By Tim Bartlett
Beidou is another Global Navigation Satellite System. Sometimes called Compass, it is being developed by China, which has launched 16 Beidou satellites since 2007. The system should eventually include 35 satellites, and when it is completed, in 2020, it should give worldwide coverage with an accuracy of about 10 meters (33 feet). Some of the frequencies used overlap with those used by GPS and Galileo, and Beidou uses similar signal formats. Though the Chinese authorities have said that the system will be freely available worldwide, they have not yet released the technical specifications required for the construction of commercial receivers.