Sunday, 18 November, 2007

Police Speed Trap Radar/Laser

(1) acronym for RAdio Detection And Ranging. (2) a remote sensor that emits electromagnetic waves (radio, microwave, or light) in order to measure reflections for detection purposes (presence, location, motion, etc.). (3) radiolocation. (4) field disturbance sensor.

All stationary microwave traffic radars measure on-coming traffic; some models also (and/or) measure going (receding) traffic. Virtually all moving mode radars can operate from a stationary position or a moving patrol car. Moving mode radars usually require a minimum patrol car speed for moving mode operation. All moving mode radars (in moving mode) measure on-coming (opposite lane) traffic; some can also (and/or) measure receding (opposite lane) traffic (requires aft antenna). Some moving mode radars can measure targets traveling in the same-lane (direction) as the patrol car (front and/or rear antenna). Same-lane radars require a minimum speed difference (2 mph or MORE) between the target and patrol car. Moving radars also measure patrol car speed (ground speed), most also display patrol measured speed. Many radars track only one target at a time; some models have the option to track and display two targets -- the strongest (may be closest or largest) target (echo) and the fastest (or a faster) target in the beam.

Most microwave traffic radars have a relatively wide beam (9 to 25 degrees) that easily covers several lanes of traffic at a relatively short range Detection range (in the beam) varies with radar and target reflectivity and may be as low as 100 feet (30 meters) or less to 1 mile (1.6 kilometers) or more. A radar may track a distant large target instead of a closer small target without any indication to the operator which target the radar is tracking

The angle between the (microwave or laser) radar and target (alpha in the figure below) must be small for the radar to accurately measure speed. 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 radar should be located as close to the road (really projected target path) as practical to minimize Cosine Effect errors . The Cosine Effect on moving mode radar is slightly more complicated and may measure target speed high under certain conditions .

Figure 1.1-1 -- Radar Set-up
Radar Measured Speed = Target Speed * cos(alpha)

Fig 1.1-1

The above figure describes Down the Road radar.

Some microwave radars constantly transmit. Some microwave and all laser traffic radars only transmit on operator command (instant-on); some microwave radars (pulsed) only transmit periodically every several seconds, and then only long enough (fractions to 1 second or so) to get a speed measurement. Instant-on and pulsed (microwave) radars are intended to defeat radar detectors by keeping transmission time short.

Some microwave radars and many laser radars have a timing mode that allows the operator to time targets (traveling between 2 points of known distance) visually instead of transmitting (defeats microwave/laser detectors). This method takes more time to set-up, requires more operator actions, is less versatile, and thus used less often.

S Band Radar (obsolete)

A connecticut firm (Automatic Signal Co.) built one of the first traffic radars in 1947 for the state police. Early radars were bulky and heavy systems (vacuum-tube technology) that usually consisted of three or more separate pieces of equipment, an antenna (sometimes 2 antennas -- separate transmit and receive), a 45 pound (20 kg) box (the tube transmitter, receiver and processor), a strip chart pen recorder for a permanent record, and a needle meter calibrated in mph. Sometimes the antennas mounted on a tripod and sometimes on the hood or fender of a patrol car. 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). Note that many microwave ovens transmit at about 2.45 GHz, and low power unlicensed wireless communications transmit from 2.400 - 2.4835 GHz. S band radar antenna beamwidths varied from 15 to 20 degrees depending on model. These radars operated from a stationary position only and measured receding as well as approaching targets to an accuracy of about ± 2 mph. The maximum detection range was an unimpressive 150 to 500 feet (45 to 150 meters); vacuum-tube receivers do not have the sensitivity of solid-state receivers. A radar with a 150 foot detection range would have less than 1.5 seconds to measure a target traveling 68 mph (100 feet/second or 109 kmh). S band radars are obsolete.

X Band Radar

Frequency Tolerance Frequency Range
10.525 GHz ± 25 MHz 10.500 - 10.550 GHz

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 K or Ka bands. X band radars tend to have wider beams than K or Ka radars.

Some European countries use X band traffic radars that transmit at 9.41 GHz or 9.90 GHz.

Ku Band Radar

The Federal Commumications 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 countries are reported to use Ku band (13.45 GHz) traffic radars.

K Band Radar

Frequency Tolerance Frequency Range
24.150 GHz ± 100 MHz 24.050 - 24.250 GHz
24.125 GHz ± 100 MHz 24.025 - 24.225 GHz

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 wider beams than Ka band radars. Detection range decreases with moisture, also see chapter 5.5 -- Interference/Natural Interference.

Side note: Some World War II radars operated in the K band around 24.1 GHz (in the limits of K band traffic radar), which also happens to be in the water vapor absorption band (centered at about 22.24 GHz). Signals in the absorption band tend to become absorbed by moisture in the atmosphere and do not have the range that other frequency bands offer. For short range applications the effects may be tolerable on relatively clear dry days.

Ka Band Radar

Traffic Radar Ka Band
33.4 - 36.0 GHz

In 1983 the U.S. FCC allocated the spectrum from 34.2 - 35.2 GHz (Ka band) for traffic 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 traffic radar use to 33.4 - 36 GHz.

Ka band radars typically have more narrow beams than X or K band radars. Target detection range depends on moisture in the atmosphere (rain or humidity), the more moisture the less range. Also see chapter 5.5 -- Interference/Natural Interference.

