For all those who have updated to iOS 11, do this test and report your results. I have been working with Brian to find a fix for this behavior on my iPad after iOS 11 update.
You do not have to be in your airplane to do this. Start iFlY without an external GPG device. Make sure you have the iFlY GPS status instrument displayed. After iFlY starts, watch the GPS instruments. My behavior is when you do not move, the instrument shows zero bars, then the “ no GPS position warning” is displayed. The moment you start to move, iFly works normally. Then stop, count off 15 seconds and see if the warning message returns. Mine is very repeatable, during a short walk down my driveway, I had the same behavior several times. 15 sec after motion stops, warning is displayed, start moving and warning goes away.