A radar speed gun (also radar gun and speed trap gun) is a device used to measure the speed of moving objects. It is used in law-enforcement to measure the speed of moving
vehicle
A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), wa ...
s and is often used in professional spectator sport, for things such as the measurement of bowling speeds in cricket, speed of pitched
baseballs
A baseball is a ball used in the sport of the same name. The ball consists of a rubber or cork center wrapped in yarn and covered with white natural horsehide or cowhide, or a synthetic composite leather. A regulation baseball is 9 to 9¼ inche ...
, and speed of
tennis
Tennis is a racket sport that is played either individually against a single opponent ( singles) or between two teams of two players each ( doubles). Each player uses a tennis racket that is strung with cord to strike a hollow rubber ball ...
serves.
A radar speed gun is a
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
unit that may be hand-held, vehicle-mounted or static. It measures the speed of the objects at which it is pointed by detecting a change in frequency of the returned radar signal caused by the
Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
, whereby the frequency of the returned signal is increased in proportion to the object's speed of approach if the object is approaching, and lowered if the object is receding. Such devices are frequently used for
speed limit enforcement
Speed limits are enforced on most public roadways by authorities, with the purpose to improve driver compliance with speed limits. Methods used include roadside speed traps set up and operated by the police and automated roadside 'speed camera' ...
, although more modern
LIDAR speed gun
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
instruments, which use pulsed
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
light instead of radar, began to replace radar guns during the first decade of the twenty-first century, because of limitations associated with small radar systems.
History
The radar speed gun was invented by John L. Barker Sr., and Ben Midlock, who developed radar for the military while working for the Automatic Signal Company (later Automatic Signal Division of LFE Corporation) in Norwalk, CT during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
. Originally, Automatic Signal was approached by
Grumman Aircraft Corporation to solve the specific problem of terrestrial landing gear damage on the now-legendary
PBY Catalina
The Consolidated PBY Catalina is a flying boat and amphibious aircraft that was produced in the 1930s and 1940s. In Canadian service it was known as the Canso. It was one of the most widely used seaplanes of World War II. Catalinas served wit ...
amphibious aircraft. Barker and Midlock cobbled a Doppler radar unit from coffee cans soldered shut to make microwave resonators. The unit was installed at the end of the runway (at Grumman's Bethpage, NY facility), and aimed directly upward to measure the sink rate of landing PBYs. After the war, Barker and Midlock tested radar on the
Merritt Parkway
The Merritt Parkway (also known locally as "The Merritt") is a limited-access parkway in Fairfield County, Connecticut, Fairfield County, Connecticut, with a small section at the northern end in New Haven County, Connecticut, New Haven County. ...
. In
1947, the system was tested by the
Connecticut State Police
The Connecticut State Police (CSP) is a division of the Connecticut Department of Emergency Services and Public Protection responsible for traffic regulation and law enforcement across the state of Connecticut, especially in areas not served by ...
in
Glastonbury, Connecticut
Glastonbury ( ) is a town in Hartford County, Connecticut, United States, formally founded in 1693 and first settled in 1636. It was named after Glastonbury in Somerset, England. Glastonbury is on the banks of the Connecticut River, southeast o ...
, initially for traffic surveys and issuing warnings to drivers for excessive speed. Starting in February 1949, the state police began to issue speeding tickets based on the speed recorded by the radar device. In 1948, radar was also used in
Garden City, New York
Garden City is a village located on Long Island in Nassau County New York. It is the Greater Garden City area's anchor community. The population was 23,272 at the 2020 census.
The Incorporated Village of Garden City is primarily located within ...
.
Mechanism
Doppler effect
Speed guns use
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
to perform speed measurements.
Radar speed guns, like other types of radar, consist of a
radio transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
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
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
, if the object is moving toward or away from the gun, the
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of the reflected radio waves when they come back is different from the transmitted waves. When the object is approaching the radar, the frequency of the return waves is higher than the transmitted waves; when the object is moving away, the frequency is lower. From that difference, the radar speed gun can calculate the speed of the object from which the waves have been bounced. This speed is given by the following equation:
:
where
is the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
,
is the emitted frequency of the radio waves, and
is the difference in frequency between the radio waves that are emitted and those received back by the gun. This equation holds precisely only when object speeds are low compared to that of light, but in everyday situations, this is the case and the velocity of an object is directly proportional to this difference in frequency.
Stationary radar
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 two different
musical note
In music, a note is the representation of a musical sound.
Notes can represent the Pitch (music), pitch and Duration (music), duration of a sound in musical notation. A note can also represent a pitch class.
Notes are the building blocks of much ...
s played together create a
beat note
In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies.
With tuning instruments that can produce ...
at the difference in frequency between them, so when these two radio signals are mixed they create a "beat" signal (called a
heterodyne
A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is us ...
