II. A World-Class Navigation Satellite System
Aiming to build a world-class global navigation satellite system, China has adhered to an innovation-oriented approach, pursued excellence and kept improving its technologies. BDS is now a top-class system characterized by cutting-edge technologies, pioneering design, and powerful functions.
1. Independently Developed Core Technologies
China has achieved innovations in the constellation configuration, technology systems and service functions of the navigation satellite system conforming to international norms. It has made breakthroughs and reached a world-leading level in several core technologies in terms of hybrid constellation, inter-satellite links, and signal structure.
Innovative constellation configuration. China is the first country to adopt a hybrid navigation constellation in medium and high earth orbits. A satellite in a high orbit covers a larger area and has a better anti-shielding capability; those in medium orbits offer global coverage, and are core to providing global services. By complementing each other, satellites in multiple orbits have a greater capability to serve not only the region, but also the entire world.
Inter-satellite links. This is China's first inter-satellite and satellite-ground joint network, thus realizing high-precision measurement and data transmission through inter-satellite links, and providing global services based on domestic ground stations.
Optimized signal structure. With breakthroughs in core technologies such as modulation, multiplexing, and channel coding, BDS takes the lead in using triple-frequency signals, in integrating navigation and positioning with communication functions, and in providing both basic navigation information and differential augmentation information. It has reached world-class level in signal ranging accuracy, anti-interference performance, and anti-multipath capabilities. It is compatible with other navigation satellite systems and capable of providing diversified and specialized services.
2. Innovative System Configuration
BDS is composed of a space segment, a ground segment, and a user segment:
– The space segment comprises 30 satellites located in three types of orbit – MEO, GEO and IGSO;
– The ground segment mainly consists of the operation control system, the telemetry tracking and command system, the inter-satellite link operation management system, and various service platforms for international search and rescue, short message communication, satellite-based augmentation, and ground-based augmentation.
– The user segment consists of terminals and applications compatible with other navigation satellite systems.
Building the BDS integrated inter-satellite and satellite-ground network, China's first aerospace system offering global network services, has led to remarkable improvements in China's aerospace R&D capability, and leapfrog progress in China's aerospace technology.
Outstanding batch production capability. Innovations have been introduced in the research and manufacturing of satellite-ground equipment as well as satellites and launch vehicles. Carrier rocket upper stages and navigation satellite platforms have been developed to enable batch production, intensive launch of satellites and rockets, and fast constellation deployment. In less than three years, 18 rockets deployed 30 satellites into orbit, a pace unmatched by any other country.
Independent development and operation of key components. Components such as aerospace-grade memory chips, satellite-borne processors, high-power microwave switches, traveling-wave tube amplifiers, and solid-state amplifiers have been developed and produced independently by China. The 100 percent independent development and operation of BDS core components has laid a solid foundation for its widespread use.
3. Quality and Diverse Services
With its powerful functions and reliable performance, BDS is able to provide a wide range of services to meet various requirements. It mainly provides positioning, navigation, timing, international search and rescue, and short message communication services to global users, while it offers short message communication, satellite-based augmentation, precise point positioning, and ground-based augmentation services to users in the Asia-Pacific.
Positioning, navigation and timing. Through 30 satellites, BDS offers free services to users across the world. Its positioning accuracy is better than 9 meters horizontally and 10 meters vertically, its velocity measurement accuracy is higher than 0.2 meters per second, and its timing is accurate to less than 20 nanoseconds.
International search and rescue. Through six MEO satellites, BDS provides, free of charge, a global distress alert service that meets international standards. Return links have been designed so that rescue requesters can receive confirmation messages.
Global short message communication. BDS is the first navigation satellite system to provide a global short message communication service. Through 14 MEO satellites, it provides a global random-access service to specific users with a maximum single message length of 560 bits (40 Chinese characters).
Regional short message communication. BDS is the first navigation satellite system to provide a regional short message communication service for authorized users. Through three GEO satellites, it provides a data transmission service to users in China and neighboring areas with a maximum single message length of 14,000 bits (1,000 Chinese characters). It can transmit texts, graphics and audio.
Satellite-based augmentation. Through three GEO satellites, BDS provides users in China and neighboring areas with a Category I precision approach service that meets international standards. It supports both single-frequency and dual-frequency multi-constellation augmentation, providing greater security for the transport sector.
Precise point positioning. Through three GEO satellites, BDS provides users in China and neighboring areas with a cost-free high-precision positioning augmentation service. Its positioning accuracy is better than 30 centimeters horizontally and 60 centimeters vertically, and its convergence time is less than 30 minutes.
Ground-based augmentation. Network ground stations have been built across China to provide real-time meter-level, decimeter-level, centimeter-level, and post-processing millimeter-level positioning augmentation services for industrial users and the general public.
