Police radars transmit a microwave signal and receive reflections simultaneously. The microwave radiates in a cone shaped beam. The beam's width varies with model from 9° to 18°. Most of the microwave misses the target, and most of the target reflection is directed away from the radar.
The scenario below is an optimum setup, radar close to traffic lanes, and the radar antenna beam angled to maximize coverage. The below radar has a 12° beamwidth that is angled 6° down-the-road.
Reflections from moving objects have a slight frequency difference from the transmit signal, this phenomena is called the Doppler Effect or Shift. The difference is based on target speed, the greater the speed the greater the frequency difference. This difference can be converted to miles per hour or kilometers per hour or knots, etc. Reflections from the ground and stationary objects are not frequency shifted.
Detection range varies with target size, shape, and reflectivity. Other factors include radar frequency and power, angle to the target, antenna factors, and weather conditions.
Police lidars (Laser Radars) transmit pulsed infrared (IR) laser radiation to measured range. The IR pulse travel time (at the speed of light) between lidar and target and back is used to determine range. The target change in range during 0.3 seconds (or more) is used to calculate speed. Lidars have very narrow beams, just under 0.2°. Practical range is limited to about 500 feet (150 yards).
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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