## 2 The Basic Process of Navigation Inter-Satellite Link Access and Orbit Determination  
The Chinese navigation satellite system consists of 30 satellites, including geostationary orbit (GEO), inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites. It has established a navigation inter-satellite link system capable of precise measurement and data transmission [12, 13]. The process by which extended users access this system involves several stages. First, extended users must submit an access request based on their application requirements. Next, a visibility analysis is conducted according to the beam scanning ranges of both parties to plan the link establishment time slots, the number of links and the allocation of time slot resources in advance. Finally, when the conditions for link establishment are met, both parties use ephemeris data to obtain each other's position and velocity information, completing spatial coordinate alignment to establish the link [14, 15, 16, 17]. The process of extended user access to inter-satellite links includes three stages: link establishment preparation, unidirectional signal reception and bidirectional link establishment with the satellite. Extended users periodically submit usage requests, prompting the navigation satellite system to generate various operational parameters and provide them to the extended users. Daily data exchanges occur between the navigation satellites and extended users, including ephemeris, clock bias parameters and broadcast ephemeris data, enabling inter-satellite link access, as illustrated in Figure 1.  
Accessing the navigation inter-satellite link requires stringent conditions, as extended users must meet specific time and frequency constraints, achieving time-frequency synchronization at the microsecond and hundred-hertz levels. Navigation satellites are equipped with high-precision atomic clocks, providing accurate time and frequency references for the entire satellite and enabling rapid bidirectional communication link establishment. Due to weight and power consumption constraints, extended users cannot be equipped with atomic clocks, resulting in time-frequency accuracy two to three orders of magni  
Figure 1: The process of an extended user accessing the navigation Ka-band inter-satellite link.  
tude lower than that of navigation satellites. Given the limited time slot resources of the inter-satellite link, the extended user leverages its wide-range acquisition capability to capture unidirectional Ka-band signals. It then uses a small number of unidirectional satellite measurements to build a clock bias model, estimate clock bias parameters and apply compensation through time and frequency correction models, achieving high-precision time-frequency synchronization and enabling bidirectional communication.  
Based on bidirectional communication, there are two methods for orbit determination: (1) The extended user collects unidirectional ranging values while in orbit and performs real-time unidirectional orbit determination. This involves acquiring 3-4 inter-satellite link time slots per minute and collecting unidirectional ranging values for one hour per day over two days, developing a dynamic model and state transition matrix for orbit determination. (2) Bidirectional ranging values between the navigation satellite and the extended user are downlinked to ground stations via respective satellite-ground links, enabling post-processing orbit determination on the ground. To ensure timely orbit determination, this study primarily focuses on the research of real-time unidirectional orbit-determination technology for extended users in orbit, achieving an accuracy better than 100 m. The process of extended user access for unidirectional orbit determination is illustrated as follows in Figure 2 [8].  
Figure 2: Orbit-determination process of extended users using the Ka-band inter-satellite link.