Perhaps INS, even "VOR calibrated" INS just aint that accurate eh. Jaba methinks yer putting up a red herring there..
Binghi,
Jaba is right, see FAA AC90-45A, the standard for area navigation. Available updating was DME/DME, DME/VOR or LOC, and the later addition of GPS as an update source didn't change the Doc. 8168 rules for an "area nav" approach.
Even when we got the GPS update in suitable INS equipped aircraft, the minima were still the same as a VOR/DME approach (generally 400'/1 nm, except in Australia).
All the original "area nav" approaches in US were overlay approaches, you just followed the profile of a VOR or NDB approach.
Indeed, there were many area nav devices available in the US from the late 1960s, we never saw then in Australia, because we never had the density of VOR or proper DME, and still don't.
All QF 747, and subsequent types in the fleet (all B767 and later B737, and, I would assume, various Airbus to the JAA/EASA equivalent) ) are certified re. AC90-45A.
Tootle pip!!
Chaser,
Maybe you are not aware, but US forces (and, I would assume, other countries) can very effectively spoof/deny GPS coverage in (sort of) specific areas, as a nornal precaution. It works well, I know, because we got caught on the edge of an exercise --- it really screwed up the GPS feed to the IRS/FMCS system.