User Manual

An appreciation of the Doppler effect can best be gained if one considers everyday sounds

produced by familiar moving objects: the auto horn, a train whistle and a jet plane in flight will all

demonstrate a marked change in tone as they pass a stationary object. This is a result of the

wave nature of sound. For example, consider the automobile horn. The horn itself is producing

waves of sound at a constant rate, say 250 waves per second. As long as the auto is sitting still,

we perceive the sound of the horn as a 250 cycle per second tone. If we next put the auto in

motion toward us at 55 mph, it becomes apparent that we no longer receive 250 waves per second

at our ear because, while the waves travel at a constant speed, each succeeding wave has a

shorter distance to travel to our ear. The waves are effectively compressed to a higher frequency

per second and consequently a higher tone is heard. The waves momentarily drop to 250 per

second at a point perpendicular to the observer and then begin to decrease in frequency as the

vehicle moves away from the observer and each succeeding wave has farther to travel to the ear.

The waves are now effectively being stretched. Moreover, if the speed of the auto is increased,

so is the compression and stretching effect upon the waves and we perceive a higher and lower

tone respectively.

The Doppler Principle as applied to velocity measurement

Up to this point, we have been using sound to demonstrate the effects of the Doppler principle.

However, as you may know, radio energy and light also exhibit a waveform and this fact opens

several interesting areas to consideration.

As we have seen earlier, it is possible to determine the existence and the location of an object at

great distance by transmitting a beam of radio energy and then receiving that small portion of the

beam that is reflected back. If it is possible to reflect radio energy from an object, and that object

is in motion toward or away from the transmitter, the reflected radio waves should be altered in

accordance with the Doppler principle. More specifically, they will be compressed to a higher

frequency as the object moves nearer to the source and, conversely, stretched as the object

moves away. Furthermore, the faster the object approaches or recedes, the greater the

compression/stretching effect upon the waves.

Therefore, if we are able to transmit a radio wave of a known frequency that travels at a constant

speed, and then construct a device to measure the frequency of the reflected waves, by

comparing the two frequencies we will know how much our beam was altered by motion, the

Doppler frequency. From here, it is a straightforward calculation to determine the velocity of our

target object. This is precisely the approach taken in all modern speed measurement devices.

Practical application of the Doppler Principle in traffic radar

Now that we have an understanding of the Doppler principle as applied to velocity measurement,

let us examine how it is used in MPH traffic radar.