pynmeagps

pynmeagps is an original Python library aimed primarily at the subset of the NMEA 0183 v4 protocol relevant to GNSS/GPS receivers - that is, NMEA 0183 'talkers' 'Gx' (standard) or 'Px' (proprietary).

The intention is to make it as easy as possible to read, parse and utilise NMEA GNSS/GPS messages in Python applications.

The pynmeagps homepage is located at https://github.com/semuconsulting/pynmeagps.

FYI There is a companion library pyubx2, which handles u-blox © UBX protocol GNSS/GPS messages.

Current Status

At time of writing the library implements a comprehensive set of outbound (GET) and inbound (SET/POLL) GNSS NMEA messages relating to GNSS/GPS devices, but is readily extensible. Refer to NMEA_MSGIDS and NMEA_MSGIDS_PROP in nmeatypes_core.py for the complete dictionary of messages currently supported. Additional NMEA 'talkers' may be added in due course.

Sphinx API Documentation in HTML format is available at https://www.semuconsulting.com/pynmeagps.

Contributions welcome, particularly any standard or proprietary NMEA message definitions not yet included - please refer to CONTRIBUTING.MD.

Bug reports and Feature requests - please use the templates provided.

Installation

pynmeagps is compatible with Python 3.6+ and has no third-party library dependencies.

In the following, python & pip refer to the Python 3 executables. You may need to type python3 or pip3, depending on your particular environment.

The recommended way to install the latest version of pynmeagps is with pip:

python -m pip install --upgrade pynmeagps

If required, pynmeagps can also be installed into a virtual environment, e.g.:

python -m pip install --user --upgrade virtualenv
python -m virtualenv env
source env/bin/activate (or env\Scripts\activate on Windows)
(env) python -m pip install --upgrade pynmeagps
...
deactivate

Reading (Streaming)

class pynmeagps.nmeareader.NMEAReader(stream, **kwargs)

You can create an NMEAReader object by calling the constructor with an active stream object. The stream object can be any data stream which supports a read(n) -> bytes method (e.g. File or Serial, with or without a buffer wrapper).

Individual input NMEA messages can then be read using the NMEAReader.read() function, which returns both the raw data (as bytes) and the parsed data (as an NMEAMessage object, via the parse() method). The function is thread-safe in so far as the incoming data stream object is thread-safe. NMEAReader also implements an iterator.

The constructor accepts the following optional keyword arguments:

nmeaonly: True = raise error if stream contains non-NMEA data, False = ignore non-NMEA data (default)
validate: bitfield validation flags (can be used in combination):

VALCKSUM (0x01) = validate checksum (default)
VALMSGID (0x02) = validate msgId (i.e. raise error if unknown NMEA message is received)

msgmode: 0 = GET (default), 1 = SET, 2 = POLL

Examples:

Serial input - this example will ignore any non-NMEA data.

>>> from serial import Serial
>>> from pynmeagps import NMEAReader
>>> stream = Serial('/dev/tty.usbmodem14101', 9600, timeout=3)
>>> nmr = NMEAReader(stream)
>>> (raw_data, parsed_data) = nmr.read()
>>> print(parsed_data)

File input (using iterator) - this example will produce a NMEAStreamError if non-NMEA data is encountered.

>>> from pynmeagps import NMEAReader
>>> stream = open('nmeadata.log', 'rb')
>>> nmr = NMEAReader(stream, nmeaonly=True)
>>> for (raw_data, parsed_data) in nmr: print(parsed_data)
...

Parsing

You can parse individual NMEA messages using the static NMEAReader.parse(message) function, which takes a string or bytes containing an NMEA message and returns an NMEAMessage object.

Note that latitude and longitude are parsed as signed decimal values for ease of use. Helper methods deg2dms and deg2dmm are available to convert decimal degrees to d°m′s.s″ or d°m.m′ display format.

Attributes within repeating groups are parsed with a two-digit suffix (svid_01, svid_02, etc.).

The parse() function accepts the following optional keyword arguments:

validate: bitfield validation flags (can be used in combination):

VALCKSUM (0x01) = validate checksum (default)
VALMSGID (0x02) = validate msgId (i.e. raise error if unknown NMEA message is received)

msgmode: 0 = GET (default), 1 = SET, 2 = POLL

Example:

>>> from pynmeagps import NMEAReader
>>> msg = NMEAReader.parse('$GNGLL,5327.04319,S,00214.41396,E,223232.00,A,A*68\r\n')
>>> print(msg)
<NMEA(GNGLL, lat=-53.45072, NS=S, lon=2.240233, EW=E, time=22:32:32, status=A, posMode=A)>

The NMEAMessage object exposes different public properties depending on its message ID, e.g. the RMC message has the following properties:

