lost gps

Hi, I been flying Spark since December with no issues; I think in that time it switched to ATTI mode once or twice. But recently while flying in my typical "test" area where I have never had a problem with GPS, it switched to ATTI mode once per short flight. Just wondering if anyone has seen...

I was flying this morning in the hills. Nothing around. No problems. Using the remote connected with OTG cable. Had 13 GPS signals. The Spark was about 100 ft up and 100 ft out when all of a sudden I get some kind of GPS disconnect notice and then the voice saying something about ATTI mode and...