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4
SOFTWARE INTERFACE
The GPS 16/17 series products interface protocol design on COM 1 is based on the National Marine
Electronics Association's NMEA 0183 ASCII interface specification. The COM 2 port can receive
differential GPS (DGPS) correction data using the Radio Technical Commission for Maritime Services'
RTCM SC-104 standard. These standards are fully defined in NMEA 0183, Version 3.0 (copies may be
obtained from NMEA,
www.nmea.org)
Service, Version 2.2, RTCM Special Committee No. 104 (copies may be obtained from RTCM,
www.rtcm.org).
The GPS 16/17 series products interface protocol, in addition to transmitting navigation information as
defined by NMEA 0183, transmits additional information using the convention of Garmin proprietary
sentences.
Binary phase data information can alternatively be output on the COM 1 port; see
Phase Output Format
for details.
The following sections describe the NMEA 0183 data format of each sentence transmitted and received by
the GPS 16/17 series products. The baud rate selection, one-pulse-per-second output interfaces and RTCM
differential GPS input are also described.
4.1
Received NMEA 0183 Sentences
The following paragraphs define the sentences that can be received on the GPS sensors' COM 1 port. Null
fields in the configuration sentence indicate no change in the particular configuration parameter. All
sentences received by the GPS sensor must be terminated with <CR><LF>, the ASCII characters for
carriage return (0D hexadecimal) and line feed (0A hexadecimal). The checksum *hh is used for parity
checking data and is not required, but is recommended for use in environments containing high
electromagnetic noise. It is generally not required in normal PC environments. When used, the parity bytes
(hh) are the ASCII representation of the exclusive-or (XOR) sum of all the characters between the "$" and
"*" characters, non-inclusive. The hex representation must be a capital letter, such as 3D instead of 3d.
Sentences may be truncated by <CR><LF> after any data field and valid fields up to that point will be
acted on by the sensor.
4.1.1
Almanac Information (ALM)
The $GPALM sentence can be used to initialize the GPS sensor's stored almanac information in the
unlikely event of non-volatile memory loss or after storing longer than six months without tracking GPS
satellites.
$GPALM,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>,<13>,<14>,<15>*hh<CR><LF>
<1>
Total number of ALM sentences to be transmitted by the GPS sensor during almanac download.
This field can be null or any number when sending almanac to the GPS sensor.
<2>
Number of current ALM sentence. This field can be null or any number when sending almanac
to the GPS sensor.
<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>
af0 clock parameter
<15>
af1 clock parameter
190-00228-20
and RTCM Recommended Standards For Differential Navstar GPS
GPS 16/17 Technical Specifications
Page 12
Appendix B: Binary
Rev. C