iFly GPS Forum

Sure--log sent.

I'd imagine that unusual attitudes would mess with AHRS instead of GPS alt, though

In almost all occurances the Terrain Alert is the symptom, not the issue.  The issue is vertical speed or altitude.  If those are wacky, this will trigger the alert.  Here are some common causes of wacky GPS:

1) Bad reception.  If you are using a tablet or phone's internal GPS, these can often not work well.  An external GPS antenna can help, or an ADSB device.

2) Cockpit interference.  Same as above.  We had a customer that whenever he tuned in his old narco radio to a certain frequency, his GPS signal would drop.  Failing electronics can cause noise.

3) If you are using a GPS Transceiver that passively receives altitude from the transponder, such as the uAvionix Echo or SkyGuard TWX, this can cause large altitude jumps - especially when you are in the middle of nowhere.  Your transponder only sends altitude data when it gets a radar ping, so if you go 10 minutes without a radar blip, the ADSB will just keep reporting the last known altitude.  You can avoid this issue by forcing the system to only use GPS altitude, do this by unchecking Menu -> Setup -> Uints of Measure -> Pressure Altitude Support.

Hope this helps,
Walter 

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