China's BeiDou navigation system (BDS) has evolved from the demonstration navigation satellite system (BDS-1) to the regional navigation satellite system (BDS-2). Now, the global BeiDou navigation system (BDS-3) is in construction and is proceeding well. The design and functions of BDS-3 are quite different from those of both BDS-1 and BDS-2. In this paper, the general design, the coordinate reference system, and the system time basis of BDS-3 are introduced. Several new payloads designed to accomplish different objectives are described as well as the platforms on which they are hosted. Since BDS-3 consists of several different constellations, the general service capabilities and special service functions provided by these different constellations are described. The performances of the initial BDS-3 platforms are evaluated based on the available eight-medium Earth orbit (MEO) satellite configuration. The results of satellite orbit determination and prediction with and without the BDS-3 inter-satellite links (ISL) are compared and analyzed.
We report on the Compass global satellite navigation system and its time reference system. China has sent three satellites into geostationary orbit since 2000, and the Compass Navigation Test System has been established. Compass time reference, named as BDT, is based on atomic time; BDT is derived from the atomic clock ensemble in Compass ground control centre and can be traced to the international time, UTC.
Search and rescue (SAR) service is one of the major services provided by BDS-3 system. Six BDS-3 MEOSAR payloads are optimally distributed on three orbits and work normally. BDS-3 system uniquely adopts the architecture of Regional Up-Link Stations and crosslinks between satellites to provide return link service (RLS) for distressed people worldwide. BDS-3 B2b signal is designed to support the return link message (RLM) broadcasting, positioning, navigation and timing. A variable-length message type of RLM is specially designed to enrich SAR operation information categories and improve SAR operation efficiency. The performance of MEOSAR, RLS and positioning is evaluated through numerical simulation and preliminary in-orbit test. According to the results, the global availability is over 98.79%, with only six BDS MEOSAR payloads. The test indexes of BDS MEOSAR payloads meet the COSPAS-SARSAT MEOSAR space segment commissioning standard requirements. BDS RLS time delay is less than 2 min, which demonstrates the efficiency of BDS RLS architecture. And B2b positioning accuracy is better than 10 m.
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