China has launched the third satellite in its Compass global satellite navigation system. It was launched from Xichang Satellite Launch Centre in southwestern Sichuan province at about 0:12 a.m. Beijing Time on 17 January.
According to the Xinhua/Qian Xian'an news service, it will join another two already in orbit to form a network which will eventually have a total of 35 satellites, capable of providing global navigation service to users around the world by 2020. The system will begin to provide navigation, time signal and short message services on the Asian mainland and in the Pacific region from 2012.
The first satellite was launched into geostationary orbit on 14 April 2007. It is also known as Compass M1, Beidou 4, or Beidou 2A. The second satellite was launched 15 May 2009. It is known as Compass M2, Beidou 5 or Beidou 2G, depending on which source one quotes, although it appears Chinese authorities have now settled on the Compass M designations.
Compass M3 will be placed in geostationary orbit. The first of the medium orbit satellites will be launched later this year.
Xinhua quoted Cao Chong, chief engineer of the China Electronics Technology Group Corporation, as estimating that the first phase of Compass, to the end of 2010, would cost more than US$1.463 billion. 'I think the Compass system might cost China several dozen billion yuan,' said Cao, who works with the China Satellite Navigation Engineering Centre. 'The first phase alone could cost more than 10 billion yuan.'
The Compass system will provide both open and authorised services, according to publicity provided by the centre. The open service will be free of charge. It will provide a resolution of 10 meters for an unaided receiver, and a time signal accuracy of 10 nanosecond. This will lead to an accuracy of 0.2 meter per second for speed measurement.
The authorised service will provide more accurate services for Chinese military users. China started to build up its own satellite navigation system to break its dependence on the GPS in 2000 when it sent two orbiters as a double-satellite experimental positioning system, known as the Beidou system.
The Beidou system, China's first-generation satellite navigation and positioning network, made the country the third in the world after the U.S. and Russia to have an independent satellite navigation system. China started to upgrade the Beidou system into the second-generation system by launching two new orbiters into space in 2007 and 2009 respectively.
The Beidou satellites are modified communications satellites and have a fully duplex communications protocol, i.e. both mobile and satellite are required to transmit and receive. The mobile receiver on the ground sends a message to the satellite requesting a position fix. There is an interaction between sender and receiver, and the satellite then sends the position to the receiver.
The system works, but it requires far more complex, and thus more expensive, receivers than the GPS. The second generation Compass system will work in more or less the same fashion as GPS, Glonass and Galileo.
Source: ASM Magazine
