Continuous-wave radar

(engineering) A radar system in which a transmitter sends out a continuous flow of radio energy; the target reradiates a small fraction of this energy to a separate receiving antenna. Also known as continuous-wave Doppler radar.

A radar in which the transmitter output is uninterrupted, in contrast to pulse radar, where the output consists of short pulses. Among the advantages of continuous-wave (CW) radar is its ability to measure velocity with extreme accuracy by means of the Doppler shift in the frequency of the echo. The detected, reflected wave is shifted in frequency by an amount which is a function of the relative velocity between the target and the transmitter-receiver. Range data are extracted from the change in Doppler frequency with time. See also Doppler radar.

In order to measure the range of targets, some form of frequency modulation (FM) of the continuous-wave output must be used. In one very effective form of modulation, the carrier frequency of the transmitted signal is varied at a uniform rate. Range is determined by comparing the frequency of the echo with that of the transmitter, the difference being proportional to the range of the target that produced the echo. Systems in which this is done are known as FM-CW radars. See also Frequency modulation.

A modified form of FM-CW radar employs long, but not continuous, transmission. This might be regarded as the same as transmitting extremely long pulses on an FM carrier. Systems of this type are referred to as pulse compression radars.

Design objectives for continuous-wave radar include: protecting the receiver from the transmitter output and close-by return echoes when a single antenna is used; resolving side-lobe ambiguity; resolving range ambiguity; distinguishing between approaching and receding targets; eliminating noise and clutter; simultaneously measuring both target range and velocity; determining the shapes of targets; handling multiple targets; measuring target acceleration; communicating with targets; and increasing the received signal-to-noise ratio.

One disadvantage of continuous-wave radar is that when a single antenna is used for both reception and transmission it is difficult to protect the receiver against the transmitter because, in contrast to pulse radar, both are on all the time. Use of isolation circuitry gives the receiver protection from a transmitter output up to 200 W. Use of magnetically biased yttrium-iron-garnet (YIG) provides three tuned, tandem-connected power-limiter stages in one X-band FM-CW radar. See also Ferrimagnetic garnets.

Applications of continuous-wave radar include missile guidance; detection of hostile targets; terrain clearance indication and ground surveillance; laser radar systems; atmospheric studies; automobile safety; surveillance of personnel; ice studies; remote sensing; and reproduction of the shape of a patient's pulse.

The principal use of continuous-wave radar is in short-range missile guidance. Typically the missile's course is tracked from the ground while the missile is simultaneously illuminated. Continuous-wave radar, in some cases the same radar, can be used for both tracking and illumination, although it is more common for pulse Doppler radar to be used for tracking.

A variant of continuous-wave radar is used to illuminate persons under surveillance by techniques employing semiconductor tracer diodes. These devices are secreted on or implanted in the subject's body without his or her knowledge, or else concealed in objects that are to be protected against theft. Despite the fact that pulsed X-band sources could provide the needed power levels more conveniently, the requirement to reduce clutter makes it desirable to use continuous-wave power.

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