@RichieRich I changed the parameters, and got it to work - thanks! I had a 3D DGPS lock, which I had assumed would be good enough, but that rejected still with 11 satellites. Once it obtained 13, it was able to work.

On our Voxl 500, I can write a mission, read it back, and everything looks good. Takeoff required is disabled, max distance to first waypoint and between waypoints is set way over what the mission is using, and yet, when I flip the switch, it just says "REJECT AUTO MISSION". Do you know what might be causing this? QGC says "Ready To Fly", and it sees 11 satellites.