CNSS (Compass/BeiDou Navigation Satellite System) / BDS (BeiDou Navigation System)
In the first decade of the 21st century, China is in the process of developing its own navigation satellite constellation known as CNSS (Compass Navigation Satellite System), or BeiDou-2 in its Chinese name. BeiDou is the Chinese name of the Big Dipper constellation. The BeiDou-2 system will be based on its current regional Compass Satellite Navigation Experimental System (BeiDou-1). 1)
Although the program was initiated by China’s military forces, China established an agency, namely CSNPC (China Satellite Navigation Project Center) in the timeframe 2006/7, to take charge of the research, building, and management of CNSS.
The BeiDou-2 program was officially started in 2005 and announced by China in October 2006. China gave also to a proper name to the rest of the world of its BeiDou-2 system, namely CNSS (Compass Navigation Satellite System) - BeiDou literally means “Compass”. The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe.
China has joined the UN-supported ICG (International Committee on GNSS) and has taken part in the ICG’s Providers Forum(founded in Sept. 2007, Bangalore, India). The members of the Forum and their current and future systems include: China (CNSS), EU (Galileo, EGNOS), India (IRNSS, GAGAN), Japan (QZSS, MSAS), Russia (GLONASS), and USA (GPS, WAAS). The goal of the Providers Forum is to foster compatibility of the services/signals and interoperability among the various navigation systems for a better service to the user community. 2) 3)
The Chinese plans call for a deployment of the full constellation by 2012. A more realistic timeframe for global service provision is probably 2015 (Ref. 12). China plans call also for a long-term and sustained navigation service and system performance for the global user community once the system is in full operation. 4) 5)
In 2009, three BeiDou-2 spacecraft are planned to be launched. More than 10 satellites will be put into service within the next two years by Long-March launchers. Around 2011, the system will be capable to offer services on a regional scale.
Background: The BeiDou program began in 1983 with a proposal by Chen Fangyun to develop a Twinsat regional navigation systemusing two geostationary satellites. The concept was proven in 1989 in a test using two in-orbit DFH-2/2A communications satellites. This test showed that the precision of the Twinsat system would be comparable to the American GPS (Global Positioning System). In 1993, the BeiDou program was officially started. Experimental launch of the first two indigenous BeiDou navigation satellites was in 2000. The final BeiDou constellation was to consist of four geosynchronous satellites, two operational and two backups. 6)
• In Sept. 2003, the EU (Europen Union) and China agreed to cooperate to develop the EU Galileo Satellite System. The United States reacted with strong skepticism since Washington was against the sharing of sensitive dual-use technology (of civilian and military applications) with China. 7)
• In the timeframe 2006/7, the BeiDou-2 program of China plans to place a portion of the Compass/BeiDou signals on the same frequency planned for Galileo’s PRS (Public Regulated Service).
It’s much the same situation as arose a few years ago when Europe originally proposed to overlay its L1 open service signals on the new GPS military (M-code) signal. That potential conflict was eventually resolved in the course of negotiating the 2004 Galileo/GPS cooperation agreement between the United States and the European Union. 8)
• In March 2009, China and Europe have made no concrete progress in negotiating how to harmonize their planned satellite navigation systems. Each side is trying to protect its abilities in jam the other's military frequencies in a time of conflict without jamming its own. Europe calls its encrypted and protected Galileo signals PRS (Public Regulated Service) and the military is considered to be the largest future market for it. China refers to its protected code as an “Authorized Service” to be used in “complex situations.” 9)
• According to the China National Space Administration (CNSA) and CSNO (China Satellite Navigation Office) , the development of the Chinese global navigation system will be carried out in three phases (Ref. 37): 10)
1) Phase-I is BeiDou Navigation Satellite Demonstration System, which has been established in the period 2000-2003. The experimental BeiDou navigation system consisted of 3 satellites.
2) Phase-II is the regional BeiDou navigation satellite system covering China and neighboring regions by 2012. By 2014, the system will be ready to initially offer high-precision positioning and navigation services to the Asia-Pacific region.
3) Phase-III is the BeiDou navigation satellite system to be established completely and provide global service by 2020.
BeiDou-1 regional satellite navigation system
The first generation regional demonstration system, referred to as BeiDou-1, is made up of 4 spacecraft, is experimental and has limited coverage and application. Unlike the GPS, GLONASS, and Galileo systems, which use MEO (Medium Earth Orbit) satellites, BeiDou-1 uses satellites in GEO (Geostationary Earth Orbit). This means that the system does not require a large constellation of satellites, but it also limits the coverage to areas on Earth where the satellites are visible. The area that can be serviced is from 70ºE to 140ºE, and from 5ºN to 55ºN.
