How does TCAS get range and bearing?

I have never been able to find out how TCAS works and I've asked several
people.  Today I tried Wikipedia and a couple of other websites, they were
no help on the subject.

It's my belief that TCAS is not a radar system. 

As I understand it, TCAS gets altitude information directly from the
interrogated transponder on the target aircraft.   But how does TCAS gets
the range and bearing of targets?

-- 
Dallas