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The successful launch of two new-generation satellites of the BeiDou Navigation Satellite System (BDS), named BeiDou M1-S and BeiDou M2-S, marks another step in expanding BeiDou into a navigation system with global coverage. An initial characterization and analysis of the navigation signals transmitted by BeiDou M2-S is presented. For data collection, a 7.3-m high-gain antenna of the National University of Defense Technology (NUDT) ground station in Changsha (China) has been used to record the signal spectrum and the modulated chip sequences. Based on a prototype receiver, code and phase observations of BeiDou M2-S have also been collected for further analysis. The signal quality and performance of this new-generation BDS satellite are described and analyzed; a backward compatibility with older-generation satellite signals is shown.

Inter-satellite communication and inter-satellite ranging are the foundation of autonomous navigation for satellite navigation systems. Due to cost limitations, it has been proposed in recent years to equip each satellite with one spot beam antenna, which points to different satellites according to a polling mechanism, resulting in an intermittently connected satellite network. This poses the problem of how to design the inter-satellite link (ISL) contact plan, which determines the evolution of network topology and has important effects on system performance. We propose a new framework for the ISL contact plan design in satellite navigation systems. Considering contact plan design as a multi-parameter and multi-objective optimization problem, the cascade optimization design (COD) is proposed as a method simple to implement and optimize the parameters of the contact plan. COD considers network load and geometry of satellites and has a good adaptability. In the proposed framework, both communication and measurement requirements are taken into account while the contact sequence and the slot length are optimized in two steps. Simulation results show that COD guarantees zero packet drops and achieves the least average delay with a selected network load. When the packet arrival rate is 0.45 packets/s, the packet drop ratio of COD is zero, while that of the traditional simulated annealing design (SAD) is 2.58%. The delay of COD is 22.97 s, which is only two-fifth of the SAD value of 58.77 s. At the same time, using COD the average autonomous navigation weighted dilution of precision decreases from 1.4408 to 0.9671. COD also has strong robustness and performance regardless of the onboard buffer size.

A technique for finding a priori position with coarse-time assistance and partial measurements is proposed to improve time-free positioning algorithm. The positioning method can be described as follows. First, three satellites are selected by considering the balance of the geometric properties and search space in estimating the rough position with the altitude hypothesis. Additionally, the relationship between the maximum pseudorange difference and inter-satellite distance is analyzed and can be used to compress the search space. Second, the most reasonable position is chosen using the residual minimum root-sum-of-squares criterion. Finally, full pseudoranges are constructed and the user position can be estimated employing time-free positioning. Experimental results showed that, for sub-millisecond measurements, high success rates of 99.93, 99.75 and 99.58 % were achieved for time offsets of 1, 3 and 5 s, respectively, using the data file `mars3650.14d.z' of the International GNSS Service, while the average number of search candidates was about 672. In addition, the success rate can still reach 100 % even if the time offset is hundreds of seconds with sub-20-ms measurements.