The BeiDou-1 spacecraft was developed on the basis of the DFH-3 satellite bus. It has a size of 2.2 m x 1.72 m x 2.0 m. The pair of solar panels has a span of 18.1 m. The spacecraft launch mass is 2,200 kg which includes 1,100 kg of propellant for its main engine. The design life is about 5 years.
Launch: All BeiDou-1 satellites were launched from the XSLC (Xichang Satellite Launch Center) in the southwestern Sichuan Province of China, using China’s Long March 3A (CZ-3A) launch vehicles.
Table 1: Overview of the BeiDou 1st generation satellites
Figure 1: Illustration of the GEO/IGSO spacecraft (image credit: DFH)
The BeiDou-1 system began to provide experimental navigation and positioning services in late 2001; it became “operational” with the launch of the third spacecraft, BeiDou-1C, in May 2003. The navigation and positioning services became available to civilian users in April 2004. This has made China the third country in the world to have deployed an operational spaceborne navigation and positioning network.
The BeiDou-1 network covers most areas of East Asia region and has both navigation and communication functions. The system will be able to provide navigation and positioning services to users in China and its neighboring countries by 2008. The satellite network comprises three BeiDou-1 satellites (2 operational and 1 backup).
The ground system includes:
• Central control station
• 3 ground tracking stations for orbit determination (at Jamushi, Kashi and Zhanjiang)
• Ground correction stations
• User terminals (receivers/transmitters).
The system provides positioning data of 100 m accuracy. By using ground- and/or spaceborne (the 3rd and 4th satellites) differential methods, the accuracy can be increased to < 20 m. The system capacity is 540,000 users per hour, and serve up to 150 users simultaneously.
BeiDou-1 location measurement scheme:
BeiDou-1 requires two-way transmissions between the user and the central control station via the satellite. Firstly, the central control station sends inquiry signals to the users via two satellites. When the user terminal received the signal from one satellite, it sends responding signal back to both satellites. The central station receives the responding signals sent by the user from two satellites, and calculates the user’s 2D position based on the time difference between the two signals. This position is then compared with the digital territorial map stored in the database to get the 3D position data, which is then sent back to the user via satellites using encrypted communications.
Figure 2: Illustration of the BeiDou-1 measurement scheme (image credit: sinodefence)
Compass/BeiDou-2 global satellite navigation constellation
Compass/BeiDou is the satellite navigation system planned by China. The following basic principles apply: 11)
• Compass/BeiDou will provide high quality open services free of charge from direct users, and its worldwide use is encouraged.
• China will widely and thoroughly communicate with other countries on satellite navigation issues to facilitate the development of GNSS technologies and the industry.
• China will develop and operate Compass/BeiDou system independently.
• Compass/BeiDou can independently provide services for global users and particularly provide high quality services in Asia-Pacific region.
• COMPASS/BeiDou will pursue solutions to realize compatibility and interoperability with other GNSS (Global Navigation Satellite Systems).
• The construction of COMPASS/BeiDou system follows a step-by-step pattern based on technical and economic evolution in China.
• COMPASS/BeiDou will provide long-term continuous services for users, improve system performance and ensure smooth transition during all life cycle.
The new system under development will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites, 3 IGSO (Inclined Geosynchronous Orbit), and 27 MEO (Medium Earth Orbit) satellites, that will offer complete coverage of the globe. The BeiDou-2 system will allow a ground receiver to calculate its position by measuring the distance between itself and three or more satellites in view, in analogy to the method of signal reception practised by the GPS and GLONASS constellations. 12) 13) 14) 15) 16) 17) 18)
As with GPS, GLONASS, and Galileo, the system is planned to provide two navigation services: an open service for (commercial) customers and an “authorized” positioning, velocity, and timing communications service. 19) 20) 21) 22)
1) Open service: Free to the global user community. The open service will provide a positioning accuracy of 10 m, a timing accuracy of 20 ns, and a velocity accuracy of 0.2 m/s.
2) Authorized service on a commercial basis: Provision of more reliable PVT (Positioning, Velocity, Timing) information and communications services as well as integrity information.
The communication access scheme of the CNSS signals will be CDMA (Code Division Multiple Access).
