**3. Conclusion and expectation**

After more than 30 years on research and exploration, the technology of FOG has achieved a high level. While guaranteeing the accuracy and meeting the current requirements, FOG is gradually developing in the direction of low cost, miniaturization, high reliability, and long life.

FOG has been mainly used in astronautics, including spacecraft, satellite, aircraft, etc., and it is also widely used in civil fields such as ship, automobile navigation, mine, and so on. Based on different zero bias stability, their applications are different. If the bias stability is greater than 10°/h, it can be employed in land vehicle navigation, robot attitude control, and camera or antenna stabilization device. And when the bias stability is small ranging from 0.001 to 0.01°/h, FOG can be used in aerospace inertial navigation system and navigation. Whereas, in precision spacecraft applications, the zero bias stability required for precise aiming and tracking is less than 0.001°/h [30].

FOG is a type of angular rate measurement instrument based on Sagnac effect. It has advantages of no moving parts and wearing parts, small size, light weight, large dynamic range, fast start-up, long life, low cost, impact-resistant structure, flexible design and simple production process, etc. [29, 31]. It is broadly used in inertial navigation systems such as aviation, navigation, and aerospace, and is not in the direction of high precision. Continuous development [29, 32] with the development of modern microelectronics technology, optoelectronics technology, and signal processing technology, FOG will continue to mature; its application will continue to expand. In the future, there will be a greater stage in the field of inertial measurement.

*Gyroscopes - Principles and Applications*
