A radar gun or speed gun is a small Doppler radar used to detect the speed of objects. A radar gun does not return information regarding the object's position or any information concerning the car e.g. Registration etc. It relies on the Doppler Effect applied to a radar beam to measure the speed of objects at which it is pointed.
Radar guns may be hand-held or vehicle-mounted. They can be used as a tool in the regulation of traffic speed by law enforcement and also to measure speeds in sports.
The radar gun was invented by Bryce K. Brown of Decatur Electronics in March 1954 , and was first used in Chicago, Illinois by Patrollman Leonard Baldy in April 1954.
There are radar detectors on the market which can detect most police radar and laser systems. Conversely, in the spirit of electronic warfare, some police radars are equipped with detectors of operating radar detectors.
A radar beam is similar to the beam of a flashlight as it spreads out as the distance from the signal origin increases. The signal then bounces off objects in the path of the beam and are reflected back to the gun. The gun uses the doppler effect to calculate the speed of the object in the beam's path. Using a comparison of frequency shift between received images instead of the frequency shift between sent and received frequencies creates what is known as moving radar, the radar must be stationary to measure speed.
All bands of radar work the same way; they simply operate on different frequencies. X band 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 and Ka band are most commonly used by police agencies.
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 antennas. 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.
For speed to be accurately calculated, the object whose speed is desired ideally should be the only moving object in the beam of the radar. If this is not the case, as in a police officer monitoring speed on a stretch of busy road, more articulation is needed in testifying to a particular vehicle's speed.
The National Highway Traffic Safety Administration (NHTSA), in cooperation with the International Association of Chiefs of Police (IACP) has developed a training program for the operation of police traffic radar. This curriculum provides radar instrument operators with the skills required to properly set up, test, and operate to identify targets. It also provides the operators with information regarding state and local laws and ordinances which apply to the user's jurisdiction. This training, while strongly recommended, is not mandatory in some states. Other states require the successful completion of this training and issue a certificate of completion.
Police officers are also trained in the practice of visual estimation of speed and distance, and the radar is thus used as a verification of the visual estimation. In addition, it is easy enough to testify that the vehicle in the offside lane was passing vehicles in the nearside lane, and is therefore the vehicle whose speed is showing on the readout as traveling above the posted limit.
The angle in which the object is in relation to the radar source can also affect the reading. This phenomenon includes angles on a horizontal and vertical plane and is known as the Cosine Effect. It is suggested that interference in the RADAR band by cellular phones, transmitters of other kinds, power wires, high tension wires, signs and even stationary walls can create erroneous readings. This is not taken into account when a photo RADAR is used and numerous discrepancies can creep in.