a system for determining orientation or position of a movable supporting structure , such as a vehicle , includes a special inertial sensor unit . this unit includes a single gyroscope hereinafter referred to as a &# 34 ; gyro &# 34 ;, with its spin axis mounted in alignment with the sensitive axis of an accelerometer . the gyro and accelerometer are mounted pendulously to rotate only about a single axis which is aligned with a direction of travel of the vehicle or with a projectile launching assembly . in additon to being pendulously mounted as mentioned above , the gyro and accelerometer combination may be rotated relative to their pendulous support , to certain fixed positions , in order to facilitate rapid initial alignment and orientation and pre - mission calibration of the inertial instruments . the pendulously mounted unit is also provided with an optical encoder to indicate the roll angle of the vehicle to which the inertial unit is secured . in an initial rapid alignment sequence after the unit is energized , the rotation of the earth is sensed and the information stored with the unit in two or three different orientations , and , using this stored information , and the stored accelerometer outputs , the azimuth of the pendulous axis relative to true north and its pitch or elevation angle may be accurately determined . when mounted in a vehicle and used in combination with the output from the vehicle odometer , an accurate indication of the movement of the vehicle may be obtained , and when combined with initial position and orientation data , the position of the vehicle may be continuously determined .