The Doppler Principle: Everyday life has a multitude of examples of the doppler effect with sound. The whistle from a train is a good example. As the train approaches a stationary listener, the pitch (frequency) of the whistle sounds higher than when the train passes by, at which point the train and the person standing are technically stationary.

Electromagnetic waves radiated by the traffic radar obey the same principle, although electromagnetic waves travel at the speed of light and audio waves at the speed of sound. The Doppler Effect that enables police radar to work is a frequency shift that results from relative motion between a frequency source and the listener. The Doppler shift is proportional to speed between source and listener, frequency of source, and the speed the waves travel at (speed of light for electromagnetic waves).

Instant ON (Pulse Radar): Intended to defeat radar detectors. Instant ON radar allows the operator to control the radar transmission. The operator only transmits after selecting the target, and only long enough to get a speed reading. In practice most police find this a difficult mode to operate in and are more likely to have the radar on all the time unless two officers are present in the car, one driving and one working the radar.

Cosine Effect on Moving Radar: Moving Radar measures closing speed between the radar and target. The radar also measures patrol car speed (from the ground echo) to calculate the target speed. (Target speed=closing-patrol car). This introduces additional sources of cosine error. In most situations the angle between the radar and target is the major error source and favours the target (measure too low). However if the antenna is misaligned (off patrol car direction) the patrol car speed may measure low resulting in target speed measured too high.

Moving Radar Variables: Target speed will only measure higher than true speed when the target is approaching the patrol car AND the cosine angle between radar and target are small, (typically less than 5%) AND the angle between the patrol car and the ground is large, (typically greater than 5%). Patrol car and target speeds are significant, patrol car speed greater than target speed increases the error. (The greater the difference the larger the error and the higher the measured speed).

Shadowing: Radars identify ground echoes as the strongest signal (most of the time). The ground echo cosine angle is a function of the radar antenna alignment and beam width. More reflective terrain in only part of the beam could change the angle of the ground return (shadowing) which can change the measured speed of the patrol car. Large and or reflective objects such as overpasses or billboards and road signs may have a momentary effect on radar. Guardrails, bridge trusses and construction zones may have a longer effect.

Ka Band Radar: Photo Radar: Automatic unattended photo radar started appearing in the late 80s and came to U.K. in 1993. With Photo radar systems a human operator does not observe any speeding violation, but is replaced by electronic circuits and a photo-recording device. No one has to see the alleged violation; the process is automatic. The registered owner of the vehicle usually receives a ticket in the mail. Photo radar is across the road radar and designed to point a narrow beam of radar (typically 5 degree horizontal beam width) across the road at an angle of 22.5 degrees. Speed measurement is then adjusted for the angle.Some units operate with an amber (orange) flash filter. This is not as bright to the human eye and causes minimum disruption to a driver even at night.

Power output is very low (2.5mW typically) which makes detection for radar detectors difficult, but not impossible.