Galileo infographic: 'A question of time'

A question of time: Why the need for these ongoing corrections? Satellite navigation relies on measuring the time taken for signals from space to reach the receiver on the surface – a minimum of four signals are needed to pinpoint a positon on the ground – but to maintain accuracy the timing must be accurate to a few billionths of a second. A single billionth of a second out equals 30 cm of positioning error. A second’s error would mean the receiver might as well be on the Moon. Similarly, any slight shift in the satellite’s orbital position also changes the signal travel time, the basis for the all-important ranging calculation.

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