As mentioned above, TCAS doesn't employ radar. Rather, it operates using transponder transmitions. The original poster's question is still valid, however: at what distance can an aircraft pick up another aircraft on its TCAS (what is the range of its 'radar')? Transponders operate on the UHF
band, so the question really boils down to "How far do UHF
signals usually propagate"? Under normal conditions, most UHF
transmitions have a range of around 100 miles. However, a number of tropospheric and general atmospheric conditions can sometimes exist which boost signal range by as much as 10 times.
So, an aircraft can detect another aircraft on its TCAS from about 100-1000+ miles away - assuming the other aircraft's transponder is operational!
Edit: I should add that while the above is the possible
range, TCAS displays typically only show traffic when it is 15-20 miles away, as Avt007 pointed out.
[Edited 2004-11-17 02:18:35]