I want to make a radar gun

I do know the basics of how one works, and the formula to calculate velocity. I had the idea to make a radar gun using a raspberry pi, but that's where my knowledge ends. Are there any USB radar transmitters and receivers? If not, is there any other way to capture data from a receiver in a way that can be translated to Python? Also what frequency should I transmit waves at? I heard something along the 3gHz range. Thanks in advance for any replies!