Different navigation message protocols are used for different navigation signal type. This is true even for a single system like NAVSTAR GPS. For example, L1C/A signals have a LNAV protocol, while L2C and L5 signals utilize a CNAV protocol. The newest L1C signal will use CNAV-2.
The protocol defines a data distribution into frames, subframes, the subframes structure, transmission intervals, data resolution and so on.
A navigation receiver use the navigation message data flow for several purposes:
1. The data flow allows to resolve a code ambiguity and set the signal time.
2. Ephemeris and clock values are used for the satellite position calculation, pseudorange corrections and coordinates computation.
3. The received data flow and navigation message data can be used for navigation symbols prediction. As result, we can use the wipe-off technique, expand discriminators and significantly increase tracking sensitivity: