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I've had many occurrences when one faulty GPS antenna can appear to work correctly but can cause other nearby (within 20m) GPS antennas to fail. This is why modern installation standards call for resilient GPS antennas to be sited as far apart as possible - I usually aim for 60+m, but I have the luxury of working on large buildings! On an aircraft, you'll be lucky to get 5m between the aerials, hence I'm wondering if you have a faulty GPS antenna. I believe you've already had one replaced - but maybe the other one needs to be replaced too.