161 lines
6.4 KiB
Markdown
161 lines
6.4 KiB
Markdown
# minmea, a lightweight GPS NMEA 0183 parser library
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[](https://travis-ci.org/kosma/minmea)
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Minmea is a minimalistic GPS parser library written in pure C intended for
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resource-constrained platforms, especially microcontrollers and other embedded
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systems.
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## Features
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* Written in ISO C99.
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* No dynamic memory allocation.
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* No floating point usage in the core library.
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* Supports both fixed and floating point values.
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* One source file and one header - can't get any simpler.
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* Tested under Linux, OS X, Windows and embedded ARM GCC.
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* Easily extendable to support new sentences.
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* Complete with a test suite and static analysis.
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## Supported sentences
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* ``RMC`` (Recommended Minimum: position, velocity, time)
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* ``GGA`` (Fix Data)
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* ``GSA`` (DOP and active satellites)
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* ``GLL`` (Geographic Position: Latitude/Longitude)
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* ``GST`` (Pseudorange Noise Statistics)
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* ``GSV`` (Satellites in view)
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* ``VTG`` (Track made good and Ground speed)
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* ``ZDA`` (Time & Date - UTC, day, month, year and local time zone)
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Adding support for more sentences is trivial; see ``minmea.c`` source. Good documentation
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on NMEA is at https://gpsd.gitlab.io/gpsd/NMEA.html
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## Compatibility
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Minmea runs out-of-the-box under most Unix-compatible systems. Support for non-Unix systems
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(including native Windows builds under MSVC) is provided via compatibility headers:
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1. Define `MINMEA_INCLUDE_COMPAT` in the build environment.
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2. Add appropriate compatibility header from under `compat/` directory as `minmea_compat.h`.
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## Fractional number format
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Internally, minmea stores fractional numbers as pairs of two integers: ``{value, scale}``.
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For example, a value of ``"-123.456"`` would be parsed as ``{-123456, 1000}``. As this
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format is quite unwieldy, minmea provides the following convenience functions for converting
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to either fixed-point or floating-point format:
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* ``minmea_rescale({-123456, 1000}, 10) => -1235``
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* ``minmea_float({-123456, 1000}) => -123.456``
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The compound type ``struct minmea_float`` uses ``int_least32_t`` internally. Therefore,
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the coordinate precision is guaranteed to be at least ``[+-]DDDMM.MMMMM`` (five decimal digits)
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or ±2cm LSB at the equator. Note that GPS modules commonly only provide four deciminal digits
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(``[+-]DDDMM.MMMM``), which equates to ±20cm (0.0001 minute is 0.0001/60 degrees and one degree
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is about 111km) at the equator.
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## Coordinate format
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NMEA uses the clunky ``DDMM.MMMM`` format which, honestly, is not good in the internet era.
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Internally, minmea stores it as a fractional number (see above); for practical uses,
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the value should be probably converted to the DD.DDDDD floating point format using the
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following function:
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* ``minmea_tocoord({-375165, 100}) => -37.860832``
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The library doesn't perform this conversion automatically for the following reasons:
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* The conversion is not reversible.
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* It requires floating point support.
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* The user might want to perform this conversion later on or retain the original values.
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## Example
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```c
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char line[MINMEA_MAX_LENGTH];
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while (fgets(line, sizeof(line), stdin) != NULL) {
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switch (minmea_sentence_id(line, false)) {
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case MINMEA_SENTENCE_RMC: {
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struct minmea_sentence_rmc frame;
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if (minmea_parse_rmc(&frame, line)) {
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printf("$RMC: raw coordinates and speed: (%d/%d,%d/%d) %d/%d\n",
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frame.latitude.value, frame.latitude.scale,
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frame.longitude.value, frame.longitude.scale,
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frame.speed.value, frame.speed.scale);
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printf("$RMC fixed-point coordinates and speed scaled to three decimal places: (%d,%d) %d\n",
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minmea_rescale(&frame.latitude, 1000),
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minmea_rescale(&frame.longitude, 1000),
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minmea_rescale(&frame.speed, 1000));
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printf("$RMC floating point degree coordinates and speed: (%f,%f) %f\n",
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minmea_tocoord(&frame.latitude),
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minmea_tocoord(&frame.longitude),
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minmea_tofloat(&frame.speed));
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}
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} break;
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case MINMEA_SENTENCE_GGA: {
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struct minmea_sentence_gga frame;
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if (minmea_parse_gga(&frame, line)) {
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printf("$GGA: fix quality: %d\n", frame.fix_quality);
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}
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} break;
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case MINMEA_SENTENCE_GSV: {
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struct minmea_sentence_gsv frame;
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if (minmea_parse_gsv(&frame, line)) {
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printf("$GSV: message %d of %d\n", frame.msg_nr, frame.total_msgs);
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printf("$GSV: sattelites in view: %d\n", frame.total_sats);
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for (int i = 0; i < 4; i++)
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printf("$GSV: sat nr %d, elevation: %d, azimuth: %d, snr: %d dbm\n",
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frame.sats[i].nr,
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frame.sats[i].elevation,
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frame.sats[i].azimuth,
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frame.sats[i].snr);
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}
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} break;
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}
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}
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```
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## Integration with your project
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Simply add ``minmea.[ch]`` to your project, ``#include "minmea.h"`` and you're
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good to go.
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## Running unit tests
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Building and running the tests requires the following:
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* Check Framework (https://libcheck.github.io/check/).
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* Clang Static Analyzer (https://clang-analyzer.llvm.org/).
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If you have both in your ``$PATH``, running the tests should be as simple as
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typing ``make``.
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## Limitations
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* Only a handful of frames is supported right now.
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* There's no support for omitting parts of the library from building. As
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a workaround, use the ``-ffunction-sections -Wl,--gc-sections`` linker flags
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(or equivalent) to remove the unused functions (parsers) from the final image.
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* Some systems lack ``timegm``. On these systems, the recommended course of
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action is to build with ``-Dtimegm=mktime`` which will work correctly as long
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the system runs in the default ``UTC`` timezone.
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## Bugs
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There are plenty. Report them on GitHub, or - even better - open a pull request.
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Please write unit tests for any new functions you add - it's fun!
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## Licensing
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Minmea is open source software; see ``COPYING`` for amusement. Email me if the
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license bothers you and I'll happily re-license under anything else under the sun.
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## Author
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Minmea was written by Kosma Moczek <kosma@kosma.pl> and Patryk Szymczak
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<patryk.szymczak@gmail.com> at Cloud Your Car, with bugs fixed by countless
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good people.
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