Quaternion.

A system of representing attitude by measuring angle of aircraft center line with respect to three orthoginal axes plus rotation about centerline; quaternions are used over Euler angles (pitch, roll, yaw) when pitch can approach 90deg because of a singularity on Euler angles at 90deg; discrete-time computations using quaternions can run more slowly than those with Euler angles while producing results of the same accuracy See Also: Euler parameters;