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equatorial satellites in the Geospace Double Star Exploration Program (DSP) using
the VLBI technique and obtaining high quality observational data.
In January 2004, the China National Space Administration declared that the
Chinese Lunar Exploration Program (CLEP), or “Chang'e” project, was officially
initiated, which meant that China's deep space exploration has entered an opera-
tional phase. In the Chang'e project, China's Unified S-Band Tracking
Telemetering and Control (USB TT&C) system and CAS VLBI measurement
system jointly monitored the orbits. Therefore, two VLBI fixed stations—one
with a 40-m antenna in Kunming and the other with a 50-m antenna in Beijing—
were formed on the basis of the Chinese former VLBI Network. Meanwhile, the
functions of the former VLBI stations in Shanghai (with a 25-m antenna) and
Urumqi (with a 25-m antenna) were transformed and extended. A rapid real-time
data delivery channel between VLBI stations and related processing centers, real-
time processors, and related processing centers were established as well. Now the
Chinese VLBI Network has become a real-time observation network with four fixed
stations and one relevant processing center (as shown in Fig.
3.23
).
Due to cost-prohibitive equipment of the VLBI, SLR, and DORIS techniques,
and the difficulties in setting observation stations, the stations realizing these
techniques throughout the world are rather few in number and are mostly unevenly
distributed. In consequence, it is difficult for them to be connected into an effective
control network within certain areas. In practice they can only form a three-
dimensional control network worldwide to realize the global three-dimensional
datum by participating in the global connection survey through international coop-
eration. Comparatively speaking, the GNSS technique is characterized by conve-
nient and rapid measurement and convenient station establishment. Therefore,
GNSS is most often used in the establishment of regional three-dimensional control
networks.
China's Three-Dimensional Coordinate Datum: High-Precision GPS
Geodetic Control Network
For regional three-dimensional datum, one method is to establish a nationwide
high-precision GPS network according to the needs of national construction and
certain scientific research. Instances are China's high-precision A- and B-order
GPS networks, the first- and second-order nationwide GPS networks, and the
Crustal Movement Observation Network of China intended for earthquake predic-
tion and plate motion research, etc.
A- and B-Order National GPS Networks
The A-order national GPS network was distributed by the SBSM, CEA and some
other agencies in combination with the international IGS92 campaign in 1992. The
entire network consists of 30 points distributed evenly in China's mainland with an
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