The launch of two Galileo satellites, marking a crucial step in Europe's planned navigation system, has been cleared for Friday, according to the European Space Agency (ESA). A Fregat-MT upper stage containing the two satellites has been moved to an assembly building at the Kourou space base in French Guiana, to be mated to a three-stage Soyuz ST-B rocket. A videographic explaining how tracking systems work. VIDEOGRAPHIC