I have been trying to understand why GA has not got a multi-sensor navigation recever after all it cant be to much of a problem to use the GPS data base to auto tune thee DME units to triangulate a fix. the (already fitted) altitude input would take the slant error out of the fix.
Something like that would no doubt meet P-RNAV by using both GPS and DME/DME.
I guess the problem is that DME/DME does not give you the continuous position reference which is needed for any kind of moving map application; one needs something else to fill in the gaps between the times you are banging the DMEs. And one can't just dead reckon for ages. GPS solves this by continuously computing the solution. Airliners use IRS (occassionally corrected by DME/DME triangulation) but inertial reference is expensive. The cheapest 3D fibre optic gyros come to about $10k in volume, and moreover are tightly export controlled.
One could fill in the gaps using solid state gyros and they are improving all the time, and would probably be good for a few minutes. This isn't really good enough though... imagine doing a descent from 5000ft to 1000ft and using the device to help find some grass field which looks just the same as the countryside around it, and 10 mins out you lose all DMEs...
In terms or marketing, "everybody" really knows since nothing short of WW3 or something close is going to switch off GPS, "nobody" is going to spend money on backing up their existing IFR GPS which works just fine. And one can always deal with total equipment failure by reverting to VOR nav or even vectors, or of course pulling out the backup GPS. There isn't really a market for something that "would be a super idea" but is not actually needed.
Google turns up nothing on Narco ND9000 - what is it??
Years ago, before GPS took over the world, somebody (Collins?) produced a moving map product which used VOR/VOR triangulation. Then, GPS killed the market and they dropped it. It would have been near-useless for European GA anyway due to poor reception distances and general VOR errors of a few degrees much of the time.