Circle will orbit a point located CIRCLE_RADIUS centimeters in front of the vehicle with the nose of the vehicle pointed at the center.
The units are centimeters from AC 3.2 (previously metres). Mission Planner reports the units as cm for all versions of the code.
Setting the CIRCLE_RADIUS to zero will cause the copter to simply stay in place and slowly rotate (useful for panorama shots).
The speed of the vehicle (in deg/second) can be modified by changing the CIRCLE_RATE parameter. A positive value means rotate clockwise, a negative means counter clockwise. The vehicle may not achieve the desired rate if this requires the acceleration towards the center of the circle to surpass the maximum acceleration held in the WPNAV_ACCEL parameter (units are cm/s/s).
The pilot does not have any control over the roll and pitch but can change the altitude with the throttle stick as in AltHold or Loiter mode.
The pilot can control the yaw of the copter, the autopilot will not retake control of the yaw until circle mode is re-engaged.
The mission command LOITER_TURNS invokes Circle mode during a mission.