How Laser Works
LASER stands for Light Amplification by Stimulated Emission of Radiation.
The laser speed gun uses a method that relies on the reflection time of light. You have probably experienced the reflection time of sound waves in the form of an echo. For example, if you shout down a well or across a canyon. The sound takes a noticeable amount of time to reach the bottom of the well and travel back to your ear. Sound travels at approximately 1000 feet (300 metres) per second, so a deep well or wide canyon creates a very apparent round-trip time for sound.
A laser speed gun measures the round-trip time for light to reach a car and reflect back. Light from a laser speed gun moves a lot faster than sound – about 984,000,000 feet per second (300,000,000 metres) or roughly a foot (30 cm) per nanosecond. A laser speed gun shoots a very short burst of infrared laser light and then waits for it to reflect off the vehicle. The gun counts the number of nanoseconds it takes for the round trip, and by dividing it by 2 it can calculate the speed of the car. By taking several hundred samples over the course of a third of a second or so, the accuracy can be very high.
The advantage of a laser speed gun is that the size of the ‘cone' of light that the gun emits is very small, even at a range like 1000 feet (300 metres.) The cone at this distance might be 3 feet (1 metre) in diameter. This allows the gun to target a specific vehicle. A laser gun is also very accurate. The officer has to aim the laser speed gun at a specific target therefore the officer will only target your vehicle if the officer deems that you are speeding.
How Does Police Laser (LIDAR) Work?
LIDAR is an acronym for Light Distance And Ranging.
A LIDAR gun emits a highly focused beam of invisible light, in the near infrared region of light that is centred at 904nm of wavelength and is only about 22 inches (56cm) in diameter at 1000 feet (300 metres.) Unlike RADAR which directly determines a vehicle's speed by measuring its Doppler shift, LIDAR calculates speed by observing the changing amount of time it takes to ‘see' reflected pulses of light over a discreet amount of time.
What is Laser?
Laser is very similar to a torch beam and it has all the same properties in comparison to a standard beam of light. The difference with laser however is that it can send out a tightly focused beam. Radar cannot single out one vehicle in a pack, so the speed reading is usually attributed to the leader. The narrow laser beam reads only the vehicle it strikes.
Laser's narrow beam imposes significant limits on its use however. It must be deliberately and carefully aimed, therefore the operator can't be moving. He must have a clear shot, preferably not through glass. As with radar, you cannot use laser to read speed from the side and there must be oncoming and departing traffic.
However since laser is a narrow beam at a close range the targeting of a car can be accurate enough not to scatter laser light onto other vehicles as the beam is only about a foot wide. This and the fact laser is only emitted when the trigger is pulled can cause problems for receiving suitably advanced warnings of laser speed traps ahead. At a longer range the beam spreads more and provides more scatter to alert other road users.
What is Radar?
Radar is a beam of microwaves, like a beam from a torch, except invisible to the eye. However a radar beam is very similar to a light beam in that it travels in straight lines and so it cannot see around corners or through hills and it can't see you when you are behind another vehicle. Like a light beam it's easily reflected and it scatters as it is passed through dust and moisture in the air. It is this scatter and reflection that Radar Detectors are sensitive to and alert you to the fact that there is a radar device in the area.
When a radar beam is pointed directly at your car the reducing time taken for it to return a signal as you approach is what is used to calculate your speed. Generally, larger vehicles return more of a radar signal than smaller vehicles. Trucks are therefore "visible" on radar further away than cars, motorbikes are hard to pick up until closer because of their relative size.
While the principle of radar microwaves is very accurate and used by the millitary and for weather forecasts, police radar equipment, on the other hand, is not so accurate. Hand held speed guns are cheaply made and simplified, and therefore vulnerable to many interferences that cause false readings. 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.
The microwave frequencies you will generally see mentioned are X, K, Ka and Ku. Laser is also considered a band. K band is the main frequency used in the UK and this is what the infamous Gatso cameras use. These are the grey boxes you see at the side of the road facing away from you and which take your picture when you pass. They are so named after Gatsometer, a Dutch company that invented them and manufactures most of them. X and Ka band are used in Europe but are not used anymore in the UK. The same frequencies are used by all traffic management systems such as traffic light monitors, pedestrian crossings, red light and bus lane cameras etc. As a result the more sophisticated radar detectors on the market have selectable bands so that you can turn off those not required to reduce the false alerts. Ku band has government approval to be used in the UK and much is written about the Ku band being used in MiniGatso cameras but our understanding is these cameras are very uncommon except in Northern Ireland. The S5R has this Ku band switched off under its "Logic 3" setting for UK use as this is considered best for optimum performance.
What is GPS?
GPS is a group of 24 satellites each orbiting the earth at around 11000 miles above the surface. Each Satellite circumnavigates every 12 hours. At all times at least four satellites will be in view at any given point on the globe. GPS has been fully operational since 1990.
As long as a GPS device can see a minimum of 3 satellites then an accurate position of the vehicle can be calculated. GPS detectors generally check their position every second and are able to work out accurately the speed of the vehicle and constantly comparing themselves to a stored database of alerts. When nearing a stored location the unit gives an audible and visual alert.
When a GPS device is first powered it must acquire satellites to know its position. The first start is known as a cold start which can take up to 45 minutes to acquire the minimum three satellites for correct operation. Thereafter, warm starts (when the unit has been switched off overnight) should take a matter of minutes and hot starts (same day) a few seconds.
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