Europe’s answer to the US Global Positioning System (GPS) is its Galileo program, a fleet of satellites that have been long delayed and over budget. And with 18 of 24 planned satellites launched, a problem has arisen. A number of clocks on them have failed, and no one knows why.

Each satellite has four highly precise atomic clocks on board. Two are rubidium clocks, where gaseous rubidium atoms are stimulated by the light of a rubidium lamp. The other two are maser clocks, which have the same basic principle but use hydrogen atoms.

In November 2016, the four latest Galileo satellites were launched. But nine clocks (six rubidium and three maser) have now failed across five of the satellites, as first reported by BBC News. This isn’t detrimental to the system, but it is obviously a cause for concern, especially with six more satellites left to launch to get the system fully operational.

These clocks are touted as Galileo’s edge over GPS, giving users on the ground a more accurate reading for their location – down to a meter (3.3 feet), compared to several meters offered by GPS. But ESA will be keen to work out the problem quickly, for fear that their other satellites might be affected.

The cause of all the failures in the rubidium clocks is not yet known, and may be a problem during the manufacturing phase. As for the maser clocks, ESA has pinpointed two possible causes, one being that the clocks struggle to turn back on when turned off for long periods.

“No individual Galileo satellite has experienced more than two clock failures, so the robust quadruple redundancy designed into the system means all 18 members of the constellation remain operational,” ESA noted in a statement.

Galileo hasn’t had the best of times since it first took shape at the turn of the century. The program is many years behind schedule, and expected to cost about 7 billion euros by the time it is complete in 2020; the original figure touted was just 1.1 billion euros by 2014.

November’s launch represented a big milestone for ESA, as the system was switched on for the first time, providing limited location services for smartphones and in-car systems. Here’s hoping they can turn things around, and get things running smoothly soon.