Firstly, it needs to be explained that there are four different RADAR bands (frequencies) used throughout the world plus LASER (which is completely different to radar).
The four radar bands include:
X-Band (10.275GHz up to 10.775GHz) - not used anywhere in Australia
K-Band (24.050GHz up to 24.250GHz)
Ka-Band (33.400GHz up to 36.000GHz)
Ku-Band (13.175GHz to 13.675GHz) - not used anywhere in Australia
Radar (short for radio and ranging) works by transmitting a pulse of electromagnetic (EM) energy, also referred to as RF energy at a moving object. Using the Doppler principle the radar gun ‘listens’ to the change in pitch of the radio waves as they reflect off the moving vehicle and thus determines the speed. Similarly, when a train blows its whistle as it passes you – you can hear the change in pitch of the whistle, as the train moves past.
The radar beam starts out as a small diameter pulse when it leaves the radar gun, but grows quite rapidly as the distance increases from the gun. At a distance of 300 metres, the radar beam is now 100 metres tall and 200 metres wide! If 6 cars were simultaneously hit by a radar beam (six vehicles driving down the freeway), 6 pulses would come back to the radar gun, and the radar gun would detect (and report the speed of) the largest signal of the 6 signals present. To overcome this shortfall, the latest radar units with Digital Signal Processing (DSP) can identify the strongest and the fastest targets to try and help the police officer identify which vehicle's speed is being displayed on the radar gun.
Because the radar beam is so large when traveling down the road, a radar detector can be located almost anywhere in or on the vehicle, and it will detect the radar signal. Radar will penetrate glass, plastic, even people, but is reflected by metal and metallic objects. Radio waves will only travel in a straight line, so you are relying on reflections of other objects to give you advanced warning if you do not have a line of sight with the radar gun.
Traffic radar uses a radar beam to measure speed. Think of the beam as a searchlight. It’s invisible because it’s made of microwaves instead of light, but otherwise it acts very much like a light beam. It travels in straight lines. It’s easily reflected. It scatters as it is passed through dust and moisture in the air. And — this is essential — it has to hit your car before it can determine your speed. Radar can’t see around corners or through hills. It can’t see you when you are behind another vehicle. When in the clear, how strongly your vehicle reflects determines at what distance the radar can detect your speed. Generally, larger vehicles reflect more strongly than smaller vehicles. Trucks are more “visible” at further distances than cars. The principle on which radar operates is absolutely reliable. Radar equipment, on the other hand, is only as good as the quality of its design and manufacture. Traffic radars tend to be unreliable. They’re cheaply made and therefore vulnerable to many interferences that cause false readings. And, compared to the military and weather radar which have rotating antennas, traffic radars are vastly simplified. This simplification means that traffic radar cannot tell one car from another. The operator has to do that, and since the operator can’t see an invisible beam any better than you can, he frequently doesn’t know which vehicle’s speed is being read. This is a source of many undeserved tickets.
As a defense against radar detectors, many police radar units can be operated in the Instant-on mode, also called the Pulse mode. This means the radar is in position, but it is not transmitting a beam. So it cannot be detected. When the target is within range, the radar operator switches on the beam and the radar calculates the speed, usually in less than a second. This calculation happens too quickly for the target (you) to respond in time. Still, you can defend against Instant-on by recognizing it when the operator zaps traffic ahead of you.
As well as radar, there is also LASER (also known as LIDAR, - LIght Detection And Ranging), which operates at a frequency of 904nm ‡ 33MHz. A police laser (LIDAR) gun emits a highly focused beam of invisible light, in the near infrared region of light, at 904nm of wavelength, with the beam being around 50cm in diameter at a distance of 300m from the laser gun. Unlike RADAR which determines a vehicle's speed by measuring its Doppler shift, police laser (lidar) calculates speed by observing the changing amount of time is takes to "see" reflected pulses of light over a discreet amount of time.
Because the laser beam's diameter is so narrow, if a laser gun is aimed at the license plate, the beam is so small, that most times, none of the laser signal would actually “spill over” up to the windshield, where your detector is mounted. Thus, a radar detector with laser detector mounted on the windscreen would not even detect the laser beam targeted on the license plate (or headlights or bumper for that matter). This means the beam of a laser gun is accurate enough to target a single vehicle out of, let’s say, six vehicles driving down the freeway. The laser gun can target and detect that single vehicle in less than ½ second after the trigger is pulled, while the surrounding vehicles would not detect the laser beam as it passed right beside their vehicle!
Other than radar and laser, there are other ways of calculating a vehicle's speed being:
None of these four are used in Western Australia, other than red light cameras, which use the sensors built into the road.