A radar sounder is used to detect the subsurface features of a medium by measuring the reflections
from different dielectric layers of that medium. It consists of a transmitter, which sends an
electromagnetic wave from the surface. This wave is reflected at different dielectric interfaces and
scattered by the objects imbedded in it. By examining the received signal, information such as the
dielectric constant of the medium and the depth of different layers can be derived. The first sounding instruments were the Apollo
Lunar Sounder Experiment
and the Surface Electrical Properties Experiment both part of the
Apollo 17 mission.