I. Introduction

The global navigation satellite system (GNSS) has developed rapidly in the past few decades. It mainly includes the global position system (GPS) of the United States, the Global Navigation Satellite System (GLONASS) of Russia, the Galileo Navigation Satellite System (Galileo) of Europe and the BeiDou Navigation Satellite System (BDS) of China [1]. These navigation systems are usually composed of many navigation satellites. The GNSS receiver can calculate the distance from the satellite to the GNSS receiver by receiving the satellite signal. After receiving the signal of at least four satellites, we can get the absolute positioning of the GNSS receiver. The positioning accuracy of the GNSS system will not diverge over time, but its positioning accuracy is not high, and the positioning output frequency is low, usually 1 Hz. The inertial navigation system (INS) is a relative positioning system, which is based on the output of the inertial measurement unit (IMU) to calculate the position, velocity, attitude, and other information of the carrier. It is not affected by the external environment and is only related to the performance of itself. It has high precision in a short time and high output frequency, usually above 50 Hz. However, because it is based on the information of the previous moment, it is easy to cause cumulative error. Therefore, INS is not suitable for long-term positioning.