gps-wizard

NMEA - National Marine Electronics Association

Overview

To facilitate the loading of NMEA data, RMC and GGA messages are combined into a single message.

The combined messages are copied into standard ndarrays, converting to standard units; degrees, m/s, etc.

RMC - Recommended Minimum Navigation Information

The RMC message contains the core data; latitude, longitude, speed and course.

NMEA Name Raw Name Type Name Type Units Resolution
UTC hhmmss f8 ts float64 s 1 x 10-3
Status status U1 - - - -
Latitude 1 lat f8 lat float64 deg 1 x 10-7
N or S ns U1 - - - -
Longitude 1 lon f8 lon float64 deg 1 x 10-7
E or W ew U1 - - - -
Speed over Ground 2 sog f4 sog float32 m/s 1 x 10-3
Track Made Good (True) 3 cog f4 cog float32 deg 1 x 10-3
Date 4 ddmmyy u4 ts float64 s 1 x 10-3
Magnetic Variation magVar f4 - - - -
E or W magVarEw U1 - - - -
FAA Mode Indicator 5 faaMode U1 - - - -
Nav Status 6 navStatus U1 - - - -

Notes:

  1. Latitude and longitude is provided in degrees and minutes in the NMEA format. The internal conversion to degrees is rounded to 7 decimal places.
  2. Speed over ground is provided in knots in the NMEA format. The internal conversion to m/s is rounded to 3 decimal places.
  3. Course over ground / track made good is automatically rounded to 3 decimal places.
  4. Two digit years are since 1 Jan 1980.
  5. FAA mode indicator is NMEA 2.3 and later.
  6. Nav status is NMEA 4.1 and later.

GGA - Global Positioning System Fix Data

The first few fields are unused because they are also present in the corresponding RMC message.

NMEA Name Raw Name Type Name Type Units Resolution
UTC - - - - - -
Latitude - - - - - -
N or S - - - - - -
Longitude - - - - - -
E or W - - - - - -
GPS Quality Indicator quality u1 - - - -
Number of Satellites numSv u1 sat uint8 - -
Horizontal Dilution of Precision 1 hdop f4 hdop float32 - 1 x 10-2
Altitude from MSL 2 alt f8 ele float64 m 1 x 10-3
Units of Altitude altUnit U1 - - - -
Geoidal Separation geoSep f4 - - - -
Units of Geoidal Separation geoSepUnit U1 - - - -
Age of differential GPS data dgpsAge u2 - - - -
Differential reference station ID dgpsId u2 - - - -

Notes:

  1. HDOP does not have a fixed precision but will rarely be more than 2 decimal places.
  2. Altitude does not have a fixed precision but will rarely be more than 3 decimal places.

Latitude and Longitude

GPS / GNSS chips outputting NMEA data typically provide a maximum precision of ddmm.mmmmm for latitude and dddmm.mmmmm for longitude. Since the mm before the decimal point is whole minutes (00 - 60), overall resolution is only 60% of the SiRF and ublox binary formats which provide 7 decimal digits.

TODO

Determining fix type from GGA quality indicator.

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