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Navstar Message Processing

The structure of the data transmitted by the Navstar satellites is divided into 5 frames of 10 words. Each word is 30 bits long and finishes with a 6 bit parity code. Each word is sent to the message thread when it has all been received and undergoes a parity check, if all 10 words in the frame pass the parity check the message is then moved to a different location for further processing.

The first 3 Frames hold the Ephemeris data, these are the values that are used to calculate the satellite position as part of the receiver position calculation. Depending on the whether this is a full or partial new message either all 3 frames are processed together or just the first frame or the second and third frames are processed together. This is required in order to keep the Ephemeris data properly in-sync.

If the fourth or fifth frames have been received they are processed individually by seperate routines depending on the particular type of data contained in that frame. The fourth and fifth frames consist of 25 pages that repeat every 12 1/2 minutes, depending on the page number the messages are decoded differently. Most of them are the 32 messages that contain Almanac data for all the 32 Navstar satellites. The almanac data is used to calculate a rough satellite position used in the warm allocation process. The other 3 message types contain satellite health information and then there are some secret message types with unknown content for civilian users.

WAAS Message Processing.

Decoding the WAAS data signal is more complicated than the NavStar data as it incorporates FEC coding and CRC parity coding as well as a frame and preamble.

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