GPS is a marvelous navigation resource and a freebee to the rest of the non-US world - but it is primarily a US military asset that could in principle be denied to TROTW thru selective availabity (SA), total denial of the CA code or high DOP due to repositioning of space vehicles ( leaving other areas without or very poor GPS coverage) to better serve a given conflict area for tactical purposes should things get politically hot as seen in 1991.
This is of course the main reason the wider civil community is hesitant to put all its huevos in one basket as a sole means navigation system. Nevertheless today GPS is indeed used by nearly all airlines as a complement to land based systems like DME/VOR/TACAN/NDB and ,of course, inertial. Aircraft FMS use a prioritised list for selection of navigation method based on its avilability and GPS heads the list for oceanic and other on-route phases. Also some FMS support EGI (embbeded GPS INS) for complementary navigation. GPS is even good for lateral 0.3 P-RNAV requirements.
When Galileo/EGNOS (together with GPS, even GLONAS) are fully established as truely civillian controlled infrastructures governed by a world agency then wider spread use down to CAT II (?)approaches will be a reality - this would necessitate a satellite navigation system that offers full integrity monitoring by an overlay system such as WAAS + EGNOS and psuedollites.
Currently non-differenctial GPS does not provide adequate integrity or accuracy in the vertical channel for vertical navigation meaning accurate trust worthy altitude remains an issue although under favourable DOP conditions it can better baro but not all the time and certainly not good enough for unaided precison approaches. Pseudollites are of course a local area augmentation that could remedy this but this solution will more likely find use as part and parcel of a widearea augmentation system. For approaches RADALT will contine to be essential as even augmented GPS will not provide height above ground accurately ( only height above Geoid) this can only be mitigated thru use of a data base holding surveyed terrain details held in the reciever - not necessarily a substitute for RADALT.
Until such time ILS and VOR/DME will continue to serve the needs of precision approaches ready to step in should GPS fail or be denied during onroute phases.
Last edited by b377; 17th Feb 2009 at 07:37.