The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.
Find out more about space activities in our 23 Member States, and understand how ESA works together with their national agencies, institutions and organisations.
Exploring our Solar System and unlocking the secrets of the Universe
Go to topicProtecting life and infrastructure on Earth and in orbit
Go to topicUsing space to benefit citizens and meet future challenges on Earth
Go to topicMaking space accessible and developing the technologies for the future
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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.
#TwelveThingsYouNeverKnewAboutGalileoSatellites