SEPTENTRIO PolaRx5TR, Справочное руководство

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4 SBF Reference
4.1.12 Decoding of SBF Blocks
In order to decode an SBF block, one has to identify the block boundaries in the data stream
coming from the receiver. This involves searching for the initial "$@" characters that mark
the beginning of each SBF block. Since the "$@" sequence can occur in the middle of an SBF
block as well, additional checking is needed to make sure that a given "$@" is indeed the
beginning of a block. The following procedure is recommended to decode SBF data stream.
1. Wait until the "$@" character sequence appears in the data stream from the receiver.
When it is found, go to point 2.
2. Read the next two bytes. It should be the block CRC. Store this value for future refer-
ence.
3. Read the next two bytes and store them in a buffer. It should be the block ID.
4. Read the next two bytes and append them to the buffer. It should be the
Length
field
of the SBF block. It should be a multiple of 4. If not, go back to point 1.
5. Read the next (
Length-8
) bytes and append them to the buffer. Compute the CRC of
the buffer. The computed CRC should be equal to the CRC stored at point 2. If not, go
back to point 1, else a valid SBF block has been detected and can be interpreted by the
reading software.
6. If the block number (bits 0 to 12 of the
ID
field decoded at point 3) is of interest to your
application, decode the SBF block.
7. Go back to point 1 and search for the new occurrence of the "$@" sequence after the
end of the last byte of the block that was just identified.
To ensure compatibility with future upgrades of SBF, it is recommended that the decoding
software observes the following rules:
• Only bits 0 to 12 of the
ID
field must be used to identify a block. Bits 13 to 15 represent
the revision number.
• The lengths of SBF blocks and sub-blocks should not be considered constant and hard-
coded in the decoding software. Instead, the decoding software must use the
Length
and
SBLength
fields encoded in the SBF block.
• Padding bytes should be ignored.
• Reserved fields and reserved bits in bit-fields should be ignored.
• SBF blocks or sub-blocks of which the satellite ID field or the signal number field is
undefined in this document should be ignored (see section 4.1.9).
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