The process starts when the radar sends out a burst of radio waves at a moving object. The radar then times how long it takes for the echo to return to the receiver. Since radio waves are light waves, they travel at the speed of light, and it's an easy calculation to determine how far away the moving object is. This portion is not particularly important to speed radar, but it is an application that finds its use in detecting missiles and airplanes. However, the next bit is where we get to have some fun. Fun with math, that is!
The equation for the Doppler Effect for an approaching source is given as:
Equation made in Microsoft Word
There is only one unknown in that
equation; the radar knows "f naught," the original
frequency of the radio waves sent out,the frequency at
which it returns to the radar, and it knows "v," the
speed at which the wave propagates through the air (or
at least a good approximation for it). It can then
solve for "v sub s," or the speed at which the object
the radio waves bounced off of is traveling at. That
is given by the equation:
Equation made in Microsoft Word
Equation made in Microsoft Word
And that is how speed radar
works. One type, anyway. Stay tuned for more
exciting physics talk (now with less math!)
Wave Physics
Radar, Part 1
Radar, Part 2
Lidar
Bibliography
Tl;dr