). An electrical circuit then measures this frequency using a
digital counter
In digital logic and computing, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock. The most common type is a sequential digital logic cir ...
to count the number of cycles in a fixed time period, and displays the number on a
digital display
A display device is an output device for presentation of information in visual or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information that is supplied has an electrical signal t ...
as the object's speed.
Since this type of speed gun measures the difference in speed between a target and the gun itself, the gun must be stationary in order to give a correct reading. If a measurement is made from a moving car, it will give the difference in speed between the two vehicles, not the speed of the target relative to the road, so a different system has been designed to work from moving vehicles.
Moving radar
In so-called "moving radar", the radar antenna receives reflected signals from both the target vehicle and stationary background objects such as the road surface, nearby road signs, guard rails and streetlight poles. Instead of comparing the frequency of the signal reflected from the target with the transmitted signal, it compares the target signal with this background signal. The frequency difference between these two signals gives the true speed of the target vehicle.
Design considerations
Modern radar speed guns normally operate at
X,
K,
Ka, and (in Europe)
Ku bands.
Radar guns that operate using the
X band
The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxim ...
(8 to 12 GHz) frequency range are becoming less common because they produce a strong and easily detectable beam. Also, most automatic doors utilize radio waves in the X band range 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.
Some motorists install
radar detector
A radar detector is an electronic device used by motorists to detect if their speed is being monitored by police or law enforcement using a radar gun. Most radar detectors are used so the driver can reduce the car's speed before being ticketed ...
s which can alert them to the presence of a
speed trap
Speed limits are enforced on most public roadways by authorities, with the purpose to improve driver compliance with speed limits. Methods used include roadside speed traps set up and operated by the police and automated roadside 'speed camera' ...
ahead, and the microwave signals from radar may also change the quality of reception of AM and FM radio signals when tuned to a weak station. For these reasons, hand-held radar typically includes an on-off trigger and the radar is only turned on when the operator is about to make a measurement. Radar detectors are illegal in some areas.
Limitations
Traffic radar comes in many models. Hand-held units are mostly battery powered, and for the most part are used as stationary speed enforcement tools. Stationary radar can be mounted in police vehicles and may have one or two antennae. Moving radar is employed, as the name implies, when a police vehicle is in motion and can be very sophisticated, able to track vehicles approaching and receding, both in front of and behind the patrol vehicle and also able to track multiple targets at once. It can also track the fastest vehicle in the selected radar beam, front or rear.
However, there are a number of limitations to the use of radar speed guns. For example, user training and certification are required so that a radar operator can use the equipment effectively, with trainees being required to consistently visually estimate vehicle speed within +/-2 mph of actual target speed, for example if the target's actual speed is 30 mph then the operator must be able to consistently visually estimate the target speed as falling between 28 and 32 mph. Stationary traffic enforcement radar must occupy a location above or to the side of the road, so the user must understand
trigonometry
Trigonometry () is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. T ...
to accurately estimate vehicle speed as the direction changes while a single vehicle moves within the field of view. Actual vehicle speed and radar measurement thus are rarely the same due to what is known as the cosine effect, however, for all practical purposes this difference in actual speed and measured speed is inconsequential, generally being less than 1 mph difference, as police are trained to position the radar to minimize this inaccuracy and when present the error is always in the favor of the driver reporting a lower than actual speed. Additionally, the placement of the radar can be important as well to avoid large reflective surfaces near the radar. Such reflective surfaces can create a multi-path scenario where the radar beam can be reflected off of the unintended reflective target and find another target and return back thereby causing a reading that can be confused for the traffic being monitored.
Radar speed guns do not differentiate between targets in traffic, and proper operator training is essential for accurate speed enforcement. This inability to differentiate among targets in the radar's field of view is the primary reason for the operator being required to consistently and accurately visually estimate target speeds to within +/-2 mph, so that, for example if there are seven targets in the radar's field of view and the operator is able to visually estimate the speed of six of those targets as approximately 40 mph and visually estimate the speed of one of those targets as approximately 55 mph and the radar unit is displaying a reading of 56 mph it becomes clear which target's speed the unit is measuring.
In moving radar operation, another potential limitation occurs when the radar's patrol speed locks onto other moving targets rather than the actual ground speed. This can occur if the position of the radar is too close to a larger reflective target such as a tractor trailer. To help alleviate this the use of secondary speed inputs from the vehicle's CAN bus, VSS signal, or the use of a GPS-measured speed can help to reduce errors by giving a secondary speed to compare the measured speed against.
Size
The primary limitation of hand held and mobile radar devices is size. An antenna diameter of less than several feet limits directionality, which can only partly be compensated for by increasing the frequency of the wave. Size limitations can cause hand-held and mobile radar devices to produce measurements from multiple objects within the field of view of the user.
The antenna on some of the most common hand-held devices is only in diameter. The beam of energy produced by an antenna of this size using X-band frequencies occupies a cone that extends about 22 degrees surrounding the line of sight, 44 degrees in total width. This beam is called the
main lobe
In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength.
