I see no reason why data from another nav system could not be mixed with GPS in or outside the GPS unit and then drive the CDI for a NP approach OR even (god & CAA forbid) a CAT 1 Approach.
You'll need WAAS or LAAS to support CAT 1 or better approaches (they are precision approaches since they have vertical guidance). WAAS is due for full USA coverage this year sometime, LAAS will have to be provided by a local provider such as an airfield operator. Which means someone will end up paying for it.
As I understand it, the UK CAA are not against GPS approaches per se. But they won't be the ones that draw up the procedures, they will merely regulate them (not as an EASA organ but as the Directorate of Airspace Policy which is a different sphere of operation). To date, no one has grasped the nettle and designed any overlays or stand alone GPS procedures, then done the safety analysis, then submitted them to the CAA for approval. The main problem being that someone will have to pick up the cost of that work and obtaining the approval. Once the first is done, then it may be simplified and cheaper for others following on, but like somone getting a type certificate for a new aircraft, then until someone with deep pockets makes the effort, everyone will just wait in the wings in hope. So there you go
IO540 .. you could blaze the trail here and design a NPA for your local field
Plenty examples in the US to use as guidance, although the US uses TERPS for their procedure design whereas the rest of the world uses ICAO PANS-OPS
NATS is doing some work on routes which might, in fact will, use GPS as a possible means of navigational compliance. These will be Precision RNAV routes in the TMA airspaces of the UK around about 2010 onwards. But these will be in pure IFR airspace (SIDs and STARs) and all probably a bit above your average PPL though
Just for the avoidance of doubt, I like GPS, I like what it offers to someone trained in its use. It offers great accuracy, and potentially (with all the right maps and databases), an electronic nav plog kind of capability which can be used to reduce workload.
My nervousness is that we go on to produce a generation of aviators who can only slavishly follow the line on a moving map, without understanding how to navigate themselves, and will end up in trouble if it goes wrong. They'll probably even have to input the circuit pattern so they can follow that around the airfield too
I also don't buy the 'it never goes wrong' argument. The fact that someone has never seen a problem in 500 hours of GPS operation is neither here nor there. Any safety assessment will have a probability of failure to look at and there is
no electronic system which simply cannot be degraded or fail at any time in its life cycle. So you have to allow a fallback which deals with that and the obvious one is for pilots to be able to navigate without any electronic aids. Ensuring they are competent in both is the test for this industry ... in terms of training requirements and ultimately cost. This is being dealt with in the other thread.
Fuji
'GoTo' is good for diversions to airfields which are already in the database. Quick and simple (as long as the line also shows CAS and Danger Areas which have to be negotiated). But these are not the kinds of diversions I see problems with. The ones I see causing problems are those where your original route through CAS (because the pilot only knows how to get a straight line on his GPS between established airfields or waypoints) is refused and you then have to 'divert' around CAS.
That gives 2 options. The pilot can visually navigate around the CAS lines on his moving map. Wouldn't be hard as long as the multiple lines are not misunderstood and there is awareness of stepped bases where appropriate. But still a small element of risk. Or the pilot has to input a lat/long which he didn't do on the ground, since he needs a line to follow. Big thumbs, turbulence, poor vis, high workload .. all can contribute to this having an error input. Interestingly, the top navigational error cause in Transatlantic Gross Navigation Errors are where the crew (2 man professionals and operating in a comfortable well equipped airliner) input a lat/long incorrectly (missing digits, wrong digits, etc). So the odds of an under pressure GA pilot making a mistake must be slightly higher.
It's how to reduce the risk involved in navigating in these kinds of operations with GPS that need to be tackled and a suitable 'warm feeling' given to the authorities.