# Development Status and Service Performance Preliminary Analysis for BDSBAS
## 1 Introduction  
The satellite-based augmentation system (SBAS) can broadcast ephemeris and clock error corrections, ionospheric delay corrections and the corresponding integrity information to users through geostationary orbit satellites (GEO), to improve the accuracy, integrity, continuity and availability of the Global Navigation Satellite System (GNSS) core constellations service. Because of the benefits of the augmentation service, including large service areas and relatively low construction and maintenance costs, many countries and regions have established SBAS systems to meet the navigation performance requirements of high real-time and integrity applications such as aviation users for all phases of flight, from en route through category I approach [1]. Current global SBAS operational systems include the Wide Area Augmentation System (WAAS) of the United States which has been operational since 2003 and can supportCAT I-like approach capability (LPV-200) [2; 3]; Japan's Multi-functional Transport Satellite-based Augmentation System (MSAS) has been operational since 2007 and supported non-precision approach operations [4; 5; 6]; the European Geostationary Navigation Overlay Service (EGNOS) system in European Union whose precision approach operations capability began in 2011 [7; 8; 9]; both the Indian Global Positioning System (GPS) Aided Geostationary Earth Orit Augmented Navigation (GAGAN) system [10; 11] and the Russian System of Differential Correction and Monitoring (SDCM) system [12; 13] are in development with plans for horizontal and vertical guidance. In addition to the system already in operation, there are some future SBAS under development: the BeiDou Satellite-Based Augmentation System (BDSBAS) by China, the Korea Augmentation Satellite System (KASS) by South Korea, the SBAS for Africa and Indian Ocean (A-SBAS) and the Australian SBAS (AUSBAS) [14]. BDSBAS is an important part of the BDS system, and provides services for users in China and the surrounding areas. It will be an important supplement to the availability of Global SBAS services. Since civil aviation is the most demanding user for SBAS, the International Civil Aviation Organization (ICAO) established standards and recommended practices (SARPs) providing overarching standards and guidance for global SBAS implementation, and organized the SBAS Interoperability Working Group (IWG) for SBAS service providers to assure common understanding and implementation of the SARPs [15; 16; 17]. Incorporation of the ICAO SARPs has become one of the most important jobs of BDSBAS system construction, and BDSBAS provides two kinds of augmentation service: single-frequency (SF) service and dual-frequency multi-constellation (DFMC) service, both in accordance with ICAO standards [18; 19]. The BDSBAS SF service augments GPS constellation and meets the RTCA minimum operational performance standards (RTCA MOPS) which defined the GPS L1 C/A signal minimum performance, functions and characteristics. The service provides GPS satellite ephemeris and clock error corrections referenced to the L1 NAV message, ionospheric grid delay corrections, integrity information of corrections, GEO navigation and almanacs, degradation factors and clock-ephemeris covariance matrix, and uses BDSBAS-BIC signal of GEO satellites to broadcast the augmentation messages to GPS/SBAS users, aiming to support APV-I precision approach [19]. The BDSBAS DFMC service is compatible with the SBAS L5 DFMC protocol and the augmentation messages are transmitted by GEO satellites through BDSBAS-B2a signal to augment BDS/GPS/Galileo/GLONASS dual-frequency signals. For BDS, it is recommended to use BIC and B2a dual-frequency signals and the ephemeris corrections are referenced to the BIC CNAV1 navigation message. For GPS, it is recommended to use L1 C/A and L5 dual-frequency signals and the ephemeris corrections are referenced to the L1 NAV navigation message.