SDI-151 is mainly used to quickly and autonomously determine the true north direction. It is composed of high-precision fiber optic gyroscope, mechanical rotating device and signal solving circuit. The rotation axis of the gyroscope keeps its orientation unchanged relative to the inertial space, while the earth rotates relative to the inertial space around the polar axis with its rotation angular velocity. If the earth is used as a reference, the rotation axis of the gyroscope will be seen to rotate relative to the earth, so the gyroscope can track and measure the angular velocity of the earth’s rotation. Calculate the angular rate signal of the earth’s rotation, calculate the angle between the main axis of the gyro and the true north of the earth, and provide azimuth information for the carrier.