Electromagnetic aspects for sub-surface sounding of planets

Some aspects of satellite-based planetary subsurface sensing are outlined. One constraint has to do with the fact that at very low frequencies, needed to penetrate the ground, it is difficult or impossible to achieve a narrow antenna beam-width and therefore a large area is illuminated, creating backscatter which interferes with the wanted signal. Another issue is the propagation through the planet's atmosphere and particularly through the ionosphere.