Novatel 01020347, User Manual

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26 WAAS GUS Signal Generator User Guide Rev 1
Chapter 5 Messages
5.1 WMP Message Structure
Command and status messages are exchanged between the WMP and the Signal Generator using a RS-232 link.
Both command and status messages are encapsulated in a fixed length packet. This packet contains a
synchronization field, a L1/L5 indication field, message data fields and a CRC field for reliable packet transfer.
Table 5 shows the format of a message packet. Binary messages (command or status) are received by the Signal
Generator in byte order [0] through [35]. The Signal Generator swaps these bytes as necessary in order to recover
the original data fields. The tables shown in this chapter indicate the order in which bytes are received and
transmitted by the Signal Generator.
Table 5: Packet Format
5.1.1 Packet Synchronization Field
The packet synchronization field consists of four (4) bytes containing the hex value 0xAA5555AA. The Signal
Generator looks for these synchronization bytes and interprets them to be the start of a new packet. Status messages
transmitted to the WMP also have these synchronization bytes encoded to the hex value 0xAA5555AA.
Table 6
shows the Packet Synchronization field. Table 7 shows the Packet Synchronization field byte and bit order.
Table 6: Packet Synchronization Field
Table 7: Packet Synchronization Field Bit Format
5.1.2 L1/L5 Indication Field
This field consists of one (1) byte containing the L1 or L5 indication flag. If a command is targeted for a L1 Signal
Generator then this byte contains the integer value 1, otherwise the integer value 5 is used to indicate a L5 target. If
a L1 signal generates a status message, this byte contains the integer value 1, otherwise the integer value 5 is used to
indicate a L5 signal generation source.
Table 8 shows the L1/L5 Indication field. Table 9 on Page 27 shows the L1/
L5 Indication field bit order.
Field Bytes Byte Position Description
1 4 Byte [0:3]
Packet Synchronization Bytes Field
(start of packet)
2 1 Byte [4:4] L1/L5 Indication Field
3 29 Byte [5:33]
Command Message Payload Fields or
Status Message Payload Fields.
4 2 Byte [34:35] 16 bit CRC Field.
Field Bytes Byte Position Description Value
1 4 Byte [0:3] Start of packet 0xAA5555AA
Byte [0] Byte [1] Byte [2] Byte [3]
D7 - D0 D15 - D8 D23 - D16 D31 - D25
D0 = LSB D31 = MSB
0xAA 0x55 0x55 0xAA