Galileo enters the next stage

Galileo will be a satellite-based navigation system under European civilian control and independent of the existing systems GPS (global positioning system, USA) and GLONASS (the Russian pendant system). After completion of Galileo 30 satellites will orbit the earth at a distance of 24 000 km. Each of them will have several atomic clocks on board and will send continuous time signals to earth. The time signals can be exploited by means of propagation time measurements to determine a position with an accuracy of a few metres. Concerning the technical features Galileo is very similar to the American and Russian systems, so that currently negotiations are being held with the goal to make the three systems “interoperable”. Signals from all satellites of the three systems are to be made receivable to enable a position determination with just one receiver.

In December 2003 the preparation phase was completed with the so-called “Galileo complete definition” and the phase for detail planning, set-up, testing and inspection began. During this phase the first four satellites of the subsequent constellation are to be placed in orbit and a first set-up stage of the ground segment is to be created by mid 2008. Operation of the satellites will be controlled from this ground segment. Additionally, the time signals of the satellites are to be produced and controlled at the ground station. This will be achieved with support from the “Precise Time Facility” together with other components. The “Precise Time Facility” will produce the “Galileo System Time” (GST). PTB is consulting the European Space Agency and the participating industry in the realisation of GST and the design of the “Precise Time Facility”.

Any Satellite-based navigation system is also a system for time distribution. At a known position receiving the satellite signals can be used to transmit GST to a user. GST is to be kept in excellent agreement with coordinated universal time, UTC, and the relation between the two is to be made known to the user. This was seen as an additional attractive feature of Galileo, however, not immediately justified through the primary task of the system. Therefore, an independent entity will be created to provide the necessary time information to the Galileo system. This so-named “Galileo Time Service Provider” is currently set-up by “Galileo Joint Undertaking”, a joint enterprise in public and private holding in Brussels, with funds from the European Commission. Together with NPL (National Physical Laboratory, UK) and IEN (Istituto Elettrotecnico Nazionale Galileo Ferraris, Italy) PTB has determined the requirements concerning organization and function of the Time Service Provider. An important part of PTB’s duties will be to organize and conduct time comparisons between several European timing institutes and the PTF.