Many models have a frequency tolerance of ± 100 MHz (200 MHz bandwidth), some models have a tolerance of ± 50 MHz (100 MHz bandwidth). An advantage of 100 MHz bandwidth over a 200 MHz bandwidth (besides less chance of interference) is more radar channels can be squeezed in the (Ka) band. The bandwidth allocated to Ka band traffic radar is 2.6 GHz (36-33.4 GHz), or 2,600 MHz. A radar with 200 MHz bandwidth has 13 channels (2600/200). A radar with 100 MHz bandwidth has 26 channels (2600/100). At a minimum 2 radars operating close to each other should be separated by at least 2 channels (the greater the separation the less chance of interference).

Wideband (Ka) Radar
Wideband radars (Ka band) operate on a single fixed frequency (operator selects one of several available), and/or in a frequency hop mode. In frequency hop mode the radar dwells on a fixed frequency for a fraction of a second (on the order of 1/10 of a second or more) and hops to another frequency. The radar cycles between a number of different frequencies. Wideband radars are intended to defeat radar detectors.

Across the Road Photo/Safety Radar

ACROSS THE ROAD (photo or safety) radars are designed to point a narrow beam (typically 5 degrees) across the road at an angle -- instead of down the road. The beam cannot cross the road at anything close to 90 degrees but something much less (typically about 22 degrees). The main beam of the radar paints only a small portion of the road. These systems, if designed properly, account for the Cosine Effect angle (based on alignment angle and beamwidth) and adjust (upward 6% to 9% for a 5° beam aligned at 22°) measured target speed.

Figure 1.1-2 -- Across the Road Radar Set-up

Fig 1.1-2

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. Photo Radar

Photo radar (constantly transmitting) automatically detects a speeding violation (auto-lock) and photographs and/or video tapes the driver, the suspect vehicle and license plate, and records vehicle speed and typically the date, time and location. In states that have only one license plate (rear) two photographs (and cameras) are required to record the license plate and driver -- one photo (front of vehicle) to get the driver, another photo (rear of vehicle) to capture the license plate. Some units function at night by using a flash. Some units use an orange flash filter -- an orange flash is not as bright (as a white light flash) and should startle the driver less. Many photo radars connect to a computer or printer to retrieve stored data statistics such as number of violations, time and speed of each violation, etc.

A police officer does not have to see (or even be near) the alleged violation -- the process is automatic. The police officer is replaced by electronic circuits and a still and/or video camera. Many drivers do not even know they were recorded by photo radar (usually hidden in a van, pick-up truck, highway maintenance vehicle, etc.) until weeks later when a ticket and photograph (usually includes license plate and driver) arrives in the mail. Note that the registered vehicle owner may not be the driver, but the owner still gets the ticket.

Safety Radars are across the road radars connected to a large display to indicate a driver's speed to the driver. Some Safety Radars only display speeds above the limit, some display all speeds measured. Some units record all speeds measured, some only record speed violations. Safety Radars usually do not record the driver or vehicle, unless connected to a camera.

When operator controlled, rarely used, the radar can be set to constantly transmit, or set to instant-on (transmit only on operator command). In either mode the operator can observe target speed on a display and/or set the radar to automatically photograph (auto-lock) violations.

Across the road radars have fractions to a couple of seconds to measure speed. Also because of the Cosine Effect error the echo is not a steady stable frequency shift (such as for down the road radar) but multiple shifts that change rapidly with time. Across the road radars are much less accurate than down the road radars.

Laser Radar

Laser radars, sometimes referred to as ladars (LAser Detection And Ranging) or lidars (LIght Detection And Ranging), were introduced in the early 1990s. These systems radiate in the upper infrared (IR) band and have extremely narrow beams (compared to microwaves). Laser radars function from a stationary position only (no moving mode) and measure approaching and/or receding targets, some also display target range. Most laser radars can also measure the range of stationary objects. Laser signals propagate best in clear dry cool atmospheric conditions

Microwave versus Laser Radar

Microwave traffic radars do not require the operator to aim exactly (only general direction) at one particular target, and are most effective when traffic is light. Many microwave radars can be used from a moving patrol car. Traffic laser radars can function in dense (or light) traffic, and require the operator to select (aim crosshairs at) a particular target. Laser radars are not designed to operate from a moving patrol car. Typically microwave radars have longer detection range than laser light systems.

Microwave Radar Laser Radar
Stationary only
Easy Aim Exact Aim Required
Intermittent (Pulsed)(1)
Instant-on only
Light-Moderate Traffic Light-Dense Traffic
Short-Long Range Targets
Detection range decreases with
rain, humidity, fog (K / Ka bands)
Short Range Targets
Detection range decreases with
fog, rain, dust, smoke, CO2, humidity
Measures Speed only
some models measure / display
Strongest and/or Fastest(1) targets
Measures Speed and Range
some models do not display Range
Inside or Outside Patrol Car
antennas can be mounted / used
inside(3) or outside(1,2) patrol car
Outside Patrol Car Use
Should not be operated from
behind glass / windshields etc.

(1) -- optional mode / feature
(2) -- Some places (United Kingdom for example) require microwave radars also be operated outside patrol vehicle.
(3) -- Do Not Use a radar, or a radar detector, from behind an electrically heated windshield such as the Ford Instaclear or General Motors (GM) Electriclear -- these windshields have metal film coatings that block microwave signals. Also note vehicles with these (reflective) windshields have a larger radar cross section -- increasing detection range for a radar.

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