Basic Radar Operation

http://www.netspace.net.au/~smack/traffic/mvradar.html accessed on 5 March 2003

Radar operates on a “saturation effect” principal. Huh? Basically… like a flashlight. Turn a flashlight on in a room. Radar is a flashlight that projects a type of light which is not visible to the human eye. Just like a “normal” flashlight, the radar illuminates everything it is pointed at. And, just like a flashlight, it can not generally penetrate solid objects. These objects will reflect the light back to the radar unit, as well as in other directions. This is also how you see… the objects you see are really just the reflections of the various wavelengths off the object.

But wait!

How does THAT equate to speed and YOUR speed? Well, moving objects cause the frequency of the transmitted beam to be “bent”. It changes in frequency. If the original frequency is known, then the frequency returned from a stationary object should be the same. But, if the object is moving, the frequency will change based precisely on the speed of the object’s motion. The faster the motion relative to the radar source, the more the frequency is changed. Measure this change and you can calculate (or rather a chip in the radar gun does) the speed of the object reflecting the radar beams. But… there is one “minor” detail… the object could be moving towards the unit… or away. There is no way to tell which. Um… another slight detail. Just like shining a flashlight in a room… What happens to all those reflections? And what about the size, shape, and color of the object? Oops….

That’s right!!! There are reflected beams.. bouncing everywhere.

Those reflected beams are supposed to be “ignored”. Only the strongest reflection is the reflection which is supposed to register in the radar reading. But, doesn’t a large object have more reflections… and thus a stronger signal… than a smaller object? YES! So, a small, fast moving car might be ignored if a larger, slower moving truck is nearby. Thus, the idea of some newer units to measure TWO signals. One being the strongest return signal… and the other the signal with the highest frequency shift… the fastest signal. They should be the same as the object generating the faster signal gets closer to the radar unit… if so… that’s a speeding object. If not, it could be a stray signal… a different object… or random error. Who knows?

And there is yet one more important detail….

Just like shining a flashlight into a room… What happens if there is already something else shining light around? Um… Uh… Oh… Eeee… Ooh… Well, the idea is that the frequency of the radar is within a specific “band”. A “color”, if you will. If a different “shade”, or frequency is in the area, the radar unit simply ignores it. Much like the difference between the color red and green. The radar can only see a specific range of frequencies… it’s radar band (like a color). It should not be affected by the other frequencies.

Unless…

Those frequencies fall into its frequency band.. what then? Ideally, there isn’t anything nearby that can transmit frequencies within the band of a radar unit. But, many things do… cell phones, intrusion alarms, store door openers, garage door openers, neon signs, and many others… Then, hopefully… these items will generate a signal so strong… or so weak… that the radar unit will ignore them. Most of these different items broadcast signals ten times or more stronger than a radar gun. But… could they affect radar? Yes. What then?