Chris
TCAS operates in several ways depending on what the other ac is equipped with; as long as the other ac is squawking (not necessarily with Mode C) TCAS will give you an early enough cue even at 420 kts+ (range of 40nm). However, you are completely correct in that TCAS only operates line of sight and will not detect the other ac round the corner of a valley until v late. Yes, the gold plated solution would be a satellite based one, however who is going to fund it? Whilst not the be all and end all TCAS/E-TCAS is a large step in the right direction and would have prevented the Tornado/Cessna collision (assuming of course it was squawking?).