Accurate timekeeping for Galileo
A specially developed hydrogen clock will be the first of its type to fly when it is installed in ESA's (European Space Agency) Galileo satellite navigation system. Accurate clocks are essential on board Galileo if the satellite's position is to be determined accurately. 'In navigation, clocks are the driving factor for determining positions accurately. With an accuracy of better than one billionth of a second in one hour, the clocks on the Galileo satellites will allow you to resolve your position anywhere on the Earth's surface to within 45 cm,' says Franco Emma, the clock expert and navigation engineer at ESTEC, ESA's technical centre in the Netherlands. Each of the 30 satellites in the Galileo system will have two clocks on board, based on the rubidium atomic frequency standard and the other using a passive hydrogen maser. Both clocks however function on the same principle: forcing an atom to jump from one particular energy state to another will cause it to radiate a microwave signal at an extremely stable characteristic frequency. ESA chose the rubidium and hydrogen maser clocks because they are very stable over a few hours and their technology can fly onboard the Galileo satellites. They will however still need to be synchronised regularly with a network of even more stable ground-based reference clocks, which will be based on the caesium frequency standard. The rubidium clock should be ready for qualification by the end of 2001. Both clocks will be put to the test properly in 2004, when the first Galileo satellites go on trial.