Continuous-wave radar
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Continuous-wave radar system is a radar system where a known stable frequency 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 they are moving.
The main advantage of the CW radars is that they are not pulsed and simple to manufacture. They have no minimum or maximum range (although the broadcast power level imposes a practical limit on range) and maximize power on a target because they are always broadcasting. However they also have 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; e.g., as radio-altimeters at the close-range end (where the range may be a few feet), and early warning radars at long range.
CW radars also have a disadvantage because they cannot measure range. Range is normally measured by timing the delay between a pulse being sent and received, but as CW radars are always broadcasting, there is no delay to measure. Ranging can be implemented, however, through a technique known as "chirping", or frequency modulated continuous-wave radar. In this system the signal is not a continuous fixed frequency, but varies up and down over a fixed period of time. By comparing the frequency of the received signal to the one currently being sent, the difference in frequency can be accurately measured, and from that the time-of-flight can be calculated.
The military uses continuous-wave radar 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 a CW radar signal, 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.
