Inter Vehicle Communication System Project
Radar is not what you want.

Radar involves sending out a signal and watching for the reflection of that signal off an object.

What you are looking for is more of a triangulation system.

There's 2 ways I can see of doing this:


A rotating parabolic dish.  This will give the strongest signal response when pointing directly towards the transmitting object.  I'm sure you've seen these in cheesy 60's sci-fi movies.
Multiple aerials and receivers.  A signal sent from one point will arrive at different points with different strengths and at different times.  Having receivers at different locations will allow you to triangulate where the signal came from.  The further apart the receivers are the better the accuracy will be.  Ideally you want a minimum of 3 receivers.  Using 2 you will be able to cover a 180 degree arc, but you won't be able to tell which side of the line joining the two receivers the signal is coming from.  A third, making a triangle, will give you the full 360 degrees.


While it would be possible to use multiple receivers on a vehicle, the distances between them will be relatively small, so the differences in signal strength and timing will be minuscule, and very very hard to calculate accurately.

A much better system would be to have each vehicle have a GPS module so it knows exactly where it is.  It can then broadcast its location to vehicles around it so they then know where it is.

This kind of system is already in use in Swedish (and probably others') military (and I think some civilian) aircraft.