities, it is possible to map the stratigraphy, which can be  
of fundamental importance to better understand the dynamics and the  
history of the first meters to kilometers of the subsurface.

MARSIS is a synthetic aperture, orbital sounding radar, carried by  
ESA's Mars Express orbiter. MARSIS is capable of transmitting at four  
different bands between 1.3 MHz and 5.5 MHz, with a 1 MHz  
bandwidth.and is optimized for deep penetration, having detected  
echoes down to a depth of 3.7 km over the South Polar Layered Deposits.

MARSIS transmits through a dipole, which has negligible directivity,  
with the consequence that the radar pulse illuminates the entire  
surface beneath the spacecraft and not only the near-nadir portion  
from which subsurface echoes are expected. The electromagnetic wave  
can then be scattered by any roughness of the surface, and areas that  
are not directly beneath the radar can scatter part of the incident  
radiation back towards it, producing surface echoes that will reach  
the radar after the echo coming from nadir, and that can mask, or be  
mistaken for, subsurface echoes.

The presence of a dispersive medium, such as a plasma, between the  
radar and its observation target can cause a distortion (dispersion)  
of the transmitted waveform. Such is the case for Mars, which is  
surrounded by an atmosphere whose upper layers become ionized and  
excited through solar radiation, producing an ionosphere.

All these effects need to be accounted for and modelled before data  
can be correctly processed and interpreted. In this talk, various  
methods used in the analysis of MARSIS data will be presented, and  
their use demonstrated. Significant results obtained by the radar will  
be reviewed.