The MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) experiment aboard Mars Express is a ground penetrating radar for the investigation of the Martian surface and subsurface structure, especially to map underground water and ice, which is thought to be essential in the search for microbial life on Mars. MARSIS has a radiating dipole antenna, which transmitts impulsive signals towards the surface of Mars. For lower frequencies these radar signals are reflected by the ionosphere (ionospheric sounding), and for higher frequencies they are mainly reflected by the Martian surface, but a significant fraction will also travel through the crust. If there are further interfaces between subsurface layers, especially water or ice, there should be a radar echo.
The radar echos coming from possible subsurface water layers are weaker and have a temporal shift compared to the direct reflections from the surface. The reflections from subsurface layers can mix with so-called clutter signals, which are side-reflections from off-nadir surface features. These clutter echoes would limit the detection performance of the instrument, and therefore a monopole clutter cancellation antenna aligned with the nadir axis should record the off-nadir clutter echoes alone. For this task it is of crucial importance to know the reception properties of the monopole clutter antenna, which is influenced by the conducting surfaces of the spacecraft body.
IWF has calculated the reception properties of the monopole clutter antenna using a wire-grid model of the Mars Express spacecraft. As the next picture shows, the monopole antenna gain crucially depends on the wiring of the dipole antenna: Red areas indicate a high, and blue areas a low sensitivity of the monopole.
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