Passive calibration satellites are objects of known shape and size.
These are generally solid or hollow spheres, since that shape will have the same cross-section regardless of viewing angle, though later passive satellites used wire grid designs.
[3] Active calibration satellites are equipped with transponders that emit a signal on command.
The ground radar station submits a transmit command and takes a measurement of the satellite's location.
The transponder's signal is received by a radar receiver and combined with the satellite's ephemeris data to calculate the satellite's actual position, and this is then compared with the position calculated by the radar measurement in order to measure the radar's accuracy.