From AMS Glossary
The procedure of calibrating a radar by measuring the power reflected by and returned to the radar from a conducting sphere of a known radar cross section. In principle, this method establishes the radar constant, including the effects of the antenna gain and any attenuation of the signal in the radar system between the antenna and the receiver, effects that are sometimes difficult to estimate by other means.
In practice, the method is sometimes difficult to apply. The approach is usually to suspend the sphere beneath a tethered (sometimes free-floating) balloon. The problem is to keep the moving sphere at the center of the radar beam during the time required for a measurement of the returned power.