The positioning mode indicator was added to several of the messages in NMEA 2.3 for Federal Aviation Administration (FAA).
It is usually a single character but some messages will contain multiple characters, one for each GNSS.
The order of the characters indicates the systems; GPS, GLONASS, Galileo, BeiDou, QZSS.
The positioning mode indicator is used by the following messages:
| Message ID | Description |
|---|---|
| GLL | Geodetic Latitude + Longitude |
| GNS | Global Navigation Satellite System |
| RMC | Recommended Minimum GNSS Data |
| VTG | Velocity and Track made Good |
Positioning modes supported by NMEA:
| Positioning Mode | Description |
|---|---|
| A | Autonomous (non-differential) mode |
| C | Caution (Quectel Querk) |
| D | Differential (DGPS) mode |
| E | Estimated (dead-reckoning) mode |
| F | RTK float mode |
| M | Manual input mode |
| N | Data not valid. Either constellation not in use, or no valid fix |
| P | Precise (NMEA 4.10 and later). No deliberate degradation (e.g. SA), and using hi-res fix (e.g. P-code). Also signifies multi-frequency, SBAS or Precise Point Positioning (PPP). |
| R | RTK integer mode, or using “course position” from almanac (SiRF) 1 |
| S | Simulator mode |
| U | Unsafe (Quectel Querk) |
Notes:
1 - SiRF - position was calculated based on one or more of the SVs having their states derived from almanac parameters, as opposed to ephemerides.