The launch of two Galileo satellites, marking a crucial step in Europe's planned navigation system, has been cleared for Friday, the European Space Agency (ESA) said on Tuesday.

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, ESA said.

"The launch is scheduled for 1815 GMT" on Friday, it said in a press release.

If all goes well, the two satellites will add to the first two in the Galileo navigation system, which were hoisted aloft on October 21, 2011.

"(They) will all together make a mini-constellation of four satellites. This would allow us to validate the full system," said Javier Benedicto, ESA's Galileo project manager.

Four is the minimum needed to gain a navigational fix on the ground, using signals from the satellite to get a position for latitude, longitude, altitude and a time reference.

Galileo will ultimately consist of 30 satellites, six more than the US Global Positioning System (GPS).

By 2015, 18 satellites should be in place, which is sufficient for launching services to the public, followed by the rest in 2020, according to ESA.

The system claims it will be accurate to within a metre (3.25 feet). The GPS, which became operational in 1995 and is being upgraded, is currently accurate to between three and eight metres (10 and 26 feet).

In May, the European Commission said the cost by 2015 would be five billion euros ($6.45 billion).

Friday's launch will be the third under a contract that transferred the Soviet-Russian workhorse of space to South America.

As a medium-sized launcher, Soyuz complements Europe's heavyweight Ariane 5 and lightweight Vega rockets.