Table 2: Compass /BeiDou-2 signal characteristics:
Table 3: Signals provided in the two service categories
• Compass/BeiDou coordinate system is referred to as BDC (BeiDou Coordinate ) system. The BDC coincides with CGCS2000 (China Geodetic Coordinate System). 23)
- CGCS2000 is referenced to CTRF2000 (China Terrestrial Reference Frame 2000)
- Global, three-dimensional
- Right-handed, orthogonal
- Frame: ITRF97
- Epoch: 2000.0
• CTRF is aligned to ITRF (International Terrestrial Reference Frame). It coincides with ITRF within a few cm; hence, for most applications the difference between CGCS2000 and ITRF can be neglected.
• Status of CTRF: IGS Stations used in CTRF computation. A total of 47 IGS stations were included in establishing CTRF 2000
Coordinating Compass/BeiDou-2 with other GNSS constellations: Interoperability is one of the most important aspects in the design of the Compass timing system; the time differences of BDT - GPST and BDT - GST (Galileo System Time) are measured in the Compass central control station and will later be broadcast as part of the navigation messages of the GNSS systems. 24)
The CNSS spacecraft series is based on the DFH-3 (DongFangHong 3) satellite bus, with a design life of 8 years (the DFH-3 bus provides a medium-capacity platform and has a communication satellite heritage). The DFH-3 bus is 3-axis stabilized providing the power as well as guidance and control needed for navigation satellite services. The DFH-3 bus is being built by CAST (China Academy of Space Technology) corporation; the Dongfanghong (DFH) Satellite Company of Beijing is part of CAST.
Figure 3: Illustration of the MEO spacecraft (image credit: DFH/CAST)
• The Compass-G is a geostationary navigation 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 Compass-G is launched on a Changzheng 3C rocket carrying a single satellite.
• The Compass-IGSO spacecraft is similar to the geostationary satellite in design, but is deployed in a 55º inclination geosynchronous orbit.
• The Compass-M satellite is deployed into MEO (Medium Earth Orbit) at nominal altitudes of 21,500~24,100 km and an inclination of 55º. An experimental Compass-M1 satellite, based on the DFH-3B bus, was launched in April 2007 to test the onboard payload.
• The operational Compass-M satellite, which is currently (2012) in development, will be based on a dedicated MEO satellite bus. The satellite has a gross (launch) mass of 800 kg, and a payload capability of 280 kg. The satellite is three-axis stabilized and has a power output of 1.5 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.
Table 4: Overview of Compass satellite parameters
Figure 4: Illustration of the Compass-GEO spacecraft (image credit: DFH/CAST)
Launch: So far, all BeiDou/Compass satellites have been launched from XSLC (Xichang Satellite Launch Center) in the southwestern Sichuan Province of China.
Figure 5: Photo of one deployed Compass solar panel (image credit: DFH/CAST)
Orbital planes of the CNSS constellation:
• The 27 MEO satellites will be deployed into 3 orbital planes with an inclination of 55.5º. The nominal orbital altitude is 21,500 -24100 km.
• Five GEO satellites at the equatorial longitudes of: 58.75ºE, 80ºE, 110.5ºE, 140ºE and 160ºE (regional system).
• Three IGSO (Inclined Geosynchronous Orbit) satellites.
Figure 6: Illustration of the future Compass/BeiDou-2 constellation (image credit: CSNPC)
Compass-M1 is an experimental satellite, the first spacecraft of the BeiDou-2 series (second generation MEO), launched on April 14, 2007 for signal testing and validation and for the frequency filing with the ITU (International Telecommunication Union). The role of Compass-M1 for Compass is similar to the role of GIOVE satellites for Galileo.
Table 6: Overview of current and future GNSS constellation parameters (Ref. 16)
Compass/BeiDou-2 mission status:
• December. 27, 2013: China will launch upgraded satellites and expand its regional BDS (BeiDou Navigation Satellite System ) to global coverage by 2020. Over the past year, positioning has been accurate to within 10 m, according to Chengqi Ran, director of the satellite navigation office. 30)
• December 27, 2013: With a constellation of 14 operational satellites in orbit, BDS has been in full service for 1 year to provide open services to the most part of the Asia-Pacific region since December 27, 2012. 31) 32) 33) 34) 35)
The current BDS space segment consists of 5 GEO satellites, 5 IGSO satellites and 4 MEO satellites. The respective positions of satellites are shown in Figure 7.
- The GEO satellites are operating in orbit with an altitude of 35,786 km and positioned at 58.75ºE, 80ºE, 110.5°E, 140°E and 160°E, respectively.
- The IGSO satellites are operating in orbit with an altitude of 35,786 kilometers and an inclination of 55° to the equatorial plane. The phase difference of right ascensions of ascending nodes of those orbital planes is 120°. The sub-satellite tracks for three of those IGSO satellites are coincided while the longitude of the intersection point is at 118°E. The sub-satellite tracks for the other two IGSO satellites are coincided while the longitude of the intersection point is at 95°E.
