User manual

ZyWAN User Manual Configuring the ZyWAN
© 2007 Eurotech Ltd Issue B 57
Arcom format for GPS messages (UDP)
When using the Arcom Format for GPS data, the position message shall be
transmitted using the User Datagram Protocol (UDP). If caching is enabled, the
ZyWAN waits a minimum of 10 seconds to receive an acknowledgement message
after transmission of a position message. If a response message is received that does
not contain the characters ACK or no message is received within the timeout period,
the position message shall be retransmitted. This process shall continue indefinitely.
The ACK should be 3 ASCII characters (0x41 0x43 0x4B).
The position message is defined below. The first 12 bytes are a header, followed by
19-byte portions containing the timestamp and position information. The number of
these 19-byte portions is determined by the Send Threshold parameter. All data shall
be in binary big-endian format, unless otherwise specified.
Message header:
som(1) snum(10) npos(1)
Position data (repeated):
sval(1) time(4) stat(1) sats(1) lat(4) lon(4) psrc(1) speed(1) head(2)
Field Bytes Description
som 1 Start of message indicator. Always set to 0x7E.
snum 10 Unit serial number in ASCII left justified and null(0) filled to the right.
npos 1 Number of positions in this message.
sval 1 Sequence value for each position transmitted. The sequence value
for the first position transmitted is 0. The sequence value is
incremented by 1 for each subsequent position transmitted. Since
this value is only one byte, the maximum sequence value is 255.
time 4 UTC timestamp of the GPS position. This value is in seconds since
1/1/1970.
stat 1 This field contains a code representing status of the position from the
GPS receiver. In addition, bits 5-7 are used as flags for other
conditions.
Status Codes:
Bit: 1 Condition: No response from GPS receiver.
Bit: 2 Condition: Error in response from GPS receiver.
Bit: 3 Condition: Almanac error response from GPS receiver
Bit: 4 Condition: Good position response from GSP receiver