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GPS 18 Technical Specifications (190-00307-00) Rev. D
Page 12
4 GPS 18 PC, GPS 18 LVC, & GPS 18-5Hz SOFTWARE INTERFACE
The interface protocol design of the GPS 18 PC, LVC, and GPS 18-5Hz products is based on the National
Marine Electronics Association’s NMEA 0183 ASCII interface specification. This standard is fully defined
in NMEA 0183, Version 2.30. Copies may be obtained from NMEA, www.nmea.org.
In addition to the standard NMEA 0183 sentences, the GPS 18 PC, LVC, and GPS 18-5Hz may also be
configured to transmit information over their serial interface using NMEA 0183 compliant Garmin
proprietary sentences. These proprietary sentences begin with the characters, “$PGRM”, instead of the
characters “$G” that are typical of the standard NMEA 0183 sentences. The characters “$P” indicate that
the sentence is a proprietary implementation and the characters and “GRM” indicate that it is Garmin’s
proprietary sentence. The letter (or letters) that follow the characters “$PGRM” uniquely identifies that
particular Garmin proprietary sentence.
It is also possible to configure the GPS 18 PC, LVC, and GPS 18-5Hz to transmit binary phase data
information over their serial interface. See Appendix B: Binary Phase Output Format for details.
The GPS 18 USB does not transmit NMEA sentences: It transmits using the Garmin USB interface. The
Garmin USB interface is discussed in the Garmin Device Interface Specification located on the Garmin
Web site at: http://www.garmin.com/support/commProtocol.html. You can configure the GPS 18 USB to
transmit binary phase data information over the USB interface. Refer to Appendix B: Binary Phase Output
Format.
The following sections describe the NMEA 0183 data format of each sentence transmitted and received by
the GPS 18 PC, LVC, and GPS 18-5Hz products.
4.1 RECEIVED NMEA 0183 SENTENCES
The following paragraphs define the sentences that can be received on the GPS sensors’ 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