NavCom Sapphire Rev.J - Manual (Page 40)

Sapphire Technical Reference Manual Rev. J
40
The CRC includes all of the fields in the binary header (9 bytes) and the binary message body
(variable number of bytes). The CRC is expressed as a sequence of four hex-ASCII digits,
preceded by an asterisk. The four hex-ASCII digits represent the binary value of a 16-bit CCITT
cyclic redundancy check computed by the C-language function shown in
Appendix A .
Table 7: Standard Sapphire Binary Header Format
Field # Bits Data Type Description
Length 16 U16
Number of bytes in the binary header plus the data block, i.e.
data block length plus 9 bytes for the length of the binary
header
GPS Week 16 U16 GPS week number
GPS time 32 U32 GPS time (milliseconds into the week)
Time Confidence 4LSB
U08
Receiver time confidence (refer to Table 8)
Version 4MSB Message version control
Table 8: Time Confidence Values
Code Description
0 Time is unknown
1 Time has been set from the real time clock
2 Time has been set from the serial port
3 Time has been obtained from a satellite
4 Time has been obtained from a navigation solution
5 Time has been obtained from a stable navigation solution
Table 9: Simplified Sapphire Binary Header Format
Field # Bits Data Type Description
Length 16 U16
Number of bytes in the binary header plus the data block; i.e.,
data block length plus 2 bytes for the length of this header
Message Updates & Software Revisions
From time to time it may be necessary for NavCom to change the format of an existing
message. This is normally accomplished by appending to the existing message (which will be
defined in a later version of this manual). Programmers should design software to be forward
compatible by recognizing that messages may be extended and the content of the extension
may be unknown to the user. In this circumstance, the message length will increase. Do not
reject the data record if the message length and checksum are valid for any given record. Allow
the program to ignore “undefined data” to ensure forward compatibility.