I'd like to build a speed detector that can measure speed of a running athlete, or the speed of serve in tennis or any other similar amateur sport. Devices like these are used during Olympics, Wimbledon, etc. Yes - it's all in spirit of inspiring a generation! :-)
I've seen Arduino based solution that relies on tripping sequence of light beams to calculate average speed - however that is too restrictive and will only work for events like running but not tennis. Hence I presume I need something like a radar sensor. Do these exist in reasonable hobbyist package? How do I get one? What're the best ways of hooking it up to the RPi?
Additionally, are there any alternatives? Would ultrasonic have a sufficient range? Will the doppler effect with ultrasonic reflection be easy enough to measure? Any other choices?
N.B. Sorry, but answers with light-beam tripping won't be accepted. :-)