19-06-2012, 05:20 PM
Radar gun
[attachment=24861]
How radar speed guns work
U.S. Army soldier uses a radar speed gun to catch speeding violators at Tallil Air Base, Iraq.
Speed guns use doppler radar to perform speed measurements.
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 v is proportional to the difference in frequency Δf between the outgoing and the reflected radio waves:
where f is the frequency of the outgoing radio waves, and c is the speed of light.
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.
'Moving' radar speed guns
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.
Limitations
User training and certification are required so that the radar operator can understand and explain limitations that influence judgement.[4]
Radar speed guns do not work reliably in traffic.
Significant vehicle separation is essential for proper operation when used for speed monitoring by law enforcement personnel.
[edit] Physical Limitations
Mobile or hand-held radar are only reliable in a sterile environment with one moving object in the field of view and no other moving objects nearby.
Mobile traffic enforcement radar must occupy a location above or to the side of the road, except when the roadway is occupied by only one vehicle. The user must understand trigonometry to "guess" vehicle speed as the direction changes while a single vehicle moves within the field of view when positioned adjacent to the roadway. Vehicle speed and radar measurement are rarely the same for this reason.
Size
The primary limitation of hand held and mobile radar is size. Antenna diameter less than several feet limits directionality, which can be improved with higher transmit frequency. This limitation is imposed by antenna aperture and radiation pattern determined by antenna geometry. Mobile weather radar is mounted on semi-trailer truck for this reason.
As an example, the antenna on some of the most common hand-held radar is 2 inches, while the wavelength at X band is about 1 inch. That kind of antenna is 2 wavelengths across, and the "beam" of RF energy produced by that antenna occupies a cone that extends about 22 degrees surrounding the line of site in the direction where the radar is pointed (44 degrees wide). This beam is called the main lobe. There is also a sidelobe extending from 22 to 66 degrees away from the line of sight, which surrounds the main beam like a donut. There are other sidelobes, including some that point backward behind the user. Sidelobes are about 20 times less sensitive than the main lobe (13dB), but sidelobes produce detection when the object in the sidelobe is close or large. The primary field of view is about 130 degrees wide, but the total field of view actually extends 360 degrees in all directions for large objects and nearby objects.
K-band reduces this field of view to about 65 degrees by increasing frequency. Ka-band reduces this to about 40 degrees. Sidelobe detections can be eliminated using sidelobe blanking which narrows the field of view, but the additional antennas and complex circuitry impost price constraints that limit this to military, air traffic control, and weather.
Compare this radar geometry with the anatomy of the human eye. We see accurately within a small region about 5 degrees wide. The fovea determines direction we are looking, which is called our visual line of sight. Accurate vision extends about 20 degrees. Our total field of view is about 100 degrees horizontally and 60 degrees vertically. The field of view for small hand-held and mobile radar devices may exceed the visual field of the user because of side-lobe detections.
Size limitations cause hand-held and mobile radar to produce measurements from multiple objects within the field of view of the user.
Distance
The second limitation is that hand-held devices are limited to continuous-wave radar to make them light enough to be mobile. Speed measurements are only reliable when evaluated at a specific distance, and distance measurements require pulsed operation or cameras when more than one moving object is within the field of view. Continuous-wave radar produces only a steady tone and not pulses. The frequency shift of this tone is used to measure speed. Continuous-wave radar may be pointed 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. Users cannot tell which object is being measured within the field of view without knowing the distance, which is impossible with continuous wave radar.
Some sophisticated devices may produce two different speed measurements from two objects within the field of view. This is used to allow the speed-gun to be used from a moving vehicle and not to discriminate between multiple vehicles within the field of view. Reliable operation cannot be achieved as more moving objects are added to the environment. Portable hand-held or vehicle-mounted radar can never produce a reliable measurements when 2 or more moving vehicles occupy the field of view if no distance measurement is produced by the radar.
[edit] Environment
Environmental influences also play a role. Using a hand-held radar to scan traffic on an empty road while occupying the shade of a large tree renders the hand-held radar sensitive to detecting the motion of the leaves if the wind is blowing hard (sidelobe detection). Airports cause a similar phenomenon.
Hand-held radar is only reliable on single vehicles when the location has been certified to be free of environmental influences that will cause false readings. Site survey must be repeated periodically for reliable operation.[5]
Corrections
Conventional radar gun limitations are corrected with a camera aimed along the line of site.
Cameras are associated with automated ticketing machines, 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.
• Automated ticketing machines
Laser devices, like LIDAR, 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 where it has been aimed.
