Police Radar Units use an unmodulated continuous wave and measure reflections or echoes of the original signal, there will be a difference in the returning signal from the original signal, the speedtrap measuring device will calculate this difference and present it in the format of a speed reading. Radar Speedtraps work on the Doppler Shift priciples. This relies on the fact that an object moving relative to a wave source will alter that wavelength by either increasing the wavelength (lower pitch) if it is receding from the source or decreasing it (higher pitch) if it is moving towards the source. If you have ever had an aircraft fly close overhead or a car travel past at high speed you may have noticed that as the vehicle approaches, the pitch of the noise is higher and gets lower as it moves away, this is doppler effect.
Police Laser units or Lidar ( Light detection and ranging ) use an infra red wavelength. They send out a beam of light which will travel at about 30cm ( 1ft ) per nanosecond then will reflect off a vehicle back to the speed gun. The time ( in nanoseconds ) it takes for this to happen ( divided by 2 ) gives the gun the distance from the vehicle, this can be done hundreds of times in half a second so the difference between the distance readings over a very short period of time can be used to calculate the speed of the vehicle, and they are very accurate.
First used in 1947 on 2.455 GHz these consisted of multiple pieces of equipment including a separate transmitter and receiver, a pen recorder and a needle type speedometer, so a very cumbersome and unwieldy unit to operate. Detection range was only 150 to 500 feet. Now obsolete.
Developed during the 1950s and in use since about 1965. X Band ranges between 10.5 and 10.55 GHz (10.525 with a tolerance of 0.025), compared to other bands it has low frequency and high output. This makes it particularly easy to detect, in fact it is possible to detect at up to 4 miles away although it's own useful range is obviously a lot less than this. Unfortunately because intrusion alarms, radio masts, and door openers use a similar signal these can be the cause of a lot of false alarms.
This came into use in 1976-8 firstly for mobile units and uses 24.150 plus or minus 100 MHz (to give 24.050 to 24.250 GHz), or 24.125 plus or minus 100MHz (to give 24.025 to 24.225 GHz). Because water will absorb wavelengths around 22.24 GHz it can be absorbed more easily by water in the atmosphere so a rainy day may give a weaker signal. K Band can be detected anything upto 2 miles away on a good clear day. Traffic light sensors and some automatic door openers use K Band transmitters so a false alarm may result when approaching these devices.
Ka Band / Ka WideBand
Came into force in 1983-7 and uses 34.2-35.2 GHz, Ka Wide Band detection range is upto 1/2 mile.
Ka Super Wide Band
1992 Developed from Ka Wideband, Superwide Band can use any frequency between 33.4-36.0 GHz.
13.45 GHz Has been tested in the UK and is used on the continent.
Laser 904 nM
A mobile unit the laser gun can pinpoint a target in a lane of traffic, it takes less than half a second to get a reading wheras a radar can take 2-3 seconds. It can't be used from behind glass, requires a very steady hand, and at 1000 feet the beam is 3 feet wide, at 1 and 1/2 miles the beam is 2 lanes wide