The second of two navigation satellites sent astray by a rocket mislaunch last year has been nudged into a better, potentially useful orbit, the European Space Agency said Friday.

Testing will be done to assess the performance of the satellite's navigation instruments, and whether it can still be used as part of the system, the agency said in a statement.

ESA's Galileo satnav programme is being developed as a rival to America's GPS.

On August 22, Galileo satellites Sat-5 and Sat-6 were placed in the wrong orbit by a Soyuz rocket launched from Europe's space base at Kourou in French Guiana.

They should have been slotted into a circular orbit at an altitude of 23,500 kilometres (14,600 miles), inclined at 56 degrees to the equator.

Instead, they were placed in an elliptical orbit that saw them travelling as high as 25,900 km above Earth and to a low point of 13,713 km, not useful for navigation.

Last November, Sat-5 was manoeuvred into a slightly better position, and managed to send a signal home.

And thanks to a six-week salvage operation that started mid-January, Sat-6 is now in the same orbit as its companion, mirroring its path on the opposite side of the planet, said ESA.

After 14 manoeuvres, the orbit is now more circular, and the lowest point has been raised by more than 3,500 km.

"The exposure of the two to the harmful Van Allen Belt radiation has been greatly reduced, helping to ensure future reliability," said an ESA statement.

Such belts are zones of charged particles around planets with a magnetic field, like Earth.

"Significantly, the corrected orbit means they will overfly the same location on the ground every 20 days," said ESA.

"This compares with a standard Galileo repeat pattern of every 10 days, helping to synchronise their ground tracks with the rest of the constellation."

Sat-6 will undergo testing until the end of the month, the agency said.

Once the results are in, it will be up to the European Commission, which owns the system, to decide whether or not to incorporate the misplaced twins in Galileo's navigation and search-and-rescue capabilities.

The multi-billion-euro Galileo project has encountered a long series of technical hitches, delays and budget questions.

The hitch of August 22 last year was blamed on frozen fuel pipes aboard the Russian Soyuz rocket's fourth stage.

By 2017, according to the Galileo schedule, all 24 operational satellites should be in place. Six backups would join the fleet by 2020, at which point the system would be fully operational.

The next two satellites are due to be launched on March 27.