738 with dual GPS receiver failure, you'll get a TERRAIN POS (or some verbiage to that effect) alert on the ND long before the ANP exceeds RNP. You're left with the magenta line (INS & DME/DME referenced) but the Terrain Display is removed (both from the ND & VSD) and I believe some EGPWS functions are degraded. With ANP gradually increasing but still less than RNP, do you continue under these circumstances?
.Boeing 737 NG QRH says continue.
The GPWS mode gets info from the GPS but also ADIRS and RA for the terrain clearance floor .
The Terrain clearance floor (TCF) will use FMC position if the GPS fails and will give alerts if the aircraft descends below that floor.
For terrain ahead,if the GPS was out more than 15 minutes,the enhanced modes should not be deemed reliable and TERR INHIBIT should be set to the override position.
.FCTM says 1 GPS required to conduct an RNP approach.
Dispatch requirement?certainly,before FAF requirement as well?
I believe to pass the FAF,1 GPS must be serviceable,after that a pilot decision indeed.