RE: Radar guns
Flapper is correct about the radar info. I use one at work, (yes, to catch speeding vehicles) and hold a certification in the use of doppler radar speed measuring devices. I have also had one used to measure the speed of my RC plane, and it does work. The radar does not pick up smaller objects like airplanes as easily as large objects, and if there are any moving cars within the scope and range of the radar, it will readily pick up that vehicle instead of your plane. There are also many types of interference that can cause you to get an inaccurate reading; however, it will be obvious. Estimate how fast you think the plane is moving, then measure with the radar. If it is much slower or much faster, you may be experiencing interference.
The cosine effect that was mentioned before refers to the radar providing a slower than actual reading due to the object moving at an angle to the radar antenna. Basically, your reading becomes less accurate the higher the angle. So, to get the most accuracy, the moving object must be moving directly toward or away from the radar antenna's signal.
I don't recommend spending all the money on a police radar. You don't need all the certification that police need to win cases in court. Just make sure it has the speed range for the planes you are going to measure. Some radars don't detect beyond 120 or so mph.
LIDAR's are just a different technology for measuring speed in the same way as the radar. You still have to worry about the cosine effect, but they are less prone to interference since the laser beam is pointed directly at the object and not at a large area encompassing the object. The GPS system may cost more than you are willing to put into it, but if you got the cash, that would probably be the most accurate way. It, however, measures speed horizontally in relation to the earth's surface, so if you are climbing or diving, your reading will be slower than actual.
None of these devices measure airspeed, mind you. You would need a miniature pitot tube and an airspeed measuring device inside the plane.
In short, I recommend you spend the $80 and get the cheaper radar.