THE simplest concept of inertial navigation is that of using an accelerometcr for measurement of vehicle acceleration, integrating once to give a measure of velocity and twice to give a measure of distance travelled (fig. 1). For this purpose, the direction of the input axis of the accelerometcr must be kept constant, or alternatively its direction at all times must be known and its changing direction must be taken into account in interpreting its output. In either case, provision of a stable attitude reference is necessary. Gyroscopes can provide this feature.