14
Software Interface
The GPS 35LP interface protocol design is based on the National Marine Electronics Association’s
NMEA 0183 ASCII interface specification, which is fully defined in
“NMEA 0183, Version 2.0”
(copies
may be obtained from NMEA, P.O. Box 50040, Mobile, AL, 36605, USA) and the Radio Technical
Commission for Maritime Services’
“RTCM Recommended Standards For Differential Navstar GPS
Service, Version 2.1, RTCM Special Committee No. 104”
(copies may be obtained from RTCM, P.O. Box
19087, Washington, DC, 20036, USA). The GPS 35LP interface protocol, in addition to transmitting
navigation information as defined by NMEA 0183, transmits additional information using the convention
of GARMIN proprietary sentences.
The following sections describe the data format of each sentence transmitted and received by the GPS
35LP sensor. The baud rate selection, one-pulse-per-second output interfaces and RTCM differential
GPS input are also described.
4.1
NMEA Received sentences
The subsequent paragraphs define the sentences which can be received on RXD1 by the GPS 35LP
receivers. Null fields in the configuration sentence indicate no change in the particular configuration
parameter.
All sentences received by the GPS 35LP must be terminated with <CR><LF>, but do not require the
checksum *hh. The checksum is used for parity checking data and it is recommended that the checksum
be used in environments containing high electromagnetic noise. It is generally not required in normal PC
environments. Sentences may be truncated by <CR><LF> after any data field and valid fields up to that
point will be acted on by the GPS 35LP.
4.1.1
Almanac Information (ALM)
$GPALM,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>,<13>,<14>,<15> *hh<CR><LF>
The $GPALM sentence can be used to initialize the receivers stored almanac information if battery back-
up has failed.
<1>
Total number of ALM sentences to be transmitted by the sensor board during almanac download.
This field can be null or any number when sending almanac to the sensor board.
<2>
Number of current ALM sentence. This field can be null or any number when sending almanac
to the sensor board.
<3>
Satellite PRN number, 01 to 32.
<4>
GPS week number.
<5>
SV health, bits 17-24 of each almanac page.
<6>
Eccentricity
<7>
Almanac reference time.
<8>
Inclination angle.
<9>
Rate of right ascension.
<10>
Root of semi major axis.
<11>
Omega, argument of perigee.
<12>
Longitude of ascension node.
<13>
Mean anomaly
<14>
afo clock parameter
<15>
af1 clock parameter
