On 2 May the Surrey Satellite Technology built Galileo In-Orbit Validation Element (GIOVE)-A spacecraft successfully transmitted its first navigation message, containing the information needed by users' receivers to calculate their position using the European Union, European Space Agency (ESA) proposed European Galileo satellite navigation service.

The navigation message is being generated for demonstration purposes only. This message was sent to GIOVE-A on 2 May from SSTL's Guildford ground station and then transmitted from the spacecraft to receivers.

The complete radio transmission from Galileo has a navigation signal and a navigation message. The signal contains the information needed to measure the distance from the satellite to the user receiver, while the message provides the timing and spacecraft orbit data needed to calculate the time and exact position of the satellite. Prior to 2 May GIOVE-A had been broadcasting only the data needed for measuring the receiver-to-satellite distance.
This first Galileo navigation message was created by GIOVE-A's Thales Alenia Space manufactured navigation signal generator unit, using content prepared by the GIOVE Mission Segment. 

The Mission Segment consists of a world-wide network of 13 Galileo experimental sensor stations and the GIOVE Processing Centre (GPC) located at ESA’s European Space Research and Technology Centre in Noordwijk, the Netherlands. The GPC is operated by European Satellite Navigation Industries. One of the main tasks of the Mission Segment is the generation of this navigation message. 
On 12 January 2006, GIOVE A transmitted its very first signal, to secure the frequencies allocated to Galileo by the International Telecommunication Union.

Source: FlightGlobal.com