China launches second GPS satellite, 28 left to go by 2015

By the year 2015, China plans to have launched a 35-satellite array which will serve as its own GPS system, one not reliant upon the US-controlled GPS or the EU-controlled GLONASS, which are used by all cell phones and mobile devices around the world today. The Chinese system is called Beidou Navigation System (BNS) and will be fully operational in 2015, though it may be partially operational before then when at least 20 of its satellites are in orbit.

This latest satellite, part of the COMPASS array, was launched from China’s southwest Sichuan province at the Xichang Satellite Launching Centre just past midnight this morning local time. The first non-experimental satellite was launched two years ago, in April, 2007, with four “experimental COMPASS” satellites launched earlier this decade. The experimental satellites are dubbed Beidou-1A, -1B, -1C and 1D, with the non-experimental satellites being Beidou-2A and -2G for the April 2007 satellite and April 2009 satellite, respectively. Only the experimental -1B, -1C and -1D satellites remain in operation in orbit, though -1A still exists in its geostationary orbit. Over the next 6 years, China plans to launch the remaining 28 satellites.

Each COMPASS satellite weighs around 2,200 pounds. The final system will reportedly have 30 medium Earth orbit satellites, with five geostationary satellites, though sources vary on the accuracy of this information. The GPS service will be offered free in China, with licensed service being given to its military.

BNS will be accurate to within 10 meters (30 feet), and operates in the following way: a series of Earth-based ground signaling stations broadcast real-time 3D position and time data to each of the satellites. They receive the data and re-transmit the information back to the Earth. Ground-based receivers (like a mobile device) record the data received from several satellites and, using a complex mathematical formula, determine the relative 3D position of the receiver, compared to the known location of the satellites. It takes 5 or more satellite signals to get an accurate reading, though the system can work with 3 satellites.

China is using rubidium atomic clocks to provide the sensitive timekeeping necessary for the system to work (accurate to 3 x 10-10 seconds per year). In addition, China originally registered 36 slots for COMPASS satellites with 14 being in geostationary orbits and 22 being in medium Earth orbits, though this appears to evolved over time.

BNS is named after the Big Dipper star constellation, with the word Beidou specifically referring to the seven brightest stars of The Great Bear constellation (Ursa Major). China plans to launch 10 satellites by the end of 2010.

See the original AFP report republished on Google News.