A continuous wave radar is a radar system in which a continuous wave radio energy is transmitted and then received from any reflecting objects. The return frequencies are shifted away from the transmitted frequency based on the Doppler effect if the objects are moving. A continuous wave radar continually transmits energy in the direction of the target and receives back reflection of the continuous wave, providing velocity information by comparing the differences in the transmitted and received waves and making use of the Doppler effect.
The armed forces use continuous wave radars to guide semi-active radar homing (SARH) air-to-air missiles, such as the U.S. AIM-7 Sparrow. The launch aircraft illuminates the target with continuous wave radar signals, and the missile homes in on the reflected radar waves. Since the missile is moving at high velocities relative to the aircraft, there is almost always a strong return. Most modern air combat radars, even pulse Doppler sets, have a CW function for missile guidance purposes.
The advantage of using a continuous wave radar is that it has no minimum or maximum range and maximize power on a target because it is always broadcasting. However, it also has the disadvantage of only detecting moving targets, as stationary targets (along the line of sight) will not cause a Doppler shift and the reflected signals will be filtered out. CW radar systems are used at both ends of the range spectrum, as radio-altimeters at the close-range end (where the range may be a few feet), and early warning radars at long range.