The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
. There is also a
side lobe
In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''.
The radiation pattern of most antennas shows a pattern of "''lobes'' ...
extending from 22 to 66 degrees away from the line of sight, and other lobes as well, but side lobes are about 20 times (13
dB) less sensitive than the main lobe, although they will detect moving objects close by. The primary field of view is about 130 degrees wide. K-band reduces this field of view to about 65 degrees by increasing the frequency of the wave. Ka-band reduces this further to about 40 degrees. Side lobe detections can be eliminated using
side lobe blanking which narrows the field of view, but the additional antennas and complex circuitry impose size and price constraints that limit this to applications for the military, air traffic control, and weather agencies.
Mobile weather radar is mounted on
semi-trailer truck
A semi-trailer truck, also known as a semitruck, (or semi, eighteen-wheeler, big rig, tractor-trailer or, by synecdoche, a semitrailer) is the combination of a tractor unit and one or more semi-trailers to carry freight. A semi-trailer ...
s in order to narrow the beam.
Distance
A second limitation for hand-held devices is that they have to use
continuous-wave radar
Continuous-wave radar (CW radar) is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Individual objects can be detected using the Doppler effect, which ...
to make them light enough to be mobile. Speed measurements are only reliable when the distance at which a specific measurement has been recorded is known. Distance measurements require pulsed operation or cameras when more than one moving object is within the field of view. Continuous-wave radar may be aimed directly at a vehicle 100 yards away but produce a speed measurement from a second vehicle 1 mile away when pointed down a straight roadway. Once again falling back on the training and certification requirement for consistent and accurate visual estimation so that operators can be certain which object's speed the device has measured without distance information, which is unavailable with continuous wave radar.
Some sophisticated devices may produce different speed measurements from multiple objects within the field of view. This is used to allow the speed-gun to be used from a moving vehicle, where a moving and a stationary object must be targeted simultaneously, and some of the most sophisticated units are capable of displaying up to four separate target speeds while operating in moving mode once again emphasizing the importance of the operators' ability to consistently and accurately visually estimate speed.
Environment
The environment and locality in which a measurement is taken can also play a role. Using a hand-held radar to scan traffic on an empty road while standing in the shade of a large tree, for example, might risk detecting the motion of the leaves and branches if the wind is blowing hard (side lobe detection). There may be an unnoticed airplane overhead, particularly if there is an airport nearby, which again emphasizes the importance of proper operator training.
Associated cameras
Conventional radar gun limitations can be corrected with a camera aimed along the line of sight.
Cameras are associated with
automated ticketing machines (known in the
UK as
speed cameras
A traffic enforcement camera (also red light camera, speed camera, road safety camera, road rule camera, photo radar, photo enforcement, Gatso, safety camera, bus lane camera, flash for cash, Safe-T-Cam, No contact apprehension camera dependin ...
) where the radar is used to trigger a camera. The radar speed threshold is set at or above the maximum legal vehicle speed. The radar triggers the camera to take several pictures when a nearby object exceeds this speed. Two pictures are required to determine vehicle speed using roadway survey markings. This can be reliable for traffic in city environments when multiple moving objects are within the field of view. It is the camera, however, and its timing information, in this case, that determines the speed of an individual vehicle, the radar gun simply alerting the camera to start recording.
Newer instruments
Laser devices, such as a
LIDAR speed gun
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
, are capable of producing reliable range and speed measurements in typical urban and suburban traffic environments without the site survey limitation and cameras. This is reliable in city traffic because LIDAR has directionality similar to a typical firearm because the beam is shaped more like a pencil that produces measurement only from the object it has been aimed at.
In media
MythBusters
''MythBusters'' is a science entertainment television program, developed by Peter Rees and produced by Australia's Beyond Television Productions. The series premiered on the Discovery Channel on January 23, 2003. It was broadcast internatio ...
did an episode on trying to get the gun to have incorrect readings by changing the surface of the passing object.
Use in Cricket
Speed gun is regularly used in
Cricket
Cricket is a bat-and-ball game played between two teams of eleven players on a field at the centre of which is a pitch with a wicket at each end, each comprising two bails balanced on three stumps. The batting side scores runs by striki ...
, to determine the speed of a
bowl
A bowl is a typically round dish or container generally used for preparing, serving, or consuming food. The interior of a bowl is characteristically shaped like a spherical cap, with the edges and the bottom forming a seamless curve. This makes ...
.
See also
*
LIDAR detector
*
Radar detector
A radar detector is an electronic device used by motorists to detect if their speed is being monitored by police or law enforcement using a radar gun. Most radar detectors are used so the driver can reduce the car's speed before being ticketed ...
References
External links
Hyperphysics - technical info and calculators for police RADARCalibration of Speed Enforcement Down-The-Road RadarsBaseball Radar Guns
{{DEFAULTSORT:Radar Gun
American inventions
Traffic enforcement systems
Measuring instruments
Radar
Traffic law