gdl39 lost gps alt

GeorgeC

GeorgeC

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GeorgeC
Twice now, I've gotten false positive "sink rate" alerts from ForeFlight when paired to my gdl39. It showed up in the track log as altitude dropping out and the lat/long getting noisy for a few minutes. I don't recall anything like this happening with the internal iphone/ipad gps. No interference testing going on, and my ads-b tracks look clean.

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Happened again today.
 
That's a portable GPS, isn't it?
My best guess is that the satellites weren't in a position to give reliable altitude. Likely, the signals were bouncing around before being received, and that threw off the timing. Somehow, it could still compute a valid fix. I've seen similar behavior with hand-held GPS. Once in a car, where it had me going the other direction for a quarter mile, when I was under a raised road. The other time was in Alaska, where it showed me to be a mile from the road I was following. This sort of thing is why portable GPS isn't for IFR.

Where is the antenna? Can you move it elsewhere in the plane?
 
Yeah, the GDL39 is a portable. It's on the hat shelf, next to the copilot side D window... and right above the ELT. Maybe I'll try the pilot side.
 
Turns out the antenna is held on with an SMA connector and was a little loose.
 
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