Nothing to do with GPS, however i suggest you check FCTM page 5.39 (Raw Data Monitoring Requirements). Raw data monitoring is recommended but not required both for classics (EFIS update 7.2 and FMC version > 7.1) and NGs. The MEL allows dispatch with the ADF inop as long as one FMC is operative even for NDB approaches.
Sorry, but you cannot do an ADF approach without an operating NDB and ADF - period !
If you do not have a ground station and the appropriate cockpit instrument, you are doing an RNAV FMC approach, not an ADF approach. And we have no RNAV approach plates in our Euro-Jepps, and so I'm presuming that RNAV (FMC/GPS) approaches are not yet certified to any of the airfields we visit.
Your reference in the 737 FCTM is referring to whether "raw data" or "map display" is selected - not whether the ADF is working or not. In other words, you can use map display mode, if you have overlaid VOR or ADF pointers for cross-checking (which many systems can do) or you flip between raw data and map display to check that the map is in the right position.
Note the note at the bottom of this section: -
"Compare VOR and ADF systems to detect possible map shifts". You cannot do a map cross-check, if the VOR or ADF is u/s or not fitted !!
This is what happens, when people use a (map-shifted) RNAV FMC approach, instead of the raw NDB/ADF.
1996 Croatia USAF CT-43 crash - Wikipedia, the free encyclopedia
Ditto the B-Med Airbus going into Addis Abeba. The FMC plot and terrain display were off by some 3 miles, leading to a go-around just 60 ft over the high terrain. Now while the VOR would give occasional signals of its unreliability (the VOR was the primary fault), the FMC gave no indication that it had a map-shift. This was the biggest complaint in the report, that the FMC knew it was getting bum VOR information, and therefore may have a map-shift, but did not bother to tell the pilots. And the pilots could not check for a map-shift, because they were comparing bum data with bum data.
Report: British Mediterranean A320 at Addis Abeba on Mar 31st 2003, wet VOR nearly causes two CFITs
Incidentally, the New Scientist report on this indicated the crew were highly influenced by the terrain display, which showed them passing nicely through the valley. However, the terrain display was likewise mapshifted. Not sure where this info came from, because the AAIB report underplays this aspect (that the crew were merely following the FMC and terrain plot, and disregarding the raw VOR signal).
One would have thought that following an erroneous and displaced VOR radial would bring you eventually to the VOR, whereas this flight paralleled the VOR inbound, which is what would happen if you followed an erroneous FMC position.
Search Results - page 1 - New Scientist
And I am not even sure that GPS-RNAV is entirely a solution at present (as was recommended in the B-Med report). It works well with high accuracy and reliability, but Europe is still concerned that the US may degrade GPS signals or switch the system off during periods of international tension. This is the primary reason for Europe developing and launching the Galileo GPS system. When that is up and running, and there is full confidence that erroneous signals can be detected and warnings given, then Europe may proceed to RNAV approaches.