- The MEO satellites are operating in orbit with an altitude of 21,528 kilometers and an inclination of 55° to the equatorial plane. The satellite recursion period is 13 rotations within 7 days. The phase is selected from the Walker24/3/1 constellation, and the right ascension of ascending node of the satellites in the first orbital plane is 0°. The current 4 MEO satellites are in the 7th and 8th phases of the first orbital plane, and in the 3rd and 4th phases of the second orbital plane respectively.
Figure 7: The BDS constellation as of December 2012 (image credit: CSNPC)
Figure 8: The BDS regional service coverage (image credit: CSNPC, Ref. 32)
• Operational overview of the BeiDou-2 satellite constellation. Up to June 2012, three BeiDou-2 satellites have been launched in 2012. 36)
Figure 9: Operational status of the BeiDou system in the summer of 2012 (image credit: CSNO, Ref. 36)
• Since a pre-operational status has been reached, the Compass/BeiDou-2 navigation signals can already be tracked by various commercial multi-GNSS receivers. This enables the determination of Compass/BeiDou-2 orbit and clock products independent of the control segment and paves the way for initial experimentation. 37) 38) 39)
- A total of 13 Compass/BeiDou-2 satellites have been launched, out of which 11 satellites are in active service in the summer of 2012. These include two satellites in medium altitude Earth orbits (MEO), as well as four satellites in geostationary orbits (GEO) and five satellites in inclined geosynchronous orbits (IGSOs). With a total of nine satellites permanently visible in the Asia-Pacific region, Compass/BeiDou is the third constellation (following the GPS and GLONASS constellations) that can offer an independent navigation service in this area.
- The unique availability of three carriers on all Compass/BeiDou-2 satellites enables novel strategies for differenced and undifferenced positioning. Based on B2 and B3 measurements, an EWL (Extra-WideLane) combination with a wavelength of 4.9 m can be formed. This enables an almost instantaneous EWL ambiguity resolution and greatly facilitates a subsequent widelane ambiguity fixing (Ref. 38).
• Since IOC, the system has performed stably. The space constellation performance has been improved apparently, and the user experience for the PNT (Positioning, Navigation, and Timing) service performance has been enhanced significantly.
• On December 27, 2011, the State Council Information Office, P.R.C. held a press conference to announce the initial service of the BeiDou System (Ref. 36). - The CNSS was activated on a trial basis. The network would initially provide coverage for China and its peripheral areas, and would offer service to the Asia-Pacific region by late 2012 (Ref. 36). 40)
Performance of IOC (Initial Operational Capability) service:
- Service coverage area: Longitudinal extend: 84ºE to 160ºE; Latitudinal extend: ±55º
- Positioning accuracy: 25 m horizontal and 30 m vertical
- Velocity accuracy: 0.4 m/s
- Timing accuracy: 50 ns
Figure 10: Illustration of the BeiDou Navigation Satellite System coverage area (image credit: CSNO)
Some system testing receiver results are: 43)
- The Compass SV (Space Vehicle) signal quality parameters are well in accordance with their specifications
- The current Compass constellation (Sept. 2011) can provide a positioning service in the Asia-Pacific region - indedendently.
- The Compass constellation helps to improve the multi-system-integrated positioning precision.
- The short-term precision of the Compass B1 (OS) signals is better than the GPS L1 C/A signals.
- The long-term accuracy and stability need to be further improved.
The Compass satellites carry two types of navigation payloads: RDSS (Radio Determination Satellite Service) and RNSS (Radio Navigation Satellite Service), Ref. 25).
More description of the navigation payloads will be provided when the information becomes available.
RDSS (Radio Determination Satellite Service):
The RDSS payload is only flown on the Compass-G satellites. 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.
In RDSS, the user position is computed by a ground station using the round trip time of signals exchanged via GEO satellite. The RDSS Long term feature further includes: 44)
- Short message communication (guaranteeing backward compatibility with BeiDou-1)
- Large volume message communication
- Information connection
- Extended coverage
- Multi-satellite communication vs. short message by GEO.
RNSS (Radio Navigation Satellite Service):
The RNSS payload is flown on all CNSS/Beidou-2 satellites. The package includes an atomic clock, an L-band transmitter, signal processor, transmitter antenna array, an L-band uplink receiver, laser corner-cube reflector for orbit determination, and a multilateration unit.
The RNSS is very similar to that provided by GPS and Galileo and is designed to achieve similar performances.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.