Beidou Navigation Satellite System
Just as the U.S. has its Global Positioning System (GPS), Russia has GLONASS, and Europe has its Galileo, China is also constructing its own navigation satellite system known as Beidou (BD, “Big Dipper”). The Beidou Navigation Satellite System, which first became operational in 2000, is being expanded into a full-scale global network to provide positioning and timing services for both military and civil applications.
The PLA Navy first proposed the idea of using Earth-orbiting satellites for navigation purpose 1967, and the conceptual evaluation of a navigation satellite named Dengta 1 (“Lighthouse”) had been completed by November 1970. The development of the satellite was carried out between 1972 and 1979, with engineering prototypes produced for structural, thermal control, and electric testing. The Dengta 1 project was cancelled by the Chinese Government at the end of 1980, as the country shifted its focus to economic development.
In 1983, Chen Fangyun, one of the leading figures in China’s space industry and science community, proposed the concept of a “twin-satellite” navigation system. The system would utilise two satellites, positioned on Geostationary Earth Orbit (GEO) and angled at 60° Longitude from each other, to provide location information for ground users. The use of geostationary satellites instead of medium-orbit satellites means that the system does not require an expensive large constellation of satellites, but it also limits the coverage to areas on Earth where the satellites are visible.
The idea attracted strong interest from the Chinese military and the “twin-satellite” navigation satellite system gained sponsorship from the PLA General Staff Department’s Survey and Mapping Bureau in 1986. On 25 September 1989, the project team successfully demonstrated the working principle of the navigation system by conducting an experiment using two Chinese communications satellites in GEO. In 1993, the Chinese government officially approved the satellite navigation programme to be included in the country’s 9th Five-Year Plan.
The twin-satellite navigation system, officially named the Beidou Navigation Satellite Experimental System, or Beidou-I, entered engineering development in the late 1990s. The first pair of navigation satellites, named Beidou 1A and 1B, was launched in 2000. The system began to provide positioning and communications service in late 2001. A third satellite Beidou 1C was launched in 2003 to serve as a backup, and also a testbed for RDSS and RNSS payloads of the next-generation regional navigation satellite system (Beidou-II). The Beidou-I service became available to commercial users in 2004.
The Beidou-I system covered the region between Longitude 70°E and 140°E, and Latitude 5°N to 55°N. Its space segment consisted of two operational satellites positioned on GEO at 80° E to 140° E, and a backup satellite positioned at 110.5°E. The ground segment included the central control station, three ground tracking stations for orbit determination (at Jiamushi, Khashi and Zhanjiang), ground correction stations, and user terminals (receivers/transmitters).
To determine a user’s location, the central station will need to first send an inquiry signal via the two satellites to the user terminal, which then sends responding signal back to the central control station via the satellites. The central station can then use the time difference between the two signals to calculate the user’s 2D position. This position information was then compared with the digital territorial map stored in the database to get the 3D position data, which was then sent back to the user via satellites using encrypted communications. The user could also transmit encrypted text messages (up to 120 Chinese characters) to the central station via the satellites.
As the system required dual-way transmissions between the user and central station via satellites on GEO, the user segment needed extra space for transmitter and a more-powerful battery. As a result, Beidou-I’s user terminal was much bigger (20 cm antenna), heavier and more expensive compared with GPS user receivers. Additionally, the number of users that could be served by the system simultaneously was limited by the communication capacity of the network.
Beidou-I satellites transmit at 2491.75+/-4.08MHz and the ground receiver can transmit back to the satellite on 1615.68MHz. The Beidou reference-frame is the Beijing 1954 Coordinate System, with time referenced to China UTC as determined in Beijing. The system was able to provide positioning data of 100 m accuracy. By using ground- and/or space-based (the 3rd and 4th satellites) differential methods, the accuracy was increased to under 20 m. The system capacity was 540,000 users per hour, and up to 150 users simultaneously.
The Beidou-I satellite was based on the DFH-3 satellite bus developed by China Academy of Space Technology (CAST). The satellite is 2.2 m x 1.72 m x 2.0 m in size, with a pair of solar panel wings at a span of 18.1 m. The satellite weighed 2,200 kg, and carried 1,100 kg fuel for its rocket motor. Designed life span was approximately 5 to 8 years. The satellites were launched by the CZ-3A launch vehicle from the Xichang Satellite Launch Centre (XSLC).
