A second pair of satellites that will help supply Europe with its own navigation system have been launched from French Guiana.
The satellites, nicknamed David and Tif, join two others launched a year ago as part of the European Union's nascent Galileo navigation system.
A minimum of four satellites are needed so officials can perform an in-orbit validation of the system, according to Arianespace, the commercial arm of the European Space Agency.
Once testing is complete, two satellites will be launched every quarter for a total of 30 satellites. By 2014, the system is expected to operate as a free consumer navigation service, and by 2020 it will be fully operational and provide more specialised services.
The taxpayer-funded programme is expected to rival the American GPS network, with EU officials touting it as more precise and reliable.
The European Commission has said development and deployment since 2003 is estimated at more than 6.8 billion US dollars (£3.9 billion). Maintaining and completing the system is expected to cost 1.35 billion dollars (£840 million) a year.