btnrf52-gps/lib/NeoGPS/extras/doc/Garmin.md

Garmin Proprietary messages

To use the Garmin Proprietary NMEA message parser, you must enable the following in NMEAGPS_cfg.h:

#define NMEAGPS_PARSE_MFR_ID
#define NMEAGPS_DERIVED_TYPES

NeoGPS implements the following additional NMEA messages:

• $PGRMF - Fix data

You may want to change the configured PGRM messages in src/Garmin/PGRM_cfg.h. It is currently configured to work with the example application, PGRM.ino. PGRM_cfg.h has the following configuration items:

#define GARMINGPS_PARSE_PGRMF

As shown in the PGRM.ino example program, you should use an instance of GarminNMEA instead of the typical NMEAGPS:

#include <NeoGPS_cfg.h>
#include <Garmin/GrmNMEA.h>
#include <GPSTime.h>

   ...

static GarminNMEA gps; // This parses received characters
static gps_fix    fix;

A few different header files are included, but all other operations are identical.

Notice that the $PGRMF message sets the global variable, GPSTime::leap_seconds. As reported in Issue #90, this is required for exact UTC calculations that span dates with different GPS leap seconds (see Wikipedia article).