In 2004, the Chinese government approved the programme to construct an operational navigation satellite system comparable to the GPS and GLONASS in capability. The system was initially to offer high-precision positioning and navigation services to users in the Asia-Pacific region by 2014, and will be expanded into a global network by 2020. The regional navigation satellite system, known unofficially as Beidou-II or Compass, was activated on a trial basis on 27 December 2011 and became fully operational in December 2012.
The Beidou-II system consists of 14 satellites, including 5 placed on GEO, 5 on Inclined (55°) Geosynchronous Orbit (IGSO), and 4 on circular 21,500 km Medium Earth Orbit (MEO). The ground segment includes a Central Control Station responsible for satellite constellation control and processing the measurements received by the Monitor Stations to generate the navigation message; two Upload Stations responsible for uploading the orbital corrections and the navigation message to Beidou satellites; and 30 Monitor Stations responsible for collecting Beidou data for all the satellites in view from their locations.
The system offers positioning data to users located between Longitude 55°E to 180°E and from Latitude 55°S to 55°N. Currently it has two types of services: a free Open Service and a fee-charging Licensed Service. The open service is designed to provide a positioning accuracy of under 10 m in central China (or 20 m in neighbouring countries), velocity accuracy of within 0.2 m/s and timing accuracy of within 50 nanoseconds in the single-way working mode (or within 2 nanoseconds in the dual-way working mode). The Licensed Service, intended for government and military users, provides positioning and timing services with higher accuracy, though no details regarding the actual accuracy have been published. The system also offers a text messaging service similar to that of Beidou-I.
Beidou-II employs three variants of satellites: Compass-G, Compass-IGSO, and Compass-M.
The Compass-G is a geostationary satellite developed by CAST based on its DFH-3B platform. The satellite consists of the service module and payload module, and has a designed lifespan of 8 years. The onboard telemetry is unified C-/S-band. The propulsion is a 490 N motor. The satellite is launched on a CZ-3C launch vehicle from Xichang, carrying a single satellite.
The Compass-IGSO is similar to the geostationary satellite in design, but is deployed in a 55° inclination geosynchronous orbit.
The Compass-M is deployed in 21,500 to 24,100 km, 55° inclination MEO (intermediate circular orbit). The first experimental Compass-M satellite launched in April 2007 was based on the DFH-3B bus, but later operational satellites were based on a dedicated MEO satellite bus, which has a gross mass of 800 kg, and a mission payload of 280 kg. The satellite is three-axis stabilised and has a power output of 1,500 kW. The satellite can be carried on a liquid-propellant upper stage (satellite dispenser), allowing two or more satellites to be launched onboard a single launch vehicle.
Compass-G Compass-IGSO Compass-M ------------------------------------------------------------------------- Satellite bus DFH-3B DFH-3B DFH-3 Mass (gross) 4,600 kg 4,200 kg 2,200 kg Propellant mass 3,050 kg 2,300 kg 2,160 kg Power output 6,800 W 6,200 W 3,000 W Designed life 8 years 8 years 8 years Launch vehicle CZ-3C CZ-3A CZ-3A
Compass satellites carry two types of payloads: Radio Determination Satellite Service (RDSS) and Radio Navigation Satellite Service (RNSS). The RDSS payload is equipped on the Compass-G satellites only. The package includes a high-power S-band transponder, an L-band low-noise amplifier, frequency generator, a large L-/S-band antenna, and a C-band antenna. The RNSS payload, equipped on all Compass satellites, includes an atomic clock, an L-band transmitter, signal processor, transmitter antennas array, an L-band uplink receiver, and laser corner-cube reflector for orbit determination.
In 2015, the Beidou Navigation Satellite System began its transition towards global coverage, with the launch of the first satellite Beidou III-I1-S on 30 March. The satellite is the 17th in the entire Beidou programme and the first of 35 high- and medium-orbit satellites to be launched before 2020 to establish a global navigation satellite network. It was designed and built by the Shanghai Engineering Centre for Microsatellites (SECM), an R&D institution under the China Academy of Sciences (CAS).
