Based on the gradualness principle, the BeiDou system will follow a step-by-step pattern in accordance with technical and economic evolution in China, provide long-term continuous services for users, improve system performance, and ensure seamless and smooth transition between the system construction phases.
Figure 1 shows the overall development planning of the BeiDou system: “three-step” deployment, from regional to global, from active to passive.
In the 1980s, in line with the condition and demands of the time, China began to conduct a study on the BeiDou demonstration system under dual-satellite active positioning mechanism. First initiated in 1994, the BeiDou demonstration system was able to provide regional active services in 2000. From 2000 to 2003, three satellites were launched, and in 2010, these satellites have been replaced by new ones.
In 1999, China began to study the BeiDou navigation satellite system under passive positioning mechanism. System construction formally started in 2004. The system will provide radio navigation satellite service (RNSS) for users in China and its surrounding areas on the basis of radio determination satellite service (RDSS) from 2012.
The BeiDou system will finally provide compatible and interoperable RNSS with global coverage and also upgraded RDSS by 2020.
The BeiDou system is comprised of three major components: space segment, ground segment, and user segment. The space segment consists of 5 geostationary (GEO) satellites and 30 non-GEO satellites. The ground segment consists of several master control stations (MCS), upload stations (US), and a network of globally distributed monitor stations (MS). The user segment includes BeiDou terminals and interoperable terminals compatible with other GNSS systems, to meet different application requirements from different fields and industries.
The BeiDou system can transmit multiple navigation signals. Generally speaking, the system can provide two kinds of global services: open service and authorized service. Open service is free to users with positioning accuracy better than 10 m, timing accuracy better than 20 ns, and velocity accuracy of 0.2 m/s. The system can also provide two kinds of regional services—wide-area differential service and short message service. Wide-area differential accuracy is better than 1 m.
The time reference for the BeiDou system uses BeiDou time (BDT), which is traceable to the Coordinated Universal Time (UTC) maintained by the National