‘GPWS mode 1-4 are radio altimeter based alert and does not rely on GPS’.
Correct, but the Enhanced components of EGPWS may still be available based on alternative position data.
Extract from Airbus operational information tetex; SE 999.0015/04/VHR, 5 Feb 2004
This outlines their policy in only certificating an external GPS, preferably as part of the MMR.
“The TAWS is an improved system over the existing GPWS (ground proximity warning system). TAWS improves on existing systems by providing the flight crew automatic advanced aural and visual display of impending terrain, much earlier warning, forward looking capability, and operability in landing configuration. These improvements provide more time for the flight crew to make smoother and gradual corrective action.
To achieve the new terrain functions, the TAWS computer uses the aircraft position to locate the aircraft on its internally loaded terrain database. This aircraft position is currently provided by the flight management system (FMS) that computes it:
1) from the ADIRUS and GPS position source for aircraft equipped with GPS sensor unit (GPSSU) or multimode receiver (MMR), when GPS primary is available, or
2) from the ADIRUS and radio position update, for aircraft not equipped with GPS, when ground radio navaids are available, or
3) from the ADIRUS only, for aircraft not equipped with GPS receiver when radio position update is not available.”
Presumably paras 1) and 2) would be part of the MEL / DDG for aircraft with GPS; if the GPS was u/s then EGPWS functions should s be available.