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4
GPS 15XH/15XL SOFTWARE INTERFACE
The factory default interface protocol of the GPS 15xH/15xL serial com port 1 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 15xH/15xL 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 "$P"
indicate that the sentence is a proprietary implementation and the characters and "GRM" indicate
that it is Garmin's proprietary sentence. The character (or characters) that follow the characters
"$PGRM" uniquely identifies that particular Garmin proprietary sentence.
It is also possible to configure the GPS 15xH/15xL to transmit Garmin binary data over their serial
com port 1 interface. See
The following sections describe the NMEA 0183 data format of each sentence transmitted and
received by the GPS 15xH/15xL.
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>.
Please note that <CR> and <LF> denote the ASCII characters for carriage return (0D
hexadecimal) and line feed (0A hexadecimal), respectively. So, <CR> and <LF> are each
transmitted as single bytes.
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 upper and
lower nibbles of the exclusive-or (XOR) parity, computed over all of the characters between the
"$" and "*" characters, non-inclusive. The hexadecimal representation uses capital letters, 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 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><L
F>
<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
Appendix B: Garmin Binary Output Format
13
for details.
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