Police Radar Information Center
The most common speed measuring devices are radar and lidar. Police radar is also referred to as microwave radar, or Doppler radar. Lidar is also referred to as laser radar or light radar. The terms radar and lidar are often mistakingly interchanged because they both measure speed, but use much different methods and are operated in a different manner.
- Microwave or Doppler Radar
- Continuous Microwave Signal
- Handheld or Car Mount
- Traffic in Beam
- Stationary or Moving Operation
- Range: up to a mile.
- Speed for strongest &/or fastest
- Laser or Light radar
- Pulsed Infrared Laser Signal
- Handheld or Tripod
- Carefully Aimed at Target
- Stationary Operation only
- Range: 2000 feet (maybe).
- Speed and Range
Radar measures the strongest reflection, some also measure the fastest. The strongest reflection is usually the closest vehicle, but not always. Lidar must be carefully aimed at a single vehicle.
COSINE EFFECT ERROR - radar and lidar.
The closer a radar or lidar is to the traffic lane, the more accurate the speed reading. A radar close to the road keeps the traffic direction angle (to radar) small, close to 0°. The greater the angle, the lower the measured speed and the greater the speed error. At 90° the error is 100%, no speed readings. When the angle changes too fast the speed is changing too fast for the radar or lidar to get a measurement. This introduces a minimum range, vehicles inside minimum range cannot be measured. The greater the angle, the larger the minimum range blind zone.
Moving mode radar measures ground reflections (patrol speed) to compute target reflection speed. Ground reflections are subject to cosine errors causing a low patrol speed reading (patrol speed shadowing). A low patrol speed reading translates directly to a high traffic speed reading.
An exception is same-lane mode with a target traveling slower than patrol car. In this case speed measures low, but will measure high if the target speeds up faster than patrol car. Same-lane mode cannot measure any target traveling close to patrol car speed, ± 3- 5 mph.
TEST - radar and lidar
Radars and lidars should be tested daily. Radar should be tested with calibrated tuning forks. Lidar range accuracy and alignment should be tested with a narrow object of known distance at several hundred feet. A lidar should also be tested against a vehicle with a calibrated speedometer traveling at a known speed.
Radars, tuning forks, and lidars should be calibrated tested at least twice a year. Some departments require once a year. Test should be completely documented with a clear paper trail.
This test is important
but often overlooked,
many times not done,
and widely misunderstood.
The Police Traffic Radar ISSUE PAPER is somewhat dated, but still very much applicable today. All the same problems and concerns exist today to some degree, and a few more. The report is detailed and has good empirical data to back up conclusions.
RF (Radio Frequency) and MICROWAVES
ON THE ROAD AGAIN
103 MPH School Bus ?
Police radars and lidars are not just point (at target) and click (transmit) devices. There are some basic setup limitations and operating procedures that must be observed. Too often procedures are not properly followed in order to save a little time or hide from motorists, resulting in speed errors. Additionally, far too many operators don't remember, or don't use, what they were taught in radar training.
Radars can and do make mistakes, even when properly setup.
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