The mission was also the first debut of the Yuan Zheng-1 (YZ-1, “Expedition-1”) upper stage, which was added to the three-stage CZ-3C rocket to provide the capability to directly insert the satellite into a high orbit without the need to use the satellite’s own propulsion. This stretched the total length of the launch vehicle to 57.126 m, with the satellite and upper stage accommodated inside an enlarged payload fairings 4.2 m in diameter. The dual-engine cryogenic third-stage boosted the stack into a transfer orbit of 194 by 25,307 km inclined at 55°. After separation from the third-stage that remained in this orbit, the liquid-fuelled YZ-1 began its first burn to raise the apogee of the orbit to 36,000 km, eventually reaching a geosynchronous orbit inclined at 55°, five hours 42 minute after lift-off.
Beidou-III M1-S and M2-S
On 25 July 2015, a four-stage CZ-3B/YZ-1 launch vehicle carried two new-generation MEO navigation satellites into space: Beidou III M1-S and M2-S. After 3.5 hours of flight, the YZ-1 upper state placed the two satellites into orbit. The two satellites were developed by the China Academy of Space Technology (CAST), based on a newly-developed MEO navigation satellite bus. They feature a phased array antenna for navigation signals and a laser retroreflector. As they have no apogee propulsion system, they require the launch vehicle to perform the final orbit insertion.
On 30 September 2015, a CZ-3C launch vehicle lofted Beidou III I2-S into orbit. The CAST-developed satellite carried China’s first hydrogen maser clock and would be used to conduct a number of testing including atomic clock, inter-satellite link, and a new type of navigation signal.
On 1 February 2016, a CZ-3C/YZ-1 launch vehicle lofted Beidou III M3-S into orbit. The SECM-developed satellite was the last of the five engineering experimental satellites to demonstrate the various technologies required for constructing a global navigation satellite system (GNSS).
|1||2000-10-31||CZ-3A||Xichang||Beidou 1A||CAST||GEO 140°E||Retired Dec 2011|
|2||2000-12-21||CZ-3A||Xichang||Beidou 1A||CAST||GEO 80°E||Retired Dec 2011|
|3||2003-05-25||CZ-3A||Xichang||Beidou 1C||CAST||GEO 110.5°E||Retired Dec 2012|
|4||2007-02-03||CZ-3A||Xichang||Beidou 1D||CAST||GEO 58.75°E||Retired Feb 2009|
|5||2007-04-14||CZ-3A||Xichang||Compass-M1||CAST||MEO, 21,500 km||Retired|
|6||2009-04-15||CZ-3C||Xichang||Compass-G2||CAST||GEO||Lost control in 2009|
|9||2010-08-01||CZ-3A||Xichang||Compass-IGSO1||CAST||55° inclination IGSO||Operational|
|11||2010-12-18||CZ-3A||Xichang||Compass-IGSO2||CAST||55° inclination IGSO||Operational|
|12||2011-04-10||CZ-3A||Xichang||Compass-IGSO3||CAST||55° inclination IGSO||Operational|
|13||2011-07-27||CZ-3A||Xichang||Compass-IGSO4||CAST||55° inclination IGSO||Operational|
|14||2011-12-02||CZ-3A||Xichang||Compass-IGSO5||CAST||55° inclination IGSO||Operational|
|16||2012-04-30||CZ-3B/E||Xichang||Compass-M3||CAST||MEO, 21,500 km||Operational|
|17||2012-04-30||CZ-3B/E||Xichang||Compass-M4||CAST||MEO, 21,500 km||Operational|
|18||2012-09-18||CZ-3B/E||Xichang||Compass-M5||CAST||MEO, 21,500 km||Operational|
|19||2012-09-18||CZ-3B/E||Xichang||Compass-M2||CAST||MEO, 21,500 km||Operational|
|21||2015-03-30||CZ-3C/YZ-1||Xichang||Beidou-III I1-S||SECM||55° inclination IGSO||Operational|
|22||2015-07-25||CZ-3B/YZ-1||Xichang||Beidou-III M1-S||CAST||MEO, 21,500 km||Operational|
|23||2015-07-25||CZ-3B/YZ-1||Xichang||Beidou-III M2-S||CAST||MEO, 21,500 km||Operational|
|24||2015-09-30||CZ-3B||Xichang||Beidou-III I2-S||CAST||55° inclination IGSO||Operational|
|25||2016-02-01||CZ-3C/YZ-1||Xichang||Beidou-III M3-S||SECM||MEO, 21,500 km||Operational|
|26||2016-03-30||CZ-3A||Xichang||Compass-IGSO6||CAST||55° inclination IGSO||Operational|