A common question that always seems to crop up is “Why can radar lock onto vehicles not of interest?”.
Well, it’s far from a simple topic, but it’s the phenomenon known as Radar Cross Section that is of interest.
What is Radar Cross Section?
To put it simply, Radar Cross Section is how “big” a target appears to the radar. (we use the word big loosely here because it’s not just dependent on the size of the target)
In an ideal world, the emitted signal from the radar would reflect off the target and return in a nice orderly fashion with only compression or expansion of the original signal, the result of this would be a nice clean doppler signal.
However, the real world doesn’t like ideal situations and so things become much more complex. Vehicles do not present a nice simple surface to reflect radar off – they have many different surfaces which can reflect the same emitted signal back in many different ways!
If you were to take two similarly sized cars but with different shapes, both will present a different doppler signature back to the radar, there is no “one size fits all”, depending on the number and size of the reflectors one car could appear to big a much bigger target.
In addition to different shapes affecting the radar cross section, the size of the object greatly affects the amount of signal reflected back to the radar. It should be obvious that a truck will present a much bigger target to the radar than a small car would – this has particular relevance in the real world as the radar will lock onto the “biggest” target that it can see.
So the shape and size of the object helps determine the radar cross section, but there are more subtleties involved here!
The material that the target is made from also greatly affects how well the signal is reflected, metal objects will reflect the signal much “harder” than some other materials.
Putting it all together
You should now understand why there is no simple way (with CW radar) to say when a radar will lock onto an object, the pickup distance varies greatly with the radar cross section. A radar may pick up a car at ~100m but will then pick up a truck a ~200m and if there are multiple objects in the beam, then the radar will lock onto the object with the biggest radar cross section.