To maybe clear up a point, I had an aircraft last week inform me they had just lost their RVSM approved status due to the TCAS packing up. Seem to remember when the trials began over the Atlantic, a/c had to have 2 of 3; INS, GPS, and one other, and only 2 out of 3 needed to work, but then I could just be dreaming.......and things obviously change!!!