I have a DIY quad with APM 2.5 with AC 2.9.1 and with uBlox GPS. Yesterday I tested to fly and log five identical missions just to compare them, see attached picture. Start of all mission was in upper left corner and it flew down, right, up and left back by RTL command. Altitude during whole flight was constant 20 meters. Look at that pictures, especialy to that violet track (first mission). Mission 2-5 were almost identical with various starting points (upper left), but first mission, violet one, was like waves, instead of linear track of next missions.
Any idea, what can cause this behavior by first mission? This is very typical for my quad, that first auto mode is like my quad is mad and then it is fixed by itself and other automissions are stable. Just to remark, I always wait with start until GPS is locked (blinking blue LED on uBlox unit and constantly lighting blue LED on APM.
What can cause it?