Radar

:This article is about the device. For the fictional character in M*A*S*H (movie) and M*A*S*H (TV series), see Corporal Walter (Radar) O'Reilly.

Distance measurement

Transit time

The easiest way to measure the range of an object is to broadcast a short pulse of radio signal, and then evaluate the time it takes for the reflection to return. The distance is one-half the product of round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal.

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Range = rac{c au}{2} where c is the speed of light in a vacuum, and au is the round trip time. For RADAR the speed of signal is the speed of light, making the round trip times very short for terrestrial ranging. For this reason accurate distance measurement was difficult until the introduction of high performance electronics, with older systems being accurate to perhaps a few percent.

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The receiver cannot detect the return while the signal is being sent out – there is no way to tell if the signal it hears is the original or the return. This means that a radar has a distinct minimum range, which is the length of the pulse multiplied by the speed of light, divided by two. In order to detect closer targets you have to use a shorter pulse length.

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A similar effect imposes a specific maximum range as well. If the return from the target comes in when the next pulse is being sent out, once again the receiver cannot tell the difference. In order to maximize range, one wants to use longer times between pulses, the inter-pulse time.

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These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast have less total energy, making the returns much smaller and the target harder to detect. You could offset this by using more pulses, but this would shorten the maximum range again. So 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.

Related Topics:
Energy - Signal - Pulse Repetition Frequency - PRF

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Frequency modulation

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.

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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.

Related Topics:
Altimeter - Sine wave

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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 the article on continuous wave radar for more information.

Related Topics:
Doppler - Continuous wave radar

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