Read in full from the ESA: Hyper-accurate clocks – the beating heart of Galileo
Galileo, Europe’s twenty-first century navigation system, also relies on clocks – but they are millions of times more accurate than those earlier timepieces.
The operational Galileo satellites will carry two types of clocks – passive hydrogen masers and rubidium atomic frequency standards. Each satellite will be equipped with two hydrogen masers, one of which will be the primary reference for generating the navigation signals, with the other as a cold (non-operating) spare...
...As a by-product of satellite navigation’s need for accurate timekeeping, Galileo will also be able to offer precision time services to be used, for example, in the time stamping of financial transactions.