“Once, when I was sailing along the coast, my navigational signal (a terrestrial one at the time) told me I was on the land," recalls Hans Fromm, retired Deputy Head of the Navigation Department at ESTEC.

"Obviously, I knew it was out and could estimate by how much. But without such an obvious marker, how would I have known the accuracy of the signal?

“Such knowledge is crucial for many applications. For example, a train needs to know which rail it’s travelling on. It’s important for safety, especially where life could be at risk."

EGNOS provides the information needed to use navigational signals from GPS satellites for such safety critical applications. It improves the accuracy of position measurements from about five metres to less than two metres, informs users of the errors in the position measurements and warns of disruption to a satellite signal within six seconds. “EGNOS will take responsibility and guarantee the service,” explains Fromm.

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Three geostationary satellites and a complex network of ground stations carry out this task. The three satellites send out a ranging signal similar to those transmitted by the GPS satellites. However, these signals are more than another opportunity for users to fix a position. They also provide information about the accuracy of position measurements delivered by GPS so that a pilot, for example, can assess whether the position is accurate enough to rely on.
 
This information, or integrity data, is modulated onto the ranging signal. It includes accurate information on the position of each GPS satellite, the accuracy of the atomic clocks on board the satellites and information on disturbances within the ionosphere that might affect the accuracy of positioning measurements. The EGNOS receiver, decodes the signal to give a more accurate position than is possible with GPS alone, and an accurate estimate of errors.