The ground proximity warning system (GPWS) uses the radio altimeter to receive information about the distance from the ground. Does the GPWS system work solely based on the information it receives from the radio altimeter? or are there other instruments involved with the system? If the radio altimeter gave an incorrect reading due to something like a terrain data base error would this trigger a GPWS alert in the cockpit?

  • 3
    $\begingroup$ Radar altimeter does not have any terrain database. It simply indicates how far to the closest obstacle straight down. Terrain database is used with GPS to also notify you of high ground (mountains) ahead where the radar altimeter is not looking. $\endgroup$
    – Jan Hudec
    Apr 4, 2022 at 17:15

1 Answer 1


GPWS is an older technology, and it uses the radar altimeter and a few other things, but not GPS, since it was developed long before GPS existed. EGPWS, the "E" standing for "Enhanced," uses GPS and a database of terrain & obstruction heights. A system that has the latter will typically, in more expensive (and thus better equipped) aircraft at least, incorporate the former, so that you have the benefit of both -- i.e. if the GPS is unavailable, you still have warnings from the radar altimeter, or if the radar altimeter fails you still have the EGPWS functionality. That integration also handles cases when both systems would be providing an alert, so the pilots are presented with one alert rather than two at once. However, for aircraft without a radar altimeter, EGPWS by itself is valuable, so it is possible to have the one without the other, and this is common in less expensive aircraft where the cost of a radar altimeter isn't justified.

As far as inputs to a GPWS system, it will consider things like aircraft configuration, and can provide warnings when, for instance, the aircraft is descending as if on a glideslope to a runway but without the landing gear down. Depending on the implementation, pitot-static inputs might be considered as well.

  • 1
    $\begingroup$ In particular, it uses the computed position generated by the FMCs or other equivalent navigation gear -- EGPWS will still function on aircraft without GPS in their nav receivers, using INS/navaid positioning, but it's vulnerable to dodgy ground navaids in that circumstance $\endgroup$ Apr 5, 2022 at 3:08
  • 2
    $\begingroup$ @UnrecognizedFallingObject In the 737 NG, that's not correct. The EGPWS has its GPS receiver, and it never sees the FMS position. I've flown when GPS jamming was active, and while the FMS position was fine (from DME/DME updates - so a slightly larger ANP), the EGPWS Terrain display was missing & we had the Fail light for the EGPWS. Other aircraft may work differently. $\endgroup$
    – Ralph J
    Apr 5, 2022 at 3:48
  • 2
    $\begingroup$ According to that linked article, the A-320 does pass the FMS position to the EGPWS. That incident illustrates the wisdom of keeping the position fixes separate! $\endgroup$
    – Ralph J
    Apr 5, 2022 at 3:58

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .