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Rotate the observer around the last point faced. Imagine the observer is on the surface of a sphere, the last point face is the center of that sphere. Up and down orbit along the lines of longitude and right and left orbit along the lines of latitude. The observer will remain facing the last point faced so the heading and pitch may change as result of orbiting. However, because we assume an absolute north pole (parallel to the positive z-axis) the roll will never change.