Galileo OS SIS ICD.indd - GSA - Europa

4. Galileo Message Structure
4.1. General Message Format Specifi cation
4.1.1. General Navigation Message Content
The Galileo signal-in-space data channels transmit different message types according
to the general contents identifi ed in Table 26 below. The F/NAV types of message
correspond to the OS and the I/NAV types of message correspond to the SoL. Note that
the C/NAV message format is not subject of this SIS ICD.
Message Type Services Component
F/NAV OS E5a-I
I/NAV OS/CS/SoL E5b-I and E1-B
C/NAV CS E6-B
4.1.2. General Navigation Message Structure
Table 26. Message Allocation and General Data Content
The complete 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. The page is the basic structure for building the
navigation message. This arrangement allows accomplishing the three different main
categories of data to be transmitted, which are repeated at fast (for urgent data, such
as integrity), medium (like data required for warm start TTF) and slow rates (like data
required for cold start TTF).
For all message types, only the message pages as the smallest unit of data are equipped
with a ‘type’ marker to identify the content of each page received by the user. There is
no management data transmitted within the navigation message to indicate subframe
and frame structures, and indeed these higher level structures are to be considered as
typical fl ow of pages refl ecting an instantaneous Galileo navigation message design,
and may evolve together with future evolutions of Galileo. This evolution may involve
also the inclusion of additional new page types beyond the types defi ned in this version
of the Galileo OS SISICD. A user receiver is expected to be able to recognize page
types and to react properly and well controlled to page types unknown to its software
as well as to variations in the fl ow of the received pages.
4.1.3. Bit and Byte Ordering Criteria
All data values are encoded using the following bit and byte ordering criteria:
● For numbering, the most signifi cant bit/byte is numbered as bit/byte 0
● For bit/byte ordering, the most signifi cant bit/byte is transmitted fi rst
26 © European Union 2010
Document subject to terms of use and disclaimers p. ii-iii
OD SIS ICD, Issue 1, February 2010