There exist two types of I/NAV pages: Nominal and Alert pages.
Nominal pages have a duration of 2 seconds and they are transmitted sequentially in time in two parts of duration 1 second each in both E5b and E1frequency bands. The first part of a page is denoted “even” and the second one is denoted “odd”.
Alert pages have a duration of 1 second and they are transmitted in two parts of duration 1 second each at the same epoch over the E5b and E1 frequency bands. Again, the first part of a page is denoted “even” and the second one is denoted “odd”. This transmission is repeated at the next epoch but switching the two parts between the frequency bands.
The layout of the I/NAV pages is the same for both types and it presents the following three fields:
Sync: The synchronization pattern (0101100000), which is not encoded, allows the receiver to achieve synchronisation to the page boundary
I/NAV page part: 114 bits
Tail: consists of 6 zero-bits enabling completion of the FEC decoding of each page’s information content in the user receiver
The I/NAV navigation message is transmitted on both E1 and E5b channels at a rate of 125 bps. In case no valid I/NAV data is to be transmitted, the satellite generates and downlinks the dummy message replacing the pages in the nominal sequencing.