Having quite a bit of experience with GPS I have witnessed (and reported) on its failures. Its not a fool proof system, and one has to remember that TAWS is just a picture that represents a map of the area around your GPS calculated position. While GPS is accurate 99.99% of the time, certain installations will occasionally interpret that position badly or not tell you particularly well when its dropped out of 'GPS' mode, or fallen back to DR mode. I've been in an airliner using GNSS when the FMS system suddenly gave us a position alert for being 7 miles off calculated track, the GPS had disconnected due to lack of satellites (it could see) and it was using the local VOR to tell us the error. How it got to 7nm before telling us, and how ATS didn't pick it up either is a mystery, reported and moved on. However at Flight levels so no nasty hills around to ruin the day. If you are relying on the system to navigate hills without the system being in 'approach' mode where it does certain checks to ensure tracking integrity you might be surprised when both the hill pops up suddenly and the TAWS says its somewhere left of you. This is why TSO installations of GPS for IFR use is essential, as the annunciators for GPS/FMS failures are included to be in the pilots eyesight if a system drops RAIM or reverts to DR.
PS I've used IPADs for IFR flight bags and while the position is usually accurate, I have seen the plane icon do circles at times when it struggles with direction, so while I trust its derived navigation plot for roundabout navigation purposes, I don't really like its chances of continually providing attitude and heading information with the fidelity to stay upright for a length of time, and I definitely would not be navigating valleys below LSALT with it. Remember that GPS units need to see satellites, at low level below ridge height you could be masking some sats normally in view, and in the southern hemisphere you usually don't have heaps of spare sats.
Last edited by 43Inches; 3rd Sep 2022 at 05:10.