A radar speed gun (also radar gun and speed gun) is a small doppler radar unit used to measure the speed of moving objects, including vehicles, pitched baseballs, runners and other moving objects. Radar speed guns may be hand-held, vehicle-mounted or static. A radar speed gun measures the speed of objects at which it is pointed by detecting a change in frequency of the returned radar signal due to the Doppler effect. Modern radar speed guns normally operate at X, K, Ka, IR Band (infrared), and (in Europe) Ku bands.
They are frequently used for speed limit enforcement although more modern LIDAR speed gun devices which use pulsed laser light instead of radar are more commonly used now. Some motorists install radar detectors which can detect signals from these devices to avoid being caught.
The radar speed gun was invented by Bryce K. Brown of Decatur Electronics in March 1954,[1] and was first used in Chicago, Illinois in April 1954. Patrolman Leonard Baldy was the first officer to issue a speeding ticket using the new device.
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Radar speed guns, like other types of radar, consist of a radio transmitter and receiver. They send out a radio signal in a narrow beam, then receive the same signal back after it bounces off the target object. Due to a phenomenon called the Doppler effect, if the object is moving toward or away from the gun, the frequency of the reflected radio waves when they come back is different from the transmitted waves, and from that difference the radar speed gun can calculate the object's speed.
The target object's speed is proportional to the difference in frequency between the outgoing and the reflected radio waves. After the returning waves are received, a signal with a frequency equal to this difference is created by mixing the received radio signal with a little of the transmitted signal. Just as when two different musical notes are played together they create a "beat note" at the difference in frequency between them, when the two radio signals are mixed they create a "beat" signal (called a heterodyne) at the difference in frequency between the outgoing and reflected waves. The circuit then converts this frequency to a number by counting the number of cycles of the signal in a fixed time interval using a digital counter, and displays the number on a digital display as the object's speed.
It is important that the radio waves leave the gun in a narrow beam that doesn't spread out much, so that the gun will get a return only from the vehicle it is aimed at, with no chance of receiving a false return from nearby objects or vehicles. To create a narrow beam with an antenna small enough to fit in a handheld gun, radar speed guns use high frequency radio waves in the microwave range. X band (8 to 12 GHz) guns are becoming less common due to the fact the beam is strong and easily detectable. Also, most automatic doors utilize radio waves on X band and can possibly affect the readings of police radar. As a result K band (18 to 27 GHz) and Ka band (27 to 40 GHz) are most commonly used by police agencies.
The above-described system measures the difference in speed between the target and the radar speed gun itself. The gun must be stationary to give a correct reading; if the gun is used from a moving car it just gives the difference in speed between the two vehicles. So a different system is used in radar speed guns designed to work from moving vehicles. In so-called "moving radar", the gun receives reflected signals from both the target vehicle and stationary background objects, such as the road, road signs, guard rails, streetlight poles, etc. Instead of comparing the frequency of the signal reflected from the target with the transmitted signal, it compares the target signal with the background signal. The difference in frequency of these two signals gives the true speed of the target vehicle.
Traffic radar comes in many models. There are hand held, stationary and moving radar instruments. Hand held units are mostly battery powered, and for the most part are used as stationary speed enforcement tools. Stationary radar is mounted in police vehicles, and may have one or two antennae. These are employed when the vehicle is parked. Moving radar is employed, as the name implies, when the police vehicle is in motion. These devices are very sophisticated, able to track vehicles approaching and receding both in front of and behind the patrol vehicle. They can also track the fastest vehicle in the selected radar beam, front or rear.