Basic Concepts of Radar Detectors
One first has to know what they are detecting, in order to understand how radar detectors function. A very simple concept is applied for measuring the speed of a vehicle with radar. A radio transmitter and receiver is combined into a single unit to form a basic speed gun. A radio transmitter causes an electric current to oscillate so the voltage moves up and down at a certain frequency. Electromagnetic energy is produced on account of this electricity, and this energy travels through the air as an electromagnetic wave, when the current is oscillated. There also is an amplifier that increases the intensity of the electromagnetic energy and an antenna that broadcasts it into the air, in a transmitter.
A radio receiver can be described as being just the opposite of the transmitter: It converts electromagnetic waves back into electric current after picking these up with an antenna. This forms the very basis of radio – transmitting electromagnetic waves through space.
Radio waves are used for detecting and monitoring various objects, through radar. To find out how far away an object is, is the simplest function of radar. A concentrated radio wave is emitted by the radar, which then listens for any echo, in order to do this. The radio wave will bounce back to the radar device if there is an object in the path of the radio wave, as it will reflect some of the electromagnetic energy. The radar device can calculate how far away the object is based on the time it takes for the radio signal to return, as radio waves travel through air at a constant speed i.e. the speed of light.
On account of a phenomenon called Doppler shift, radar can also be used for detecting the speed of a moving object. As in the case of sound waves, radio waves also have a particular frequency, which can be described as the number of oscillations per unit of time. The echo will have the same wave frequency as the original signal, when the radar gun and the car are both at rest. The reflected signal mirrors the original signal exactly, as each part of the original signal is reflected when it reaches the car.
However, each part of the radio signal is reflected at a different point in space, when the car is moving, which causes the wave pattern to change. The second segment of the signal has to travel a greater distance to reach the car than the first segment of the signal, when the car is moving away from the radar gun. This either causes the wave to "stretch out", or leads to the lowering of its frequency. If the direction of motion of the car is towards the radar gun, the second segment of the wave gets reflected after travelling a shorter distance than the first segment does, before getting reflected. The crests and troughs of the wave are squeezed together, as a result. This leads to an increase in frequency.
Based on the level of change in the frequency, a radar gun can measure the speed of motion of a vehicle, whether towards it or away from it. Its own movement also has to be taken into account, if the police car is moving when the radar gun is used. For instance, if the police car is travelling at 60 miles an hour and the gun measures the speed of the target to be 30 miles an hour, as it moves away in a straight line, the target must be driving at 90 miles an hour. If, according to the measurements made by the radar gun, it is determined that the speed of the target is zero, than the target is driving at exactly 60 miles an hour.
Therefore, police cars in motion can correctly detect the speed of a car approaching it or moving away from it. So, in order to detect speedsters by making use of radar guns, police do not have to remain standing at one place, by the side of the highway.
For more than 50 years, police officers have caught speedsters in this way. Many police departments have recently acquired a new sort of speed detector and it uses light instead of radio waves.
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