LessGPS

This is a Python library for parsing NNEA (GPS currently) sentences. It was inspired by Tim Savage Python NMEA Tolkit but made more friendly and easily configurable by using Pyparsing library and YAML description files for NMEA sentences.

Introduction

NMEA format is widely used in navigational software, chart plotters, GPS drivers, etc. In the past this was a job for embedded systems hosting mostly ANSI C programs because of hardware limitations and productivity expectations. But now more and more navigation is made using general purpose laptops, pads and smartphones. Hardware is not an issue any more, and we can use more advanced languages and technologies for navigation tasks.

LessGPS utilizes a fact that NMEA sentences have really simple structure, thus it is possible to define sentence structure in human understandable format and once having core parsing engine add new definitions without actually modifying the code.

Usage

Grammar

There is a ton of NMEA sentences, many of them are proprietary, but there is a limited set of elements, like lat, lon, utc, that form every single message. This is a core of LessGPS grammar, and it is defined in internal grammar_factory class of grammar.py. Then, for convenience, it is possible to make named groups of grammar elements and give names to particular elements.

Configuration

Before the parser can start working, you should provide it with grammar YAML file. The basic grammar is provided in lessgps/data/nmea.yaml. You should populate it with any sentences you need, and than pass the file path to Parser constructor. Everything in the YAML file is very staightforward, for example lets see the definition of GLL:

GLL:
  - Lat: lat
  - Lon: lon
  - UTC: utc
  - GPSStat: gpsstat
  - FixKind?: fixkind

This defines GLL sentense that consists from lat, lon, utc, gpsstat grammar elements and optional fixkind (added in 2.3 version of NMEA protocol). After parsing this GLL string will be translated into python dictionary with keys 'Lat', 'Lon', 'UTC', 'GPSStat' and 'FixKind' accordingly.

Example

# -*- coding: utf-8 -*-
import lessgps
from pprint import pprint

gga = '$GPGGA,084037,2727.9487,S,15305.3408,E,1,07,02.0,-00000.9,M,039.4,M,,*77'
gsa = '$GPGSA,A,3,07,08,11,13,17,27,28,,,,,,05.3,02.0,04.8*0E'
gsv = '$GPGSV,2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,14,22,228,45*75'
rmc = '$GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A'
vtg = '$GPVTG,054.7,T,034.4,M,005.5,N,010.2,K*48'
gll = '$GPGLL,4916.45,N,12311.12,W,225444,A*31'

strings = [gga, gsa, gsv, rmc, vtg, gll]

if __name__ == "__main__":
    parser = lessgps.Parser('lessgps/data/nmea.yaml')
    for sentence in strings:
        print sentence
        pprint(parser.parse(sentence))