General Operational Considerations
Understanding traffic radar
A historical perspective
The development of RADAR (an acronym for Radio Detection and Ranging) cannot be attributed to a single
inventor or even an identifiable group of inventors. Its basic concepts have been understood as long as those of
electromagnetic waves have. As long ago as 1886, it was known that radio waves could be reflected from solid
objects. Although use of a radio echo for detection purposes was discussed for many years in the literature, it
took the imminent threat of war in Europe in the late 1930's to bring about serious research and development.
The original purpose of radar was to provide advance warning of approaching enemy aircraft. Consequently, a
technique of transmitting radio waves and listening for the reflection was developed in Germany, Great Britain,
and the U.S. almost simultaneously. This search and detection system measured the length of time it took for a
reflection to come back, and from that, distance could be calculated. Using this technique, many familiar devices
were developed during the war years, often under great secrecy. These include aircraft and ship navigation, the
aircraft altimeter, and radar mapping.
With the lifting of military security restrictions in 1946, the level of research in radar declined and attention was
turned to the development of civilian applications such as radio astronomy and weather radar. Although a method
of velocity measurement using a theory of physics called the Doppler principle was well known, it was never
applied to radar until this post-war period. One of the first applications in 1948 was in primitive traffic radar to
measure the speed of autos. While these early units were an improvement over the time distance stopwatch
technique, they were bulky, difficult to operate and suffered from certain technical limitations. It was more than
twenty years before a significant breakthrough was made to enable the development of the modern-day radar as
we now know it.
The Doppler Principle
As we have seen, a wide variety of radar devices have been developed over the years to perform an even wider
variety of tasks. Let us turn our attention to how this technology is being applied to velocity measurement.
In 1842, an Austrian physicist and mathematician by the name of Christian Johann Doppler postulated a theory
that connects the frequency of a wave with the relative motion between the source of the wave and the observer.
This today is known as the Doppler principle and is used to determine the velocity of everything from a pitched
baseball to the largest galaxies in space.
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.
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