Gps 18X Pc, Gps 18X Lvc, & Gps 18X-5Hz Software Interface; Received Nmea 0183 Sentences; Almanac Information (Alm) - Garmin GPS 18x Series Technical Specifications

4 GPS 18x PC, GPS 18x LVC, & GPS 18x-5Hz SOFTWARE INTERFACE
The interface protocol design of the GPS 18x PC, LVC, and GPS 18x-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 18x PC, LVC, and GPS 18x-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 18x PC, LVC, and GPS 18x-5Hz to transmit binary data information over
their serial interface. See Appendix B: Garmin Binary Output Format
The GPS 18x 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 found in the Garmin Device Interface SDK
located on the Garmin Web site at:
You can configure the GPS 18x USB to transmit binary data information over the USB interface. Refer to
B: Garmin Binary Output Format.
The following sections describe the NMEA 0183 data format of each sentence transmitted and received by the GPS
18x PC, LVC, and GPS 18x-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
190-00879-08
www.garmin.com/support/commProtocol.html.
GPS 18x Technical Specifications
Page 12
for details.
Appendix
Rev. B