China's Beidou GPS rival gets more satellites

China has launched two more satellites for its Beidou constellation, a rival to the US-controlled GPS navigation system.

The Compass M3 and M4 satellites took off from the Xichang Satellite Launch Center in Sichuan province in the early hours of Monday morning, with both satellites sharing the same rocket. According to reports, three more Beidou satellites will follow this year.

While the GPS constellation is available for use by anyone around the world, it remains controlled by the US military. This state of affairs has spurred on the development of alternative systems, including Beidou.

Beidou became operational at the end of last year for users in China. The constellation, of which Compass M3 and M4 are the 12th and 13th elements, is intended to end up as a global system that will be able to rival GPS.

Russia also has a GPS rival in GLONASS. The European Union's Galileo system is at an earlier stage, as only a few satellites have been launched so far. India is also building a navigation satellite system, albeit one with only regional ambitions.