## 1 Introduction  
A spatio-temporal network is a strategic symbol of comprehensive national strength, and GNSS and mobile communication networks are important components of the spatio-temporal network system. The global location-based service market size will grow from USD 70.27 billion in 2022 to USD 88.42 billion in 2023, with a compound annual growth rate of 25.8% [1]. The promotion of the deep integration of GNSS system and mobile communication network, the establishment of a new type of ubiquitous, reliable, and integrated location service system platform, and the formation of a highly reliable spatio-temporal network have become an inevitable trend. In recent years, many countries have carried out a major deployment of space-time service systems using GNSS, next-generation mobile communications, and big data to strengthen comprehensive information service capabilities and comprehensively improve the level of the space-time infrastructure'sdevelopment. Building a highly dependable spatio-temporal network is urgently needed to solve the problems of network heterogeneity, data-heterogeneity fusion, and trusted positioning under the spatio-temporal network architecture. The scenario of GNSS and 5G hybrid positioning is shown in Figure 1.  
GNSS can provide users with real-time position, speed, and time information. However, with users' increasing requirements with respect to positioning accuracy and robustness, the existing satellite positioning technology can no longer meet the needs of users in the face of complex scenes with high occlusion, high multipath interference effects, and high Non-Line-Of-Sight (NLOS) characteristics such as dense buildings and indoor/outdoor switching [2]. The fusion of GNSS systems with other systems, such as inertial navigation systems [3; 4; 5], visual navigation systems [4; 5], local sensor networks [6; 7], or wireless communication networks [2; 8; 9; 10; 11; 12], is a feasible option at this time. Numerous scholars have conducted intensive research on this issue. Additional cumulative errors will lead to a decrease in accuracy in the inertial navigation system. The light environment drastically restricts the visual navigation system. Local sensor networks such as Bluetooth and UWB should be deployed independently. The 5G network is a mobile communication system, without the aforementioned deficiencies, that has a high base station density, broad indoor and outdoor coverage, and a wide communication bandwidth. Therefore, it is capable of precisely measuring the relative distance between the base station and the user terminal. The combination of 5G and GNSS signals can effectively address the problems of limited GNSS coverage and poor positioning accuracy in urban canyons. The GNSS has fully served many key industries, such as national defense and military, mass fields, transportation, public safety, disaster relief and mitigation, agriculture, forestry, animal husbandry and fishery, and urban governance. However, the service is limited or even blocked in indoor, underground, urban canyon, and other sheltered environments. The resources of the 5G mobile communication network are an important supplement to GNSS, and the deep integration of 5G and GNSS is an effective way to realize a ubiquitous and credible spatio-temporal information service. The positioning standards of R16 and R17 have been established in the 5G mobile communication network, and the R18 standard is being studied, which has the potential of sub-meter-level positioning, and its positioning signals can cover indoor and outdoor environments, underground environments, tunnels, urban canyons, and other environments and can become a supplement to the GNSS under a fully occluded environment (indoor, underground, tunnels, etc. ), a means of integration and enhancement between a semi-occluded (urban canyons, etc.) and open environment, thus expanding the service scope and scope of GNSS. The GNSS and 5G joint-positioning methods are mainly divided into three types.