radar gun

U.S. Army soldier uses a radar gun to catch speeding violators at Tallil Air Base, Iraq.

Microdigicam radar in use in Brazil

A radar gun or speed gun is a small Doppler radar unit used to detect the speed of objects, especially trucks and automobiles for the purpose of regulating traffic, as well as pitched baseballs, runners or other moving objects in sports. A radar gun does not return information regarding the object's position. 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.

Most of today's radar guns operate at X, K, Ka, IR Band (infrared), and (in Europe) Ku bands. An alternative technology, LIDAR, uses pulsed laser light.

The radar gun was invented by Bryce K. Brown of Decatur Electronics in March 1954^[1], and was first used in Chicago, Illinois by Patrolman 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 detectors.

How radar guns work

Radar guns are, in their most simple form, radio transmitters and receivers. They send out a radio signal, then receive the same signal back as it bounces off the objects. However, the radar frequency is different when it comes back, and from that difference the radar gun can calculate object speed.

A radar beam is similar to a beam of light in that 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. Unless the radar system has a provision for converting own-vehicle-speed to an appropriate receiver frequency offset then the radar must be stationary to measure speed.

All bands of radar operate on different frequencies, work differently, and are very complicated. 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 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.

Problems with radar guns

Although radar is an effective tool to acquire an object's speed, there are problems with it. Today's technology is relatively effective and error free so most inaccurate speed measurements are caused by the user.

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 is known as the Cosine Effect. 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.^{[dubious – discuss][citation needed]} This is not taken into account when a photo RADAR is used and numerous discrepancies can creep in.

Radar works by emitting invisible electromagnetic waves at a certain frequency, rather than visible light. In shining a light beam from a flashlight over an object, the light is reflected by objects, allowing a person to see the object in the dark. The same principle applies to radar beams. Radar emits microwaves, which will reflect off most metallic objects, concrete, trees, wood etc. It will however go through grass, bushes etc, just like light will go though glass and transparent objects.

The radar unit uses an antenna to "read" the microwave that is reflected back from an object (such as a car). If the microwave from the radar hits a moving target, the waves' frequency will change based on the "doppler shift" principle. With the reflected wave's changed frequency, and the known frequency that the radar emits, the computer inside the radar unit will be able to calculate the target speed. Note that radar cannot determine the target speed when the target is traveling perpendicular to the radar beam, since there will be no doppler shift relative to the radar antenna.

However, there are many shortcomings to traffic radar. First of all, radar is prone to interference. The radar's antenna will accept any microwave that it is able to "hear". The antenna might not be "hearing" the waves which originated from its own unit. High voltage power lines, telephone lines, power stations, even neon lights emit electromagnetic waves. The specific source of the wave cannot be identified. Secondly, radar cannot determine which target reflected off the waves. If there are two cars both travelling in a close proximity, either car could be responsible for the resulting radar reading. Identifying the target is a job left to the radar operator, who frequently makes mistakes. At a distance of 200m, the width of the radar beam can usually cover all 4 lanes of traffic travelling in both direction, causing a large error in determining which car is responsible for the radar reading. Even if there is only one vehicle on the road, radar has a typical range of approximately 400m (1200 feet), which is beyond the visual range of most human eyes. The radar might be clocking a speeder at a distance of 300m, but the officer can only see an innocent driver coming towards him at a distance of 100m.

Furthermore, radar does not necessarily pick up the fastest moving object.^{[citation needed]}The strongest return signal can be affected by a number of different factors, such as the target's size, the cross-sectional area facing the radar antenna, its relative speed and the distance from the radar unit. For example, if a Corvette is travelling at 80 km/h 50m from the radar, and a truck is speeding 110 km/h at 2000m away, the radar may pick up the return signal from the truck and register a speed of 110 km/h. The officer however might think that the Corvette is the one responsible for the violation and therefore issues an undeserved ticket. In addition, when traffic radar is used near an international airport, the radar will pick up the return signals from airborne objects. In this case, extremely fast speeds such as 300–400 km/h will be registered on the radar. Even if an aircraft is flying at a few km away, the radar will still be capable to pick up the return signal since an aircraft is much greater in size and has much faster speed than any ground vehicle.^{[citation needed]}

There are radar units which can identify multiple targets, determine their speeds, range, shape, altitude, direction of travel etc., but these radar units cost millions of dollars and are properties of the military and airports. Traffic radar units have to be cheap, small, and thus error-prone.^{[citation needed]}

There is a variation of traffic radar, which is called moving radar. It works in essentially the same way as the type described above (stationary radar), with the exception that moving radar is used when the police car is moving (for the purpose of highway patrol, for example). There are two readouts on the radar unit, one shows the target speed, one shows the police car speed. The strongest return signal, usually reflected from highway signs, bridges or other objects, is assumed to be the police car speed. The next strongest signal, is assumed to be from the target. Since the target speed will only be relative to the police car speed, therefore by adding the police car speed to the relative target speed, one will get the absolute speed of the target. Moving radar is subject to all the errors stationary radar has, plus the error of determining the speed of the police car.

There is another type of traffic radar, which is photo radar. Photo radar is used in many other provinces and in the States. Strictly speaking, photo radar is not a variation of traditional radar. It is basically a stationary radar with the capability of taking photos. Photo radar is never used in moving mode. A police officer often sets up the photo radar on the shoulder of a highway, pointing the radar towards oncoming traffic. The radar unit is often hidden in a vehicle like a minivan. A camera is mounted on top of the vehicle and connected to the radar unit and will take readings without the control of the police officer. The radar will monitor traffic speed continuously, if a violation speed above the threshold speed is detected, the camera will automatically take a picture of the violating vehicle's license plate. At the end of the day, all the pictures will be developed, and a ticket will be mailed to the respective registered owners of the vehicles.

Radar is not infallible as most people and courts believe. Although the technology has advanced a lot during the years, and the radar units have become more and more user friendly, radar still makes mistakes especially at the hands of a poorly trained operator.

Information is still required about the amount of radio-frequency power radiated by radar speed guns, in order to estimate the personal radiation-hazard effects associated with these devices.

A father and son team, at high expense, beat the traffic radar in New South Wales, Australia, using expert evidence and GPS to do so.^[2]

See also

• Radar detector
• Radar detector detector

Notes

This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (January 2009)

External links

• Hyperphysics - technical info and calculators for police RADAR
• Radar Guns approved in the USA and currently in production