With the arrival of affordable telemetry, the need for radar-guns to measure a model's speed will probably disappear in the next few years.
However, since there's possibly going to be an attempt on the world jet-powered RC model speed record around here in the not-too-distant future, I figure we'll probably need two different ways of confirming the model's speed.
At speeds of 600kph (400mph), a radar gun becomes somewhat difficult to use (many don't even read that high). In order to remain "in range" long enough, the pilot of the model has to fly a very precise line, fairly close to the ground (where the gun will be).
So, I was contemplating doing something a little different.
I figure that if I use a couple of doppler radar models aimed at right-angles to the flight-path, I should be able to use them to act as triggers for a simple timer.
In theory, the radar units should produce a tone with a pitch that drops in frequency as the aircraft approaches a position exactly in line with the radar beam. At that point, the frequency of the tone will be 0Hz. The frequency will decay/rise in accordance with a sine-function as it approaches and flies away from the imaginary line the radar will paint across the flight path. So, if I space the radar sensors apart by a suitable distance (perhaps 300 yards) and use the 0Hz point to trigger the start and stop of the timer, the speed can be measured pretty accurately.
What I'll have to experiment with is exactly how accurately I can detected the null-frequency which indicates that the plane is exactly opposite the radar units. This will be an interesting project and I welcome input from others who might have other ideas of how to measure the speed of a model moving this quickly.