GALILEO introduction - The operating principle how a position of a user is taken by the GALILEO satellite radio navigation system is simple:
GALILEO determines distance between a GALILEO satellite and a GALILEO receiver by measuring the amount of time it takes a radio signal to travel from the satellite to the
receiver. Radio waves travel at the speed of light, which is about 300,000 kilometer per second. So, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance
from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal's travel time can be
The GALILEO satellites in the constellation are fitted with an atomic clock measuring time very accurately. The satellites emit personalised signals indicating the precise time the signal leaves the satellite. The
ground receiver, incorporated for example into a mobile phone, has in its memory the precise details of the orbits of all the satellites in the constellation. By reading the incoming signal, it can thus recognise
the particular satellite, determine the time taken by the signal to arrive and calculate the distance from the satellite. Once the ground receiver receives the signals from at least four satellites simultaneously,
it can calculate the exact position.
Fig. 1: Principle of the satellite navigation with GALILEO
In order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. The GALILEO
satellites emit personalised signals indicating the precise time the signal leaves the satellite. The ground receiver, incorporated for example into a mobile phone, has in its memory the precise
details of the orbits of all the satellites in the constellation. The code received from the satellite can be compared with the code generated by the receiver. By comparing the codes, the time
difference between when the satellite generated the code and when the receiver generated the code can be determined. This interval is the travel time of the code. Multiplying this travel time, in
seconds, by 300,000 kilometer per second gives the distance from the receiver position to the satellite in kilometre.
GALILEO is based on a constellation of 30 satellites and ground stations providing information concerning the positioning of users in many sectors such as transport (vehicle location, route
searching, speed control, guidance systems, etc.), social services (e.g. aid for the disabled or elderly), the justice system and customs services (location of suspects, border controls), public
works (geographical information systems), search and rescue systems, or leisure (direction-finding at sea or in the mountains, etc.).
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