To keep the global navigation satellite system functional during extreme conditions, it is a trend to employ autonomous navigation technology with inter-satellite link. As in the newly built BeiDou system (BDS-3) equipped with Ka-band inter-satellite links, every individual satellite has the ability of communicating and measuring distances among each other. The system also has less dependence on the ground stations and improved navigation performance. Because of the huge amount of measurement data, the centralized data processing algorithm for orbit determination is suggested to be replaced by a distributed one in which each satellite in the constellation is required to finish a partial computation task. In the present paper, the balanced extended Kalman filter algorithm for distributed orbit determination is proposed and compared with the whole-constellation centralized extended Kalman filter, the iterative cascade extended Kalman filter, and the increasing measurement covariance extended Kalman filter. The proposed method demands a lower computation power; however, it yields results with a relatively good accuracy.

The continuous full operation of the constellation of BeiDou navigation satellite system (BDS) provides favorable conditions for the performance evaluation of the BDS inter-satellite links (ISLs). The primary features of ISLs that affect the precision of precise orbit determination (POD) and time synchronization include (i) the spatiotemporal coverage or continuity of observations; (ii) the observational accuracy, such as observation noise and bias; and (iii) the observational geometry represented by dilution of precision. After comparing some technical features and the current status of the Global Navigation Satellite Systems ISLs, the measurement principle of dual one-way ISLs for BDS and its data processing methods are presented. The performance evaluation involving the above three aspects was carried out using 14 days of ISL data, with some typical indicators derived. POD based on data fusion of ISLs and ground-based L-band monitoring stations was conducted, with root-mean-square of posterior residuals of about 5.0 cm, and MEO radial accuracy better than 1.0 cm. The results show that ISLs offer crucial support for BDS to provide global high-precision services under regional monitoring network conditions.

The main functions of the space information system, such as providing the backbone transmission, broadband access, and global connectivity, are realized based on the network topology. Thus, it is necessary to recognize the temporal dynamics of the network topology. A temporal continuity expression method is proposed to describe the topological dynamic characteristics of the network in space information systems. Based on orbit dynamics, a time-dependent adjacency matrix of the space information system can be established by introducing the geometric linkable factor, the link distance intensity factor, and the relative angular velocity factor of the node. The adjacency matrix describes the dynamic characteristics from two layers: one is the physical layer using a time-dependent function, which represents the feasibility of inter-satellite link construction in the system cycle; the other one is the transport layer, described by a piecewise continuous function that varies with time, which characterizes the link quality during the connection period between two satellites. The results show that compared with the existing network topology description methods, the proposed method describes the network topology more accurately, which can distinguish the network topology characteristics at any time, and is more conducive to the understanding and application of the network topology of the space information system.

The Ka-band Inter-Satellite Links (ISL) has been introduced as an innovative measurement technique for orbit determination (OD) and time synchronization (TS) of the global BeiDou Navigation Satellite System (BDS-3), complementing traditional L-band ground-satellite tracking. For the first time, OD and TS have been conducted for the entire full-operational constellation of BDS-3 using raw one-way ISL data. The normal equation (NEQ) accumulation technique is employed to estimate critical parameters, such as the ISL hardware delays (Ka-biases), to derive long-term average solutions. With the support of ISL, an orbital accuracy of 8.0 cm for Medium Earth Orbits (MEOs), 11.6 cm for Inclined Geosynchronous Orbits (IGSOs), and 24.0 cm for Geostationary Orbits (GEO) can be achieved using a ground network consisting of 20 stations. An extended observation model was developed for ISL heterogeneous data at both ends due to the presence of a specialized GEO satellite C61 which exclusively provides ISL data without broadcasting civil L-band signals. Subsequently, the OD and TS capabilities were validated using one-way ISL for GEO C61, resulting in an OD accuracy of approximately 3.7 cm in the radial direction, 7.9 cm in the normal direction, 27.7 cm in the tangential direction, and 29.0 cm in 3-D position; with a TS accuracy of around 0.1 ns. In most cases, one-way ISL transmission and reception Ka-biases remain stable over considerable periods, achieving an accuracy of 0.2 ns. However, apparent abnormal jumps in Ka-bias were observed, which were determined to be sudden changes in L-band hardware delay from navigation signals rather than jumps in ISL hardware delay itself. This finding marks the distinction between L-band hardware delay jumps and atomic clock jumps for navigation satellites. Additionally, the residuals of one-way ISLs exhibit periodic terms. Investigations indicate that these residuals vary with the antenna azimuth and elevation angles of each link and are specific to individual satellites. This variation may be attributed to small deviations in the on-board ISL receivers or antennas. This discovery is valuable for modeling ISL observations.