Conclusions The quaternion scheme for rigid-body attitude determination has been developed starting with the Euler axis and angle. The axis/angle parameters, by themselves, provide a basis for determining attitude [Eqs. (7) and (15)], but they are not as clean computationally as the quaternion parameters [Eqs. (19) and (20)]. An explicit solution for the initial quaternion in terms of the initial values of the direction cosines has also been presented.