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How do Radar Guns Work

How do Radar Guns Work

A radar gun is an equipment that is used to measure the speed of moving objects. These radio transmitters and receivers function on the principle of Doppler effect.
Narayani Karthik
A radar is an electronic device which is used to detect and locate moving objects with the help of electromagnetic radiations. The name itself means - Radio Detection and Ranging. The simplest example of where you can find a small radar unit, is police vehicles. Radar guns are an integral part of police equipment. Most of the police vehicles are well equipped with Decatur SpeedTrak Police Radar, which helps in tracking down speed of moving vehicles on road. So what are they? How they work? A radar gun is a small electronic unit which is extensively used in detecting the speed of moving objects, like automobiles. Its principle of functioning is based on Doppler effect.
Working Mechanism
Working of a radar gun solely depends on the Doppler effect principle, but then, what is Doppler effect? When you hear a high pitched siren of an approaching ambulance, you notice that the pitch drops suddenly, when the ambulance passes by you. This effect is known as Doppler effect. As per this principle, there is always a significant change in the frequency of the sound wave perceived by an observer, who is also moving relative to the source of the sound wave. Precisely, the velocities of the observer and the source, are relative to the medium through which the sound waves are being transmitted.
A radar gun emits radio waves of specific frequencies in a particular direction. These waves are bound to hit many moving objects on the way, which may include vehicles as well. When these waves return to the receiving station of the radar gun, a measurable frequency shift is observed in the radio waves received. This is where the Doppler effect springs into action. As per the basics of the principle, when a radar signal hits an object that is moving towards the direction of the radio wave, the returning frequency of the bounced back wave will be higher than that of the original wave.
Similarly, if the radio wave hits an object which is moving away from the direction of the source of the wave, the returning frequency of the returned radio wave will be lower than the original one. This frequency shift is often known as the Doppler shift. This is used for determining the speed of the target which the radio wave has hit. The calculation of the speed of the target (which may be a moving vehicle), is done by a computer. Remember, the change in frequency calculated in the Doppler shift is always directly proportional to the speed of the target (moving vehicle).
A radar gun receiver is sensitive enough to detect even a weak signal from 3/4th of a mile. The calculation happens quite fast, in about 1/5th of a second. However, when there are too many vehicles on the road, it will only detect the signals returning with the highest amplitude. The radars used by the traffic police is usually a low level electromagnetic radiation with a non ionizing radio frequency. However, when there are obstacles encountered by the electromagnetic beam, it is important for the radar operator to understand and regulate the effective range of the gun and adjust the beam.
There are three possible ways to control the radar beam to avoid any errors in target identification.
  • Reducing sensitivity control function
  • Setting the angle of antenna in stationary mode
  • Identifying environmental interferences (Hindrances like hills and curves are unavoidable, but they do reduce the effective range)
So the next time, when you are driving your vehicle a tad too fast and the traffic police catches up with you, don't blink! That's what radar guns do in a blink of a second - calculating the speed of a moving target, which could be your vehicle too!