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Tuesday, October 17, 2017

Chinese Navigation Satellite Systems

Posted to Maritime Musings (by on September 20, 2013

Two constellations – one experimental and the second under construction

China has launched two constellations of navigation satellite systems.  The first satellite in the BeiDou constellation was launched in 2000.  BeiDou means “Northern Dipper”, the Chinese name for the astronomic constellation referred to in the West as the Big Dipper and used for navigational purposes since ancient times.   The BeiDou satellite constellation consists of four satellites that have been placed in geostationary orbits and has been used primarily for experimental purposes, although it does provide positional, navigational, and timing coverage of East Asia.  The second system, Compass (or BeiDou-2) is intended to eventually provide worldwide coverage on a par with the US Global Positioning System (GPS).  Its first satellite was launched in 2007.  To date, sixteen of the planned 35 Compass satellites have been launched.  When completed, there are to be five geostationary satellites, three geosynchronous satellites, and 27 non-geosynchronous satellites in medium earth orbit.  Like GPS, Compass offers two levels of service.  The open civilian service provides a position accuracy of 10 meters, a speed accuracy of 0.2 meters/second, and a timing accuracy of 10 nanoseconds.  The encrypted military service is more precise, with claimed positional accuracy of 10 centimeters.  Compass commenced operation in 2011, but full service is currently limited to the Eastern Hemisphere from near Iran to near the International Date Line.  Global service is expected to be provided in 2020.  Global navigation satellite system (GNSS) receivers with Compass capability (in addition to GPS, GLONASS, and Galileo) are now commercially available.  It should also be noted that there is a simmering dispute between China and the European Union (EU) regarding their satellite navigation systems.  China initially entered into cooperative arrangements with regard to the Galileo system, but pulled out and decided to go forward its independent Compass service.  The two services operate on similar electromagnetic frequencies.  Under international rules, the service that initially commenced using a particular frequency has primacy.  While planning for Galileo preceded planning for Compass, the Compass satellites were placed into orbit and commenced operation first.  Only time will tell if this potential issue develops into a serious problem.


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