I was watching a presentation on a radar system. At the end, he showed us a live demo.
In the demo, he shows it working in a Range-finder mode, where it shows returns from chirps at various distances. He also showed a video of a 2D plane with a ball with another ball being thrown in front of it.

There was some interference as the ball passed in front of the other, which he explained as “Multipathing”. Multipathing is something IBCrazy talks about in his FPV videos. He gets around it by using the concept of circular polarization. Considering that I build a CP 2.4GHz antenna recently, it's on my mind. If you set up your radar to TX on one polarization, and your RX on the reverse polarization, the only thing the antenna will pick up is signals that bounce off of other objects.
This is perfect for radar systems.

I did some web searches, and it seems that CP RADAR is useful for detecting hail, so this is obviously not a new concept.

I also thought about how his range-finder was working. It seems like a simple spectrum analyzer. I thought about how I could use what I have to do something similar. It seems to me that the OpenBee might be extremely useful in this department. It had an analyzer tool that I used to run antenna diagnostics.

If I modify the OpenLRS analyzer tool to listen on a small frequency range, and another to broadcast blips, that I might be able to build a simplistic radar system. This is worth exploring.

As I understand it, the frequency shift of the returned signal will tell you the speed of the object that's reflecting the ping. The timing will tell you the distance.

The disadvantage I anticipate is that 433MHz goes around objects better than 2.4 or 5.8GHz. This would only be useful for detecting large object. Then again, that has its uses.

I recently attended a lecture on autonomous cars. They covered their use of long-range and mid-range radar systems to help create a world map that the car uses to navigate.

It was said that the LRR systems that they are using for autonomous systems are used in Adaptive Cruise Control created by Bosch. I'll see if I can pick up one from the scrapyard.

I also found the SUCCESS project, which is promising a cheap radar system in the future.