Speed detection – radar and lidar
Radar traps have been the bane of many a lead-foot’s life since the early 1960s. More recently, laser speed guns – lidar – have added a new dimension to speed-limit enforcement.
First coined during World War II, the term radar stands for ‘radio detection and ranging’. Using it to measure distance is pretty easy. High-frequency radio waves, a type of electromagnetic radiation, are transmitted towards the object of interest and are reflected back. We know that radio waves travel at the speed of light – about 300 million metres per second. If we know the time taken for the radio wave to reach the object, bounce off, and arrive back at the radar device, we can calculate the distance to the object (using the formula: distance = speed of light × time between emission and reception ÷ 2).
Measuring the speed of the object is a little trickier and relies on something called the Doppler effect. This was first proposed by Christian Doppler in 1842. He realised that the pitch of a sound emanating from a moving source varies for a stationary observer depending on the speed of the source and the direction in which it is moving.
Imagine you are on a train in a station and you can hear the signals ringing at a rail crossing just down the track. Since both you and the signals are stationary, the signals sound normal. They continue to ring at the same rate as the train starts to move, but now because you are travelling towards them they seem to get faster. In effect, the time between arrival of pulses of sound is being compressed (or shortened) and the apparent frequency is increasing. The result is that the signals sound higher-pitched. This change in frequency is called a 'Doppler shift'.
The radio waves emitted by a radar device propagate ******ds at a predetermined frequency. When they strike a moving vehicle and are reflected, the frequency is 'shifted'. The radio waves bounce back to the radar device, where the change in frequency is recorded and used as data in a formula that calculates the vehicle’s speed.
Radar used from the roadside usually employs something called ‘slant beam’. This means that the radar beam is projected across the road at a pre-set angle; speed is still determined using the Doppler shift. Slant beam radar is used in speed cameras (although some speed cameras use laser guns); slant beams were first introduced in 1991 and are now commonplace around Australia. Attached to the radar or speed gun is a camera that is activated when a vehicle exceeds the speed limit. The re******t photograph records the numberplate of the vehicle, along with the date, time and speed travelled.
The radar device need not be stationary itself: it is the relative movement between the radar and the targeted vehicle that is important. Radar devices are often mounted on patrol cars, the speed of which can be determined from the Doppler shift against stationary objects such as houses or trees. The speed of the targeted vehicle can be determined simply by calculating the net speed after deducting the speed of the patrol car.
Laser speed guns – known as lidar, or ‘light detection and ranging’ – also use electromagnetic radiation. The term laser stands for ‘light amplification by stimulated emission of radiation’. Lasers are devices that can control the way energised atoms release photons of light: these photons form a very narrow beam of light.
The narrowness of this light beam gives rise to the label ‘speed gun’, because it must be aimed at the vehicle by the operator. When the trigger is pulled, the gun sends an invisible infrared laser light pulse. It then records the time it takes for the pulse to strike the target and return to the receiver mounted on the gun. From this time it is possible to calculate the distance to the object (range) in the same manner as for radar. The gun sends out hundreds of pulses per second; if the target is moving in respect to the laser, then the rate at which the distance to the target is changing is used to derive the speed of the target from a number of successive range measurements. The speed of the target is then displayed to the operator.
All laser guns in operation in Australia are tested to ensure that the laser light being transmitted complies with the appropriate Australian standard so that it cannot injure a person's eyes if they happen to look directly into the beam. This is done using an optical power meter certified by the National Measurement Laboratory with a certificate issued under the National Measurement Act.
The accuracy of all radar and lidar instruments to measure range and speed is tested using equipment called a delay generator, which must also be certified under the National Measurement Act.
The measure of success
In the early years of radar, many people caught speeding contested the scientific basis of the evidence. These days, the courts rarely entertain such challenges. The science, while still evolving, is sufficiently sound to satisfy most objective judges. And the reliability of the instruments can now be verified. That’s probably bad news for speeders – but good news for road safety.