SIS accuracy and service performance of the BDS-3 basic system

On December 27, 2018, the basic system of the third-generation BeiDou navigation satellite system (BDS-3) completed the deployment of its constellation of 18 MEO networking satellites as well as the construction of the operation control system (OCS) and began to provide basic navigation services to users worldwide. Compared with BDS-2, BDS-3 aims to offer users better navigation signals and higher precision with a series of new technologies. For example, the spaceborne atomic clock of BDS-3 is upgraded for higher performance, the Ka-band inter-satellite link is adopted for inter-satellite ranging and communication, and new B1C and B2a signals are broadcast in addition to B1I and B3I signals (compatible with BDS-2). In addition, a 9-parameter model based on a spherical harmonic function is employed for ionospheric delay corrections. Using the observation data from 18 satellites of the basic system, this paper conducts a comprehensive evaluation of the pseudorange measurement characteristics, signal-in-space (SIS) accuracy of navigation messages and global service capability of BDS-3. The results indicate that the pseudorange measurement multipath effect and observation noise of BDS-3 satellites are better than those of BDS-2; additionally, with the support of inter-satellite links, the user range error (URE) of the BDS-3 satellite broadcast ephemeris is better than 10 cm, the precision of the broadcast clock parameter is better than 1.5 ns, and the SIS accuracy is better than 0.6 m overall. Different from the traditional Klobuchar model, the BeiDou global broadcast ionospheric delay correction model (BDGIM) can provide ionospheric delay corrections better than 70% for worldwide single-frequency users. The service capability evaluation of the basic system consists mainly of the accuracy improvement of the B1I and B3I signals according to BDS-2 as well as the global positioning accuracy of the new signals. These results prove that the BDS-3 basic system has achieved the design goal; that is, both the horizontal and the vertical global positioning accuracies are better than 10 m (95%). In the future, 6 MEO satellites as well as 3 GEO satellites and 3 IGSO satellites for regional enhancement purposes will be deployed for full operation; consequently, BDS-3 will definitely provide a higher SIS accuracy and better service capability.