The Doppler radar is an airborne radar which measures the speed of an object by means of a Doppler shift, which is a shift in the frequency of a returned echo proportional to the speed of the echoing object. Thus, a Doppler effect is the alteration in the electromagnetic frequency, which takes place when the source of echoing radiation and its observer move toward or away from each other. The shift is a fraction of the basic frequency equals to the speed as a fraction of the speed of light. Therefore, 600 mph gives a frequency shift of only 0.00008 % (per cent), 160 cycles in the 200 million of a P-band radar.
The Doppler effect is used in two main forms: without pulsing at all (through CW, or continuous wave mode) and in a specialized pulse doppler form. The latter is quite common in airborne radars, which must overcome the clutter effect of the ground. However, there are also several important naval pulse Doppler radars in service. One of the most widely used naval pulse Doppler transmitter has been the Tartar guidance radar AN/SPG-51, which is a pulse doppler tracking illumination radio transmitter employed with the RIM-66 standard missile.
Below, an AN/APG-65, a pulse Doppler radar mounted in nose of a combat aircraft. It is an all-weather airborne radar developed by Hughes Aircraft Company for the F/A-18 Hornet aircraft.
