The Galileo satellites transmit different message types: F/NAV type correspond to the Open Service (OS),  I/NAV message correspond to the Safety of Life (SoL) and C/NAV to Comercial Service (CS).

The navigation message data are transmitted on each data component as a sequence of frames. A frame is composed of several sub-frames, and a sub-frame in turn is composed of several pages. Being this last one the basic structure for building the navigation message.

A user receiver is expected to be able to recognize page types and to react in a controlled way to unkown page types as well as to variations in the fl ow of the received pages.