Ring laser gyro (RLG) is an angular rate sensor based on the Sagnac effect and can measure the angular rate of a moving vehicle relative to inertial space. RLG inertial navigation system is widely applied in the defense military and commercial fields. In the defense military field, RLG inertial navigation systems are widely employed in launch vehicles, cruise missiles, ships, unmanned aerial vehicles, new fighter aircraft, transport aircraft, tanks, and the in-service transformation of armed systems. In the commercial field, different levels of RLG inertial navigation systems are also choosed in transportation, commercial spaceflight, deep sea survey, coal mine drilling, geological mapping, and other fields. RLG has the highest market share in medium and high precision gyro due to its high accuracy, high reliability, and mature engineering technology. RLG is characterized by high reliability and strong resistance to shock and acceleration without rotating parts. Additionally, it is unnecessary for RLG to turn at high speeds since the time required to reach a constant speed, with short start-up time. As RLG has a wide dynamic range, theoretically there is no upper limit to the angular velocity measurement range, and RLG has a long life of up to 100000 h or more. With the development of aviation, aerospace, navigation, and other fields, the RLG inertial navigation systems have put forward the requirements of high precision and long flight time. According to China's relevant research plan, the development of a high-precision, long-endurance RLG inertial navigation system is an important development direction in inertial technology.

To improve the accuracy of the inertial navigation system of RLG and meet the long-endurance requirements, we should carry out two aspects of the research. First, from the perspective of the inertial device itself, new material and improvement in the assembly process and sensitive characteristics should be employed to improve the performance of RLG and accelerometer. The second is inertial navigation system technology. From the perspective of the error propagation characteristics of the system, the error is comprehensively compensated and prevented according to the error model. The main technical approach is rotational modulation technology, in which the rotational mechanism drives the inertial measurement unit to rotate according to the proposed scheme to realize the error modulation function. After the accuracy of RLG reaches a high level, it is difficult to improve the measurement accuracy by enhancing the processing technology, with a long development period and high costs. The utilization of inertial navigation system technology to provide the system with higher accuracy and stronger self-sustaining power is a low-cost and efficient method to effectively improve the navigation capability of long endurance. The key technologies of the long-endurance RLG inertial navigation system can be summarized as five aspects in Fig. 5.

According to the five key technical directions listed in Fig. 4, the studies of relevant scholars in recent years are summarized. The future development of long-endurance RLG inertial navigation system technology can be mainly considered in the following aspects:

1) Further exploration of error types, models, and calibration methods for error calibration techniques

The existing error models can no longer meet the high-precision and long-endurance requirements. For the RLG inertial navigation system, more precise and perfect full-parameter error models should be built, and the corresponding error calibration method needs to be studied.

2) Further research on the error decoupling method of initial alignment technology

Since the existing initial alignment methods seldom consider the influence of errors in inertial devices, it is necessary to consider the influence of errors on the initial alignment accuracy based on a more refined and perfect full-parameter error model. Additionally, a reasonable compensation scheme, initial alignment scheme and decoupling of errors and misalignment according to the characteristics of each error should be designed.

3) Optimization of rotation modulation scheme

It is of great significance to optimize and design a reasonable tri-axis rotational scheme to overcome the limitations of single-axis and dual-axis rotational modulation techniques, eliminate the influence of earth rotation angular velocity on rotation modulation effect, and pursue high-precision navigation in long-endurance conditions.

4) Exploration and optimization of system level and device level redundancy scheme, fault diagnosis, isolation, and reconstruction method improvement

The real-time estimation method of device error parameters by joint rotary modulation of multiple inertial navigation system should be explored, and position error dispersion and long-time stability in the long-time autonomous navigation of multiple inertial guide systems need to be solved. Thus, it is also necessary to design a reasonable device-level redundant configuration to achieve a balance between mass, volume, and reliability. Considering the influence of failure signal and noise signal characteristics of the inertial navigation system and devices on the decision conclusion, the failure mode classification is extended in-depth, and the selection of detection threshold and adaptive adjustment need to be improved.

5) Building of high-precision earth gravity field model and earth rotation parameter mode

High-precision and long-endurance inertial navigation systems urgently need high-precision geophysical field parameter models. The geophysical field compensation of the inertial navigation system requires multidisciplinary integration, which is combined with the frontier knowledge of geophysics, astronomy, and other disciplines to build a more accurate and comprehensive model of the earth's gravity field and rotation parameter.

We introduce the working principle and research progress of RLG and RLG inertial navigation systems, and summarize the study of the six aspects in the long-endurance RLG inertial guidance system technology including error calibration technology, initial alignment technology, rotation modulation technology, high-reliability fault tolerance technology, multi-inertial guidance cooperative positioning technology, and geophysical field compensation technology. With the development of national defense science and technology, the demand for high-precision, long-endurance, and strong self-sustainability navigation systems is becoming increasingly urgent. With the maturity of laser gyro manufacturing and processing technology and further research and analysis on laser gyro error mechanism, the technology system of long-endurance RLG inertial navigation system will be developed more perfectly.