[attachment=24861]
How radar speed guns work
U.S. Army soldier uses a radar speed gun to catch speeding violators at Tallil Air Base, Iraq.
Speed guns use doppler radar to perform speed measurements.
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 v is proportional to the difference in frequency Δf between the outgoing and the reflected radio waves:
where f is the frequency of the outgoing radio waves, and c is the speed of light.
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.
'Moving' radar speed guns
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.
Limitations
User training and certification are required so that the radar operator can understand and explain limitations that influence judgement.[4]
Radar speed guns do not work reliably in traffic.
Significant vehicle separation is essential for proper operation when used for speed monitoring by law enforcement personnel.
[edit] Physical Limitations
Mobile or hand-held radar are only reliable in a sterile environment with one moving object in the field of view and no other moving objects nearby.
Mobile traffic enforcement radar must occupy a location above or to the side of the road, except when the roadway is occupied by only one vehicle. The user must understand trigonometry to "guess" vehicle speed as the direction changes while a single vehicle moves within the field of view when positioned adjacent to the roadway. Vehicle speed and radar measurement are rarely the same for this reason.
Size
The primary limitation of hand held and mobile radar is size. Antenna diameter less than several feet limits directionality, which can be improved with higher transmit frequency. This limitation is imposed by antenna aperture and radiation pattern determined by antenna geometry. Mobile weather radar is mounted on semi-trailer truck for this reason.
As an example, the antenna on some of the most common hand-held radar is 2 inches, while the wavelength at X band is about 1 inch. That kind of antenna is 2 wavelengths across, and the "beam" of RF energy produced by that antenna occupies a cone that extends about 22 degrees surrounding the line of site in the direction where the radar is pointed (44 degrees wide). This beam is called the main lobe. There is also a sidelobe extending from 22 to 66 degrees away from the line of sight, which surrounds the main beam like a donut. There are other sidelobes, including some that point backward behind the user. Sidelobes are about 20 times less sensitive than the main lobe (13dB), but sidelobes produce detection when the object in the sidelobe is close or large. The primary field of view is about 130 degrees wide, but the total field of view actually extends 360 degrees in all directions for large objects and nearby objects.
K-band reduces this field of view to about 65 degrees by increasing frequency. Ka-band reduces this to about 40 degrees. Sidelobe detections can be eliminated using sidelobe blanking which narrows the field of view, but the additional antennas and complex circuitry impost price constraints that limit this to military, air traffic control, and weather.
Compare this radar geometry with the anatomy of the human eye. We see accurately within a small region about 5 degrees wide. The fovea determines direction we are looking, which is called our visual line of sight. Accurate vision extends about 20 degrees. Our total field of view is about 100 degrees horizontally and 60 degrees vertically. The field of view for small hand-held and mobile radar devices may exceed the visual field of the user because of side-lobe detections.
Size limitations cause hand-held and mobile radar to produce measurements from multiple objects within the field of view of the user.
Distance
The second limitation is that hand-held devices are limited to continuous-wave radar to make them light enough to be mobile. Speed measurements are only reliable when evaluated at a specific distance, and distance measurements require pulsed operation or cameras when more than one moving object is within the field of view. Continuous-wave radar produces only a steady tone and not pulses. The frequency shift of this tone is used to measure speed. Continuous-wave radar may be pointed 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. Users cannot tell which object is being measured within the field of view without knowing the distance, which is impossible with continuous wave radar.
Some sophisticated devices may produce two different speed measurements from two objects within the field of view. This is used to allow the speed-gun to be used from a moving vehicle and not to discriminate between multiple vehicles within the field of view. Reliable operation cannot be achieved as more moving objects are added to the environment. Portable hand-held or vehicle-mounted radar can never produce a reliable measurements when 2 or more moving vehicles occupy the field of view if no distance measurement is produced by the radar.
[edit] Environment
Environmental influences also play a role. Using a hand-held radar to scan traffic on an empty road while occupying the shade of a large tree renders the hand-held radar sensitive to detecting the motion of the leaves if the wind is blowing hard (sidelobe detection). Airports cause a similar phenomenon.
Hand-held radar is only reliable on single vehicles when the location has been certified to be free of environmental influences that will cause false readings. Site survey must be repeated periodically for reliable operation.[5]
Corrections
Conventional radar gun limitations are corrected with a camera aimed along the line of site.
Cameras are associated with automated ticketing machines, 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.
• Automated ticketing machines
Laser devices, like LIDAR, 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 where it has been aimed.