Most recent aircraft installations use the FMC derived position for EGPWS position.
If your FMC’s have map shift issues (read: non GPS updated position), the EGPWS will predicate its warnings (or more importantly, lack of) on this false “map shifted” position.
It has happened…..an Airbus nearly hit a hill in Africa due to map shift caused by rain damaged VOR equipment. EGPWS position was derived from FMC’s.
Some airlines have resolved this by installing a dedicated GPS to feed position information to the EGPWS. Many/most/perhaps all FMS’s not designed with GPS as part of their core design cannot be “upgraded” to accept GPS inputs.
A suggestion for any airline considering this addition/modification…..a comparator to annunciate FMC/GPS position difference greater than 1.0 n.m.…..a warning to crews they may be in a “map shift” scenario.
Be careful out there…..