Originally Posted by
mad_jock
doesn't have to be INS to have DME /DME etc.
Most Turbo props won't have INS and alot of the regional jets won't have either. So no map shift, you can get the difference between GPS position and DME /DME derived if you go through a few pages but the unit bins the gps position if there is a conflict and goes with DME/DME.
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The idea that the world s commercial aircraft are flying around putting all there trust in GPS is rather stretching it. Its a tool but most of the time it comes down very low on the list of Nav data. Hence why there really isn't that much of a push from commercial operators to have GPS approaches. So there is no one to pay for the surveying and production which is why nothing is really progressing on that front.
However, when the unit bins GPS you Appear to have to have INS or your RNAV capability is degraded (at least in FAA world) and you need to have three DMEs to use it for approaches without GPS. So GPS does seem to be rather higher up the list than you indicate. However, the essence of yournpntnis of course correct, there are DME/DME and DME/VOR FMSs without INS (and not requiring GPS) that will achieve the European BRNAV requirement. (There was an interesting near accident in North Africa where a corrupt VOR signal resulted in a a commercial aircraft being in the wrong location and coming within 59 feet of the ground while maneuvering - without a GPW due to the lack of GPS signal, INS not yet resynched and the corrupt VOR signal)
As an aside, I don't believe there are any GPS approaches - I believe they are all RNAV, GNSS, or RNP. So the issue with a lack of RNAV approaches in Europe is not that commercial operators don't use GPS. It is much more likely that the overlay concept was not approved by the Various NAAs and a rather burndonsome approach to approving new procedures.
PS
I believe that GPS and GLONASS are compatable but not interoperable as they have different bit rates and different (but similar) carrier frequencies. It is unlikely a box designed 15 years ago would support a speculative Russian system.