radar (rã´där), noun. (1) acronym for RAdio Detection And Ranging. (2) a remote sensor that emits electromagnetic radio waves, microwaves, or infrared laser light in order to measure reflections for detection purposes such as presence, location, motion, speed. (3) radiolocation. (4) field disturbance sensor. (5) proximity sensor.
Police microwave radars continuously transmit while simultaneously receiving and processing reflections. Reflections from moving vehicles are frequency shifted by the Doppler Effect and proportional to speed. Police radars have minimum sample times, time required to process one speed measurement. Sample times vary with model from about 1/4 to 1/3 seconds or more. Typically multiple sample periods are required when an echo is first detected or if the radar has just started transmitting. [Timing Animation]
Police radars are available in 2 basic configurations with various options and modes.
Hand Held Radar - operates from a stationary position
- Models with a moving mode also have a radar mount
inside the patrol vehicle for moving mode operation.
Fixed Mounted Radar - stationary or moving mode
- Some have optional second rear antenna.
Stationary Mode - Tracks approaching traffic.
- Tracks receding traffic. optional
Moving Mode - Tracks opposite direction traffic.
- Tracks same-direction traffic. optional
Cannot measure traffic within ± 2 to 5 mph of patrol vehicle speed.
Transmit and Traffic Modes
All radar track and display the strongest reflection.
Transmit Mode - Transmits continuously.
- Instant-on. optional
- Pulsed. Unreliable, optional
- Tracls amd displays 2 targets, the Fastest Vehicle
and the Vehicle with the strongest reflection.
Most microwave traffic radars have a relatively wide beam, 9° to 25°, that easily covers several lanes of traffic at a relatively short range. Detection range in the beam varies with radar and vehicle reflectivity and may be as low as 100 feet or less to 1 mile or more. A radar may track a distant large vehicle instead of a closer small vehicle without any indication to the operator which vehicle the radar is tracking. [Chapter 3.2 -- Antenna Beam]
Cosine Effect Angle
The angle between the microwave or laser radar and target must be small for an accurate speed measurement. The angle is referred to as the Cosine Effect angle because measured speed is directly proportional to the cosine of this angle. The larger the angle the lower the measured speed. The angle also imposes a minimum range on the radar or lidar, vehicles inside minimum range are too close to measure. A radar in moving mode can measure speed high in some situations. The radar should be located as close to the road as practical to minimize cosine effect errors and limitations. [Chapter 2.1 -- Cosine Error Geometry] [Chapter 2.2 -- Cosine Effect Minimum Range ]
Some microwave and laser radars have a timing mode that allows the operator to time targets. The operator measures the time it takes a vehicle to travel between 2 points of known distance. The radar is not transmitting rendering detectors useless. This method takes more time to set-up, requires more operator actions, is less versatile, and thus used less often. [Chapter 1.2 -- Timing Systems / Distance / Time Computer]
Some roads have unattended fixed mounted dummy radars that constantly transmit a signal in one of the Federal Communications Commission frequency bands approved for police radar. The FCC requires unattended transmitters to radiate less power than police radar. These transmitters do not have receivers and cannot measure speed or anything else. Dummy radars are intended to set off radar detectors to alert and fool drivers with detectors to travel at the speed limit. Locals usually figure out where the dummies are and ignore them, but be careful because police like to operate radar on a different frequency in those areas. A popular place to mount dummy radar is on overhead highway signs and mobile road electric signs.
S Band Radar |
In 1947 Automatic Signal Company in Connecticut built one of the first traffic radars for state police. Early radars were bulky and heavy systems, vacuum-tube technology. The radar consisted of 3 or more large components, an antenna, a 45 pound box with transmitter and processor, and an ink pen strip chart paper recorder. The radar also had a needle meter calibrated in mph. Some systems had 2 antennas, one for transmitting and the other for receiving. The antennas were mounted on a tripod, patrol vehicle hood, or fender. Some of the early 1960s' models mounted the antennas in the back windshield of the patrol car.
The first traffic radars transmitted at 2.455 GHz in the S band (2 - 4 GHz). The antenna beamwidth varied from 15 to 20 degrees depending on model. These radars operated from a stationary position only and measured receding as well as approaching traffic to an accuracy of about ± 2 mph. The maximum detection range was an unimpressive 150 to 500 feet, vacuum-tube receivers do not have the sensitivity of solid-state receivers. S band radars are obsolete.
X Band Radar |
X band radars have been around since 1965 and operate on a single frequency, one 50 MHz channel. Radars in the X band have better all weather performance, less signal attenuation in bad weather than higher frequency systems in K, Ka, and infrared bands. The X band radars tend to have slightly wider beams.
Some European countries use X band traffic radars that transmit at 9.41 GHz or 9.90 GHz.
Ku Band Radar |
The Federal Communications Commission (FCC) has allocated 13.45 GHz in the Ku band for traffic radar use in the United States, however Ku radars are not sold or used in the U.S. Some European and Middle East countries use Ku band traffic radars.
K Band Radar |
K band radars have been around since 1976 and operate on a single frequency, one 200 MHz channel.
These radars generally have more narrow beams than X band radars, and slightly wider beams than Ka band radars.
