safetypee, your references to RA are very valid but have no relation to the EGPWS input source for terrain awareness.
BOAC, thanks for those references.
EGPWS takes its data from GPS position, in short, thus independent of barometric settings and not prone to a QNH blunder error. VSD (if installed) uses EGPWS inputs to display its terrain data as well, again providing a safeguard for blunder error (as runway symbol will either float in air or be buried under ground reference)
As mentioned in the Honeywell documentation:
Units are avail- able with an internal GPS receiver for required GPS data when another GPS source is not available.