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Continuous-wave RadarContinuous-wave radar system is a radar system where a continuous-wave is transmitted by one antenna and a second receives the radio energy reflected from an object. A very pure signal of a known frequency is transmitted by one antenna. Return signals received by the second atenna from targets are shifted away from this base frequency via the Doppler effect. The main advantage of the CW radars is that they have no pulsing, and thus no minimum or maximum ranges (although the broadcast strength imposes a practical limit on the latter) as well as maximizing power on the target. However they also have the disadvantage of only being able to detect moving targets, as motionless ones (along the line of sight) will not cause a Doppler shift and the signal from such a target will be filtered out. Military aircraft often attempt to avoid detection by CW radar by 'orbiting' around the transmitter. CW radar systems thus find themselves being used at either end of the range spectrum, as radio-altimeters at the close-range end (where the range may be a few feet) and long distance early-warning radars at the other. CW radars have the disadvantage that they cannot measure distance, because there are no pulses to time. In order to correct for this problem, frequency shifting methods can be used. When a reflection is received the frequencies can be examined, and by knowing when in the past that particular frequency was sent out, you can do a range calculation similar to using a pulse. It is generally not easy to make a broadcaster that can send out random frequencies cleanly, so instead these frequency-modulated continuous wave radars (FMCW), use a smoothly varying "ramp" of frequencies up and down. For this reason they are also known as a chirped radar. The military uses continuous-wave radar to guide semi-active radar homing (SARH) air-to-air missiles, such as the US AIM-7 Sparrow. The launch aircraft illuminates the target with a CW radar signal, and the missile homes in on the reflected radar waves. Most modern air combat radars, even pulse Doppler sets, have a CW function for missile guidance purposes. The disadvantage of CW radar and SARH weapons is that the launch aircraft must continue to point its radar (and thus its nose) at the target for the entire duration of the missile's flight, leaving the attacker vulnerable to a counterattack. In addition, most mechanically steered radar sets can attack only one target at a time, and cannot search for other targets (or imminent attacks) while guiding a SARH missile.
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