Circle mode is similar to LOITER, but doesn’t attempt to hold position. This is primarily meant to be used as an initial failsafe mode to allow the vehicle to turn in order to hopefully regain radio control signal and pilot control. See failsafe documentation for more information.
Circle mode is deliberately a very conservative mode, and doesn’t rely
on GPS positioning as it is used when GPS fails. It will do a large
circle, The bank angle is set to the
ROLL_LIMIT_DEG divided by 3, to
try to ensure the plane remains stable even without GPS velocity data
for accelerometer correction. That is why the circle radius is so large.
Circle mode uses throttle and pitch control to maintain altitude at the altitude where it started circling.