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

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# 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](https://github.com/SlashDevin/NeoGPS/issues/90), this is required for exact UTC
calculations that span dates with different GPS leap seconds
(see [Wikipedia article](http://en.wikipedia.org/wiki/Global_Positioning_System#Leap_seconds)).