DFH-3 Credit - via Chen Lan

Beidou ('Big Dipper') was the satellite component of an independent Chinese satellite navigation and positioning system. This was to be achieved by launching a satellite constellation in stages during 2000-2010 while developing the relevant application systems. The end result would be a Chinese indigenous satellite navigation and positioning industry.

Beidou began in 1983 with a proposal by Chen Fangyun to develop a Twinsat regional navigation system using 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 Global Positioning System. In 1993, the Beidou program was officially started. Beidou used the DFH-3 bus and had similar basic performance. 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.

The two satellite system was based on an iterative determination of a user's altitude. Each of the satellites continuously broadcast signals to all parts of the Earth that were visible to it (the entire Asian landmass). A user's station excerpted a certain portion of those signals and transmitted it back to the satellite. The satellite, in turn, forwarded the received signal to the system's control center. Computers at the system's control center then determined the distance between the user and the satellite by inferring the time-of-flight difference between satellite and user from the time the signal was originally broadcast, which was stamped with the broadcast time, and when the relayed user's signal reached the control center.

The control center then combined an initial guess of the user's altitude (and hence the distance from the Earth's center), either from the last reported altitude or perhaps by using an arbitrary guess such as sea level, with the distances from the two satellites to get three distance estimates for the user's position. This determines the first estimate of the user's latitude and longitude. A map of the user's region (presumably stored digitally) was then used to make an improved estimate of the user's altitude, which could then be fed back into the latitude and longitude calculation to make an improved estimate of those quantities.

Although such a system could achieve GPS-like accuracies (except in steep terrain), it obviously had serious operational constraints. The user had to emit in order to achieve a location fix. And the control center provided a single point target that would bring the entire system down. However it was a way to obtain, in only two satellite launches, an indigenous, independent, high-accuracy military navigation system that would function in anything less than total war with a major power. By comparison Navstar, Glosnass, or Galileo medium-earth orbit systems required launch of dozens of satellites.

The US Department of Defense estimated that the Beidou system had an accuracy of 20 meters over China and surrounding areas. It was also reported that Beidou had an active communications capability, allowing the leadership to send secure orders and receive confirmations and status reports.

Nevertheless, a private company, BDStar Navigation, was formed to develop the Beidou ground segment and to market receivers to commercial operators. The company had originated as a joint venture with Canada Novatel in October 2000 to develop and market GPS receivers. A year later the draft project for the Beidou 1 Information Service System was completed, which provided the basis for open applications of the Beidou navigation system. A joint state-industrial committee approved the final project plan in January 2003.

Funding from the Ministry of Science and Technology led to work beginning on a Beidou Satellite Integrated Information Application Service as part of the Chinese National 863 Development Plan. The system passed acceptance testing in December 2005, leading to potential application of Beidou receivers for Chinese ocean fishery vessels. In June 2006 the commercial demonstration project for a Beidou Ocean Fishery Secure Production and Transaction Information Service was started.

However BDStar's most lucrative projects still used Navstar GPS signals for such applications as port container management. Given the operational constraints of the geostationary Beidou system, it was perhaps unsurprising when China announced a supplementary medium-earth-orbit constellation for Beidou in 2006. The Beidou-2 operational system was now defined as a constellation of 35 satellites, five in geostationary orbit and thirty in medium Earth orbit (12-hour, 55 deg inclination, 21,000 km altitude circular orbits). The medium orbit satellites would use the same navigation principle as Navstar, GLONASS, and Galileo, with internal high precision clocks and orbital information beaming the precise position information of the satellites to the passive receivers of users. Combining signals from multiple satellites allowed the user receiver to calculate its position on the earth with high precision.

Two levels of service were to be provided. The public service for civilian use would be free to Chinese users and would have a 10 meter location accuracy, provide clock synchronization signals with an accuracy of 50 ns, and measure speeds within 0.2 m/s. The licensed military service would be more accurate than the free service, also provide system status information, and a military communications capability.

Mass: 2,200 kg (4,800 lb). Main Engine Propellants: 1,100 kg (2,400 lb).

• 2000 October 30 - Beidou 1A - Launch Site: Xichang. Launch Complex: LC2. Launch Vehicle: CZ. Mass: 2,200 kg (4,800 lb). Perigee: 35,772 km (22,227 mi). Apogee: 35,803 km (22,246 mi). Inclination: 0.10 deg.
  Beidou was China's first experimental navigation technology satellite, developed by CAST/Beijing. The satellite was placed in an initial 195 x 41889 km x 25.0 deg orbit geostationary transfer orbit before entering its final geosynchornous orbit at around 0500 GMT on November 6. Stationed at 140 deg E, still maintaining its position within 0.1 deg as of 2007.

• 2000 December 20 - Beidou 1B - Launch Site: Xichang. Launch Complex: LC2. Launch Vehicle: CZ. Mass: 2,200 kg (4,800 lb). Perigee: 35,753 km (22,215 mi). Apogee: 35,821 km (22,258 mi). Inclination: 0.00 deg. Period: 1,436.10 min.
  Second Beidou geosynchronous navigation satellite. The CZ-3A rocket's third stage put Beidou in geostationary transfer orbit at around 1642 GMT. The Beidou satellite was based on the DFH-3 comsat and had a mass of around 2200 kg including its FY-25 solid apogee motor. On December 25 Beidou was in a 190 x 41870 km x 25.0 deg transfer orbit. The launch of this second Beidou completed the prototype two-satellite navigational system which was to provide positional information for highway, railway and marine transportation. Positioned in geosynchronous orbit at 80 deg E, still maintaining its position within 0.1 deg as of 2007.

• 2003 May 24 - Beidou 2A - Launch Site: Xichang. Launch Complex: LC2. Launch Vehicle: CZ. Mass: 2,200 kg (4,800 lb). Perigee: 35,760 km (22,220 mi). Apogee: 35,836 km (22,267 mi). Inclination: 0.30 deg. Period: 1,436.70 min.
  Navigation technology satellite, joined Beidou 1A and 1B launched in December 2000. This third satellite was considered a back-up element, Positioned at 110 deg E, still maintaining its position within 0.1 deg as of 2007.

• 2007 February 2 - Beidou 2A - Launch Site: Xichang. Launch Complex: LC2. Launch Vehicle: CZ. Mass: 2,200 kg (4,800 lb). Perigee: 35,326 km (21,950 mi). Apogee: 36,248 km (22,523 mi). Inclination: 6.20 deg. Period: 1,436.10 min.
  Fourth Beidou satellite. It did not reach geostationary orbit until early April following deployment problems with its solar panels and reports of US detection of a debris cloud at the time of the original expected apogee firing.

• 2007 April 12 - Beidou 5 - Launch Site: Xichang. Launch Complex: -. Launch Vehicle: CZ. Mass: 2,200 kg (4,800 lb). Perigee: 21,519 km (13,371 mi). Apogee: 21,544 km (13,386 mi). Inclination: 55.30 deg. Period: 773.40 min.
  The fifth Beidou satellite, but the first in the 12-hour, 55 deg inclination MEO portion of the constellation. All previous launches had been to populate the geostationary portion of the system.