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Radio Detection and Ranging (RADAR) Speed detection instruments send out a pulse of radio waves which is then reflected back by the target. Radar speed detection is dependent on the fact that moving objects will echo the radio waves back at a different frequency than stationary objects. This phenomenon is known as the doppler shift..
Part of the signal sent out by the radar antenna hits the ground in front of the patrol car, this signal is returned and interpreted as the patrol cars speed.
The rest of the signal goes on towards the target. Because of the nature of the emission, a broad area becomes the target. This results in the possibility of many objects returning a signal to the radar antenna. The antenna then selects the largest signal return and interprets this data so that a speed is shown on the instrument panel.
For convergence of the two vehicles, the radar receives a signal with a high frequency such as 120MPH, the instrument subtracts the patrol car speed ie. 55 MPH and returns the second vehicles speed as the difference.
For stationary patrol cars targeting approaching cars, the instrument is told that the patrol cars speed is 0 MPH and just determines the speed of the target vehicle, as shown below:
Vs = (1-(lf/f'))c
Where vs= speed of target, c =speed of light, l=wavelength, f'=returned frequency, and f= original frequency.
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