"Chris B" wrote in message
...
Due to the demand for high level flight routes (initially over the
Atlantic) there was a major exercise to reduce this separation to 1000ft
(Reduced Vertical Separation RVSM) - but this has very specific
requirements on the proven accuracy of the aircraft altitude (and other)
systems). If you cant meet the requirements you cant fly in RVSM airspace.

I am reminded of the comments that David Gunson made in his legendary
after-dinner speech "What Goes Up... Might Come Down". I'm doing this from
memory, so I may not be word perfect, but it was something like...

"The chances of two aircraft being at the same place, at the same height, at
the same time is so mathematically remote as to be not worth bothering
about. Air-traffic controllers force them down narrow corridors to increase
the chances of a collision, thereby justifying their jobs in keeping them
apart."