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Another form of distance measuring radar is based on frequency modulation. Frequency comparison between two signals is considerably more accurate, even with older electronics, than timing the signal. By changing the frequency of the returned signal and comparing that with the original, the difference can be easily measured.
This technique can be used in radar systems, and is often found in aircraft radar altimeters. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared.
Since the signal frequency is changing, by the time the signal returns to the aircraft the broadcast has shifted to some other frequency. The amount of that shift is greater over longer times, so greater frequency differences mean a longer distance, the exact amount being the "ramp speed" selected by the electronics. The amount of shift is therefore directly related to the distance travelled, and can be displayed on an instrument. This signal processing is similar to that used in speed detecting doppler radar. See also the section on Continuous Wave radar below.
Each radar uses a particular type of signal. Long range radars tend to use long pulses with long delays between them, and short range radars use smaller pulses with less time between them. This pattern of pulses and pauses is known as the Pulse Repetition Frequency (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars now can change their PRF.
However there is another effect that can be used to make much more accurate speed measurements, and do so almost instantly (no memory required), known as the Doppler effect. The Doppler effect is the change in frequency of any signal due to the finite speed at which the signal travels compared to the motion of the object. For instance, sound travels at the fairly slow speed of around 300 m/s, which is why you hear the Doppler effect of an ambulance siren as it passes you at 3 m/s or so. Although this results in a small 1% change in frequency, the human ear is very good at detecting this change.
In the case of radar the speed of light is much faster than sound and thus the resulting shift much smaller. However modern electronics are even better at detecting this change than the human ear is for sound. Speeds as slow as a few centimeters per second can be easily measured, an accuracy typically much better than for the measurement of distance. Practically every modern radar system uses this principle, and is generally referred to as Pulse Doppler Radar.
The major use of Doppler is to separate moving objects from clutter. It's common for Doppler radars to have a frequency range adjust control to reject low speeds. Another form color-code s returns by their speed.
Doppler measures the speed only along the direction from the reflection to the radar antenna. In order to measure the object's true speed and direction, the radar set or operator had to remember a return's location. Military organizations traditionally used a manual plotting board for this purpose. Computers in the radar systems have made this even more convenient.
It is possible to make a radar without any pulsing, known as a continuous-wave (CW) radar, by sending out a very pure signal of a known frequency. Return signals from targets are shifted away from this base frequency via the Doppler effect, so they can be picked up at another antenna even if it is physically close to the broadcaster.
The main advantage of the CW radars is that they have no pulsing, and thus no minimum or maximum range s (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. Such 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 radar s (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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