The angular orientation of a spacecraft as determined by the relationship between its axes and some reference line or plane or some fixed System of axes. Usually, "y" is used for the AXIS that defines the direction of flight, "x" for the "cross-track" axis, perpendicular to the direction of flight, and "z" for the vertical axis. roll is the deviation from the vertical (the angle between the z-axis of the vehicle and the vertical axis, or angular rotation around the y-axis). pitch is the angular rotation around the x-axis. yaw is rotation around the z-axis.