Radar is a device to send and receive Radio signals that bounces them off targets to obtain information about them. Radar is used on planetary missions and has been used from Earth to the Moon and Mars, and by space probes in Earth orbit as well as those going to other planets.
Radar in the tens of megahertz (10MHz-2.6GHz) can penetrate the ground (when intended for this use, Ground-penetrating Radar or GPR) and offer information about the structure of moons and planets. Varying materials, e.g., underground layers, vary the transparency to radio and create partial reflections, analogous to optical effects.
Circular Polarization Ratio (CPR)
Frequency Modulation (FM)
Lunar Reconnaissance Orbiter (LRO)
Mars Reconnaissance Orbiter (MRO)