BeiDou Navigation Satellite System
Overview
The BeiDou Navigation Satellite System (BDS) is a global navigation satellite system (GNSS) operated by the People's Republic of China. Named after the Big Dipper constellation, known as BeiDou in Chinese, the system began providing services to customers in China and the Asia-Pacific region in December 2012, and globally in December 2018. As of 2020, it consists of 35 operational satellites, making it the fourth operational GNSS, after the Global Positioning System (GPS) of the United States, GLONASS of Russia, and Galileo of the European Union.
Development and Deployment
The development of the BeiDou system has been carried out in three phases. The first phase, known as BeiDou-1, consisted of three satellites and offered limited coverage and applications. The second phase, BeiDou-2, also known as Compass, became operational in China in December 2011 with a partial constellation of 10 satellites in orbit. The third phase, BeiDou-3, began deployment in 2015 and was completed in 2020 with a total of 35 satellites in orbit, providing global service.
System Structure
The BeiDou Navigation Satellite System consists of three segments: the space segment, the ground segment, and the user segment. The space segment is composed of satellites in different orbits, including Geostationary Orbit (GEO), Inclined Geosynchronous Orbit (IGSO), and Medium Earth Orbit (MEO). The ground segment consists of master control stations, time synchronization and frequency standard stations, and monitoring stations distributed around the world. The user segment includes various types of user receivers.
Satellite Design
The BeiDou satellites are based on a modular design, with a hexahedral satellite body. The satellites are three-axis stabilized and use sun sensors, star sensors, and inertial measurement units for attitude determination. The satellites are equipped with rubidium and hydrogen maser atomic clocks for on-board timekeeping. The satellite payload includes a navigation antenna, a laser retroreflector array, and various types of transponders.
Signal Structure
The BeiDou system uses several carrier frequencies in the L-band, with the main signals transmitted at 1.561098 MHz, 1.589742 MHz, and 1.26852 MHz. The system uses Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), and Binary Offset Carrier (BOC) modulation schemes. The navigation message includes ephemeris data, clock correction parameters, ionospheric delay model parameters, and almanac data.
Applications
The BeiDou system provides a variety of services, including positioning, navigation, and timing (PNT) services, short message services (SMS), and international search and rescue (SAR) services. The system is used in many fields, such as transportation, agriculture, fishing, disaster relief, and scientific research.
Performance
The performance of the BeiDou system is comparable to that of other GNSS systems. The system provides positioning accuracy of better than 10 meters, velocity accuracy of better than 0.2 meters per second, and timing accuracy of better than 20 nanoseconds. The system also provides a short message service with a message capacity of 120 Chinese characters.
Future Development
China plans to continue the development and improvement of the BeiDou system. The next generation system, known as BeiDou-4 or BDS-4, is planned to be a more advanced global navigation satellite system with better performance and more services. The BDS-4 system is expected to be operational around 2035.