In fact, not only underground tunnels, but also many indoor spaces, such as urban rail transit and shopping malls, have no signal or extremely inaccurate positioning due to satellite signal drift, untimely updating of map POI data and other situations.

　　In particular, with the extensive and in-depth development of satellite navigation applications, its inherent vulnerability will become a lingering barrier in indoor navigation and other application services.

　　On the one hand, the global satellite navigation system, including Beidou, is inherently vulnerable due to its circular orbit system (the orbit height reaches about 20000 km). Due to the high orbit height, the satellite signal is already weak when it reaches the ground. The signal environment is a major event and a worldwide problem.

　　On the other hand, various kinds of covering and shielding in the environmental section, such as landform, hill vegetation, housing construction, urban canyon, can affect or interfere with satellite signals, especially indoors, under tunnels, viaducts, and in all shielded environments, or even in the case that satellite signals cannot enter and reach.

　　Complexity of indoor environment

　　The indoor space environment is complex, and there are more equipment involved in positioning. The requirements for positioning accuracy are much higher than those for outdoor. Especially in some industries involving high-speed mobile equipment positioning, the requirements for accuracy and positioning delay are more stringent. In addition, the application scenario of indoor navigation is fragmented, which greatly increases the cost of updating and maintaining indoor maps.

　　Indoor positioning is one of the core technologies of artificial intelligence in the future, and plays an important role in the coming era of artificial intelligence. The development of effective new indoor positioning technology is a research hotspot in industry and academia. However, it is still a great challenge to achieve accurate, reliable and real-time indoor positioning and meet all kinds of positioning requirements due to the impact of complex indoor environment, spatial layout, topology variability, etc.

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