When doing a survey pattern in auto mode and say at 15 mph, I am always seeing the copter about 1 mph less in speed during the mission and on each line in the grid pattern. The survey pattern is 3-4 acres so it is a straight line for a while. GPS is good and healthy the whole flight. What might cause this?
Not saying I can help much, but we’d definitely need to see a .bin log file. Upload one to dropbox or google drive (or similar) and share the link to it.