This method is employed by the more conventional types of radars and by some of the radio telescopes used in radio astronomy. Another method uses the principle of the interferometer to compare the phases of signals received in separate antennas on well-established baselines. The frequency of the return signal from a vehicle being tracked depends not only upon the transmitted frequency but also on the relative motion of the vehicle and the tracker. This doppler effect makes it necessary to design the tracker to automatically follow the changing frequency. By the same token it is also possible to use this frequency change to measure the relative velocity of the vehicle and the tracker.
Optical systems make use of the visible-light portion of the electromagnetic spectrum.
They all consist essentially of a telescope mounted on gimbals to permit rotation about two axes.