Recognising vehicles in same and opposite direction using FMCW Radar

Most of us knew that FMCW Radar is being used in Automotive applications to measure distance and relative velocity of the vehicle going in front.

I'm not able to understand how the oncoming vehicle (vehicle coming in opposite direction of the subject vehicle) is differentiated from vehicle going in the same direction as that of subject vehicle but with velocity lesser than that of subject vehicle.

Below are the 2 typical cases:

Case 1) Subject vehicle is travelling at 100 km/h. Another vehicle coming directly towards the subject vehicle at 80 km/h.
Relative velocity = 20 km/h
Direction: opposite
Target: approaching. Distance getting reduced with time

Case 2) Subject vehicle is travelling at 100 km/h. Another vehicle is going in the front & same direction as that of subject vehicle at 80 km/h.
Relative velocity = 20 km/h
Direction: same
Target: approaching. Distance getting reduced with time

How the above 2 cases will be identified correctly with signal processing. Please explain with Doppler & beat frequency derivations

positive and negative frequencies? Over sample X4 (i.e. twice nyquist rate) to see them as I recall

Whether Doppler frequency shift for both the cases are same since both are approaching target with same relative velocity?

sorry, misread. If both are approaching, they will have the same Doppler frequency

using basic relativity theory, both of your scenarios are identical, so you can not figure it out.

If you knew groundspeed of both (you would need a wireless link subcarrier in the other car) you could figure it out, or if you had differential gps data on both vehicles

there might be a way of using and RFID repeater on the target vehicle to see the difference. you would process the closing distance Doppler beat note. But there might be a low level trace of ground speed Doppler shift of the outbound signal, and another re-transmitted Doppler shifted ground speed on the return. You might be able to solve all the equation unknowns with those 3 measurements? yes no?

Most of the automotive radars use several adjacent sensors. The measurements of the target distances and direction use a trilateration algorithm.
Some manufacturers (Valeo-Raytheon) developed a multibeam phased array antenna that provides the same information.

http://www.mathworks.in/help/phased/...echnology.html