History
Beidou 1A was launched on 30 October 2000, Beidou 1B followed on 20 December 2000, Beidou 1C was put into orbit on 25 May 2003[1], Beidou-1D in February 2007, Compass-M1 (to validate frequencies for the Beidou-2 constellation) in April 2009. The second Beidou-2 constellation satellite (Compass-G2) was launched on 15 April 2009 [2].
In September 2003, China intended to join the Galileo positioning system project. China was to invest €230 million (USD296 million, GBP160 million) in Galileo over the next few years.[3] (Galileo is not yet operational, although two experimental satellites have been launched.)
[4]. China has since abandoned its relationship with Galileo.
On November 2, 2006, China announced that from 2008 Beidou would offer an open service with an accuracy of 10 meters.[5]
In 2007, the official Xinhua News Agency reported that the resolution of the Beidou system was as high as 0.5 metres, considerably better than unaided GPS. [6]The user terminals are described here. It appears that the calibrated accuracy is rather 20m (100m, uncalibrated).
[edit] Satellites
(as of November 2009)
Date Launcher Satellite Orbit Usable
10/31/2000 LM-3A Beidou-1A GEO 140°E No
12/21/2000 LM-3A Beidou-1B GEO 80°E (unclear)
5/25/2003 LM-3A Beidou-1C GEO 110.5°E Yes
2/3/2007 LM-3A Beidou-1D deorbited No
4/14/2007 LM-3A Compass-M1 MEO 21,500 km Yes
4/15/2009 LM-3C Compass-G2 GEO drifting [7] No
Note: all dates are based on China Standard Time
[edit] Current technology (Beidou-1)
Unlike the GPS, GLONASS, and Galileo systems, which use medium Earth orbit satellites, Beidou 1 uses satellites in geostationary 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 70°E to 140°E, and from 5°N to 55°N.
The Beidou satellites themselves were based on the Chinese DFH-3 geostationary communications satellite and had a launch weight of 1,000 kilograms (2,200 pounds) each.[8]
To calculate a position, the following procedure is used:
A signal is transmitted skyward by a remote terminal.
Each of the geostationary satellites receive the signal.
Each satellite sends the accurate time of when each received the signal to a ground station.
The ground station calculates the longitude and latitude of the remote terminal, and determines the altitude from a relief map.
The ground station sends the remote terminal's 3D position to the satellites.
The satellites broadcast the calculated position to the remote terminal.
Additionally, the terminal can communicate with the ground station by sending and receiving short messages.
[edit] Future Plans (Beidou-2 or Compass)
Main article: Compass navigation system
The four satellites (1A,1B,1C and 1D) mentioned above were designed as experimental satellites.
The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites, for backward compatibility with Beidou-1, and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe. There will be two levels of service provided; free service for those in China, and licensed service for the military.
The free service will have a 10 meter location-tracking accuracy, will synchronize clocks with an accuracy of 50 ns, and measure speeds within 0.2 m/s.
The licensed service will be more accurate than the free service, can be used for communication, and will supply information about the system status to the users.
Two satellites for Beidou 2 have been launched in early 2007 and in early 2009. In the next few years, China plans to continue setting up the system for global operation from 2015.