Detection range decreases with moisture. The water vapor absorption band is centered at 22.24 GHz, signals in this band tend to become absorbed by moisture in the atmosphere. For short range applications the effects may be tolerable on relatively clear dry days.
Ka Band Radar |
In 1983 the United States Federal Communications Commission allocated the spectrum from 34.2 - 35.2 GHz for police radar use. That same year Ka band photo across the road radars started appearing in the United States. Nine years later in 1992 the FCC expanded the Ka band spectrum allocated for police radar to 33.4 - 36 GHz.
Ka band radars typically have more narrow beams than X or K band radars. Detection range depends on moisture in the atmosphere, the more moisture the shorter the detection range.
Many models have a channel bandwidth of ±100 MHz, a 200 MHz wide channel. Some models channel band is ±50 MHz, a 100 MHz wide channel. This gives Ka band radar multiple channels, 13 to 26 channels depending on channel bandwidth. Most Ka band police radars operate on one frequency channel, a few have 2 channels an operator can select.
Wideband Ka Radar Wideband Ka radars operate on a single fixed frequency, or hops between one or more other frequencies. In frequency hop mode the radar dwells on one frequency for one or more sample periods then switches to another. The frequency hopping mode is intended to defeat radar detectors, however is seldom if ever used due to problems to numerous to mention. Only a few police radar models have this mode.
Photo radars are Across the Road Radars that intentionally aim a narrow beam across the road instead of down the road. The main beam of the radar radiates only a very small segment of the road. These systems account for the cosine effect angle and adjust measured speed upwards 6% - 9%.
Across the road radars are inherently less accurate than down the road radars because the beam is angled to traffic direction. The angle causes the Doppler reflection to spread out as a vehicle passes through the beam, a built-in speed error. At speeds as low as 20 mph the speed spread is 6 mph, the faster the speed the greater the spread. The radar attempts to process out the spread, but there still exists more uncertainty compared to down the road radars.
Photo radars can be mounted in patrol vehicles but can only be used from a stationary position. Some are designed to be mounted to poles, portable tripods, or inside ground fixtures sometimes disguised as trash containers, billboards or other common objects. Photo radars have also been hidden in highway maintenance and construction vehicles, tractor-trailers, box trucks, and unmarked passenger vehicles including vans.
Photo radars must be properly aligned to the road for the radar to correctly process the cosine effect. If the alignment angle is shallow measured speed is high, if the angle is larger measured speed is low.
Most if not all fixed mounted photo radars operate unattended, no police officer or operator present. Mobile photo radars can operate either unattended or attended. In unattended operation the radar is constantly transmitting. When an operator is present the radar can run all the time, or the operator can use the instant-on feature to transmit only on command.
Violations are photographed and superimposed with speed, date, time and location and mailed to vehicle owner. Photo radar setup to measure approaching traffic gets the front license plate, the driver may or may not be identifiable. Some systems take a second photograph to get the rear license plate. Photo radar setup to measure receding traffic only gets the rear license plate.
Photo radars, or camera radars as they were first called, were in experimental stages of development as early as 1954 using S band radars. In 1983 the state of Texas tried a French-manufactured Ka band radar for a time but discontinued its use because the units were being stolen right off the road. Many communities use photo radar because of the revenue it generates, some communities have outlawed photo radar because of public pressure.
[Chapter 4 -- Photo Radar]
Safety radars are across the road radars that measure speed of approaching traffic and display the speed to alert drivers how fast they are traveling. These radars are constantly transmitting. Most safety radars display all speeds measured, some only display speeds above a preset limit. Many safety radars do not record anything, some record all speeds measured and some only record speeds above a preset limit.
Laser radars or lidars were introduced in the early 1990s. These systems radiate in the upper infrared band and have extremely narrow beams compared to microwaves. Lidars transmit a narrow infrared laser pulse to measure pulse round trip time to a target to calculate target range. Speed is calculated from the change in range with time.
Laser radars are hand held and function from a stationary position only, no moving mode, and measure speed and range of approaching or receding traffic. Laser radars can also measure the range of stationary objects.
Lidars have shorter detection range and are much more sensitive to weather conditions than microwave radars. Laser signals propagate best in clear dry cool atmospheric conditions.
|Microwave versus Laser Radar|
Police microwave radars do not require the operator to aim exactly at a particular vehicle, only the general direction plus or minus half a beamwidth (±4° to ±10°). These radars are most effective in light to moderate traffic at short and long ranges. Many microwave radars can operate from a moving patrol vehicle.
Police laser radar must be aimed exactly using crosshairs or aim dot at at flat surface on a particular vehicle. These radars are most effective at short ranges in light or dense traffic. Laser radars are not designed to operate from a moving vehicle.
|Microwave Radar||Laser Radar|
|Operation||Stationary or Moving||Stationary only|
|Aim||Easy Aim||Exact Aim Required|
|Traffic Conditions||Light - Moderate Traffic||Light or Dense Traffic|
|Range||Short or Long Range Traffic||Short Range Traffic|
|Measurements||Speed only||Speed and Range|
|Location||Inside or Outside
|Outside Patrol Vehicle|
Should not be operated from
behind glass / windshields etc.