Review from UK user:
When you know how to use it's spot on every time I'm a cop and it gives same speed as my radar gun very happy. also used it on a rc car with great results ;)

For those which were always interested in how fast is an airplane flying around them, a car driving or RC model driving.
Speed gun uses sound, emitted by moving object to determine its speed. It uses Doppler effect.
What you need to do is to stand with your phone, and noisy object has to pass around you with constant speed.
Perfect for RC airplanes, you just need to fly above your head, and speed gun will say you speed.

Application supports MPH and KPH, and is saving measuring history.