The Circular Polarization Ratio (CPR) is a measure of a quality of the reflected Radar signal received by Circular Polarization Radar. This type of radar transmits a circular-polarized signal, and the CPR is the ratio of the amount of received/reflected signal that is polarized in the same direction as transmitted to the amount polarized in the opposite direction.
Circular Polarization is the form of a wave signal (e.g., EMR) such that at each point in the oscillation, the wave remains at its peak, which is constant, the oscillation occurring regarding the peak's direction (perpendicular to the direction of the signal), rotating around the line-of-direction like a clock hand. Circular Polarization Radar is used for its ability to emphasize and ignore certain wanted or unwanted signals and determine something about the texture of the reflecting material. One use is in detecting water Ice, which has been used in probing the Moon and other astronomical bodies.