Mitel GPS Orion-S/-HD Receiver - Manual (Page 25)

Document Title: 21
User’s Manual for the GPS Orion-S/-HD Receiver
Document No. Issue 1.0
GTN-MAN-0110 June 22, 2003
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DLR/GSOCNo part of this document shall be reproduced in any form or disclosed to third parties without prior authorization.
4.2.2 NMEA Format
For compatibility reasons, a limited set of NMEA output messages is available in the Orion-S
and –HD receivers. Even though the NMEA format could likewise be applied for command-
ing, this option is not presently supported.
According to the NMEA-0183 standard, each message is initiated by a dollar (‘$’) character
and terminated by a carriage-return (CR, ASCII 0x13) and line-feed (LF, ASCII 0x10) record
delimiter (Fig. 4.2). The message header provides a unique five character identifier, which is
separated from the data field by a comma (‘,’). Commas are likewise used to separate indi-
vidual items in the data field. This is followed by a footer comprising an asterisk (‘*’) and a
two character This checksum is calculated as the exclusive-or of all characters in the header
and data field, (i.e. in between but excluding the ‘$’ and ‘*’ characters) and expressed in up-
percase hex format.
‘$’ C C C C C ‘,’ x x x … x x ‘*’ H H CR LF
C = Alphabetic character (uppercase).
x = Data field.
H = Hexadecimal checksum character (uppercase).
CR = Carriage return (0x13).
LF = Line feed (0x13).
Fig 4.2 NMEA format definition
Aside from the overall protocol, the NMEA standard specifically defines a set of default mes-
sages ($GPxxx) for use in common GPS receivers. Out of these, only the $GPGGA mes-
sage is presently available in the Orion-S/HD receivers. Manufacturer specific NMEA mes-
sages supported by the receiver are designated by a $PASHR (Proprietary Ashtech Re-
sponse) or $PDLRM (Proprietary DLR Message). In these cases, the first item of the data
field is a three character code that further specifies the message contents (e.g. $PDLRM,IIP
for instantaneous impact point coordinates).
NMEA messages typically do not provide a date field and are therefore ambiguous with a
24h period. If absolute timing is required, the WinMon messages should be preferred. All
time stamps in the NMEA messages are given in hours, minutes, and seconds referred to the
UTC system.