>>> print(msg)
<NMEA(GNRMC, time=22:18:38, status=A, lat=52.62063, NS=N, lon=-2.16012, EW=W, spd=37.84, cog=, date=2021-03-05, mv=, mvEW=, posMode=A)>
>>> msg.msgID
'RMC'
>>> msg.lat, msg.lon
(52.62063, -2.16012)
>>> msg.spd
37.84

Generating

class pynmeagps.nmeamessage.NMEAMessage(talker: str, msgID: str, msgmode: int, **kwargs)

You can create an NMEAMessage object by calling the constructor with the following parameters:

talker (must be a valid talker from pynmeagps.NMEA_TALKERS)
message id (must be a valid id from pynmeagps.NMEA_MSGIDS or pynmeagps.NMEA_MSGIDS_PROP)
msgmode (0=GET, 1=SET, 2=POLL)
(optional) a series of keyword parameters representing the message payload

The 'msgmode' parameter signifies whether the message payload refers to a:

GET message (i.e. output from the receiver - NB these would normally be generated via the NMEAReader.read() or NMEAReader.parse() methods but can also be created manually)
SET message (i.e. command input to the receiver)
POLL message (i.e. query input to the receiver in anticipation of a response back)

The message payload can be defined via keyword arguments in one of two ways:

A single keyword parameter of payload containing the full payload as a list of string values (any other keyword parameters will be ignored).
One or more keyword parameters corresponding to individual message attributes. Any attributes not explicitly provided as keyword parameters will be set to a nominal value according to their type.

e.g. Create a GLL message, passing the entire payload as a list of strings in native NMEA format:

>>> from pynmeagps import NMEAMessage, GET
>>> pyld=['4330.00000','N','00245.000000','W','120425.234','A','A']
>>> msg = NMEAMessage('GN', 'GLL', GET, payload=pyld)
print(msg)
<NMEA(GNGLL, lat=43.5, NS=N, lon=-2.75, EW=W, time=12:04:25.234000, status=A, posMode=A)>

e.g. Create GLL (GET) and GNQ (POLL) message, passing individual typed values as keywords, with any omitted keywords defaulting to nominal values (in the GLL example, the 'time' parameter has been omitted and has defaulted to the current time):

>>> from pynmeagps import NMEAMessage, GET
>>> msg = NMEAMessage('GN', 'GLL', GET, lat=43.5, NS='N', lon=2.75, EW='W', status='A', posMode='A')
>>> print(msg)
<NMEA(GNGLL, lat=43.5, NS='N', lon=-2.75, EW='W', time='12:04:25.234745', status='A', posMode='A')>

>>> from pynmeagps import NMEAMessage, POLL
>>> msg = NMEAMessage('EI', 'GNQ', POLL, msgId='RMC')
>>> print(msg)
<NMEA(EIGNQ, msgId=RMC)>

NB: Once instantiated, an NMEAMessage object is immutable.

Serializing

The NMEAMessage class implements a serialize() method to convert an NMEAMessage object to a bytes array suitable for writing to an output stream.

>>> from serial import Serial
>>> from pynmeagps import NMEAMessage, POLL
>>> stream = Serial('COM6', 38400, timeout=3)
>>> msg = NMEAMessage('EI','GNQ', POLL, msgId='RMC')
>>> msg.serialize()
b'$EIGNQ,RMC*24\r\n'
>>> stream.write(msg.serialize())

Examples

The following examples can be found in the \examples folder:

nmeadump.py is a simple command line utility to stream the parsed NMEA output of a GNSS/GPS device on a specified port.
nmeastreamer.py illustrates how to implement a threaded serial reader for NMEA messages using pynmeagps.NMEAReader, and send poll requests for specific NMEA message types.
nmeafile.py illustrates how to implement a binary file reader for NMEA messages using the pynmeagps.NMEAReader iterator function.
gpxtracker.py illustrates a simple CLI tool to convert an NMEA data dump to a *.gpx track file using pynmeagps.NMEAReader.

Extensibility

The NMEA protocol is principally defined in the modules nmeatypes_*.py as a series of dictionaries. Additional message types can be readily added to the appropriate dictionary. Message payload definitions must conform to the following rules:

1. attribute names must be unique within each message class
2. avoid reserved names 'msgID', 'talker', 'payload', 'checksum'.
3. attribute types must be one of the valid types (IN, DE, CH, etc.)
4. repeating groups must be defined as a tuple ('numr', {dict}), where:
   'numr' is either:
     a. an integer representing a fixed number of repeats e.g. 32
     b. a string representing the name of a preceding attribute containing the number of repeats e.g. 'numSv'
     c. 'None' for an indeterminate repeating group. Only one such group is permitted per payload and it must be at the end.
   {dict} is the nested dictionary of repeating items