Originally Posted by
andrewr
Do you ever consider why ATC can't use these separation standards? Odds are it is because they are not reliable enough.
Yes. ATC can use ADSB for separation, so its not the "reliability" of the solution as all the ADSB equipped aircraft I have flown source their position from the primary GNSS. All IFR aircraft are required to have at least one TSO C129 GNSS, which complies with RNP2 (the enroute navigation standard), so accuracy/integrity performance is known. What is an issue is the different areas of containment between a ground based position and a GNSS position. Close to a reference waypoint, GNSS integrity will not flag significant errors in azimuth accuracy, so this has to be considered. Out in the GAFA away from ground based aids and surveillance coverage ATC separation is indirectly predicated by GNSS anyway.
Originally Posted by
andrewr
"theoretically impossible to collide" might be a particularly unreliable standard for aircraft maneuvering in 3 dimensions. I suspect it does actually comply with ATC separation standards, if it can be reliably determined.
A lot of the time it does, but not always. If clear and diverging it is impossible to collide - 3nm and 5nm only gives people a warm fuzzy feeling.