gps
Here I have a question for the whizzkids: while flying last week over Africa I brought a "backup" GPS in addition to our on board UNS-1C GPS. Lateral position always agreed, but 3-D alt read out on back-up GPS (on board gps has no alt info) differed 1800 ft at fl 330 with altimeters. regional qnh 1016..isa +12...
anyone??