minmea, a lightweight GPS NMEA 0183 parser library

Minmea is a minimalistic GPS parser library written in pure C intended for resource-constrained platforms, especially microcontrollers and other embedded systems.

Features

• Written in ISO C99.
• No dynamic memory allocation.
• No floating point usage in the core library.
• Supports both fixed and floating point values.
• One source file and one header - can’t get any simpler.
• Tested under Linux, OS X, Windows and embedded ARM GCC.
• Easily extendable to support new sentences.
• Complete with a test suite and static analysis.

Supported sentences

• GBS (Satellite Fault Detection)
• GGA (Fix Data)
• GLL (Geographic Position: Latitude/Longitude)
• GSA (DOP and active satellites)
• GST (Pseudorange Noise Statistics)
• GSV (Satellites in view)
• RMC (Recommended Minimum: position, velocity, time)
• VTG (Track made good and Ground speed)
• ZDA (Time & Date - UTC, day, month, year and local time zone)

Adding support for more sentences is trivial; see minmea.c source. Good documentation on NMEA is at https://gpsd.gitlab.io/gpsd/NMEA.html

Compatibility

Minmea runs out-of-the-box under most Unix-compatible systems. Support for non-Unix systems (including native Windows builds under MSVC) is provided via compatibility headers:

1. Define MINMEA_INCLUDE_COMPAT in the build environment.
2. Add appropriate compatibility header from under compat/ directory as minmea_compat.h.

If your GPS receiver outputs very long sentences, consider increasing MINMEA_MAX_SENTENCE_LENGTH in your build environment.

Fractional number format

Internally, minmea stores fractional numbers as pairs of two integers: {value, scale}. For example, a value of "-123.456" would be parsed as {-123456, 1000}. As this format is quite unwieldy, minmea provides the following convenience functions for converting to either fixed-point or floating-point format:

• minmea_rescale({-123456, 1000}, 10) => -1235
• minmea_float({-123456, 1000}) => -123.456

The compound type struct minmea_float uses int_least32_t internally. Therefore, the coordinate precision is guaranteed to be at least [+-]DDDMM.MMMMM (five decimal digits) or ±2cm LSB at the equator. Note that GPS modules commonly only provide four decimal digits ([+-]DDDMM.MMMM), which equates to ±20cm (0.0001 minute is 0.0001/60 degrees and one degree is about 111km) at the equator.

Coordinate format

NMEA uses the clunky DDMM.MMMM format which, honestly, is not good in the internet era. Internally, minmea stores it as a fractional number (see above); for practical uses, the value should be probably converted to the DD.DDDDD floating point format using the following function:

• minmea_tocoord({-375165, 100}) => -37.860832

The library doesn’t perform this conversion automatically for the following reasons:

• The conversion is not reversible.
• It requires floating point support.
• The user might want to perform this conversion later on or retain the original values.

Example

char line[MINMEA_MAX_SENTENCE_LENGTH];
while (fgets(line, sizeof(line), stdin) != NULL) {
    switch (minmea_sentence_id(line, false)) {
        case MINMEA_SENTENCE_RMC: {
            struct minmea_sentence_rmc frame;
            if (minmea_parse_rmc(&frame, line)) {
                printf("$RMC: raw coordinates and speed: (%d/%d,%d/%d) %d/%d\n",
                        frame.latitude.value, frame.latitude.scale,
                        frame.longitude.value, frame.longitude.scale,
                        frame.speed.value, frame.speed.scale);
                printf("$RMC fixed-point coordinates and speed scaled to three decimal places: (%d,%d) %d\n",
                        minmea_rescale(&frame.latitude, 1000),
                        minmea_rescale(&frame.longitude, 1000),
                        minmea_rescale(&frame.speed, 1000));
                printf("$RMC floating point degree coordinates and speed: (%f,%f) %f\n",
                        minmea_tocoord(&frame.latitude),
                        minmea_tocoord(&frame.longitude),
                        minmea_tofloat(&frame.speed));
            }
        } break;

        case MINMEA_SENTENCE_GGA: {
            struct minmea_sentence_gga frame;
            if (minmea_parse_gga(&frame, line)) {
                printf("$GGA: fix quality: %d\n", frame.fix_quality);
            }
        } break;

        case MINMEA_SENTENCE_GSV: {
            struct minmea_sentence_gsv frame;
            if (minmea_parse_gsv(&frame, line)) {
                printf("$GSV: message %d of %d\n", frame.msg_nr, frame.total_msgs);
                printf("$GSV: satellites in view: %d\n", frame.total_sats);
                for (int i = 0; i < 4; i++)
                    printf("$GSV: sat nr %d, elevation: %d, azimuth: %d, snr: %d dbm\n",
                        frame.sats[i].nr,
                        frame.sats[i].elevation,
                        frame.sats[i].azimuth,
                        frame.sats[i].snr);
            }
        } break;
    }
}

Integration with your project

Simply add minmea.[ch] to your project, #include "minmea.h" and you’re good to go.

Running unit tests

Building and running the tests requires the following:

• CMake
• Check Framework (https://libcheck.github.io/check/).
• Clang Static Analyzer (https://clang-analyzer.llvm.org/).

If you have both in your $PATH, running the tests should be as simple as:

mkdir build
cd build
cmake ../
make
make test

Limitations

• Only a handful of frames is supported right now.
• There’s no support for omitting parts of the library from building. As a workaround, use the -ffunction-sections -Wl,--gc-sections linker flags (or equivalent) to remove the unused functions (parsers) from the final image.
• Some systems lack timegm. On these systems, the recommended course of action is to build with -Dtimegm=mktime which will work correctly as long the system runs in the default UTC timezone.

Contributing

1. Use the GitHub pull request system.
2. Make sure to follow to existing style (naming, indentation, etc.)
3. Write unit tests for any new functionality you add.
4. Be aware you’re submitting your work under the repository’s license.

Licensing

Minmea is open source software; see COPYING for amusement. Email me if the license bothers you and I’ll happily re-license under anything else under the sun.

Author

Minmea was written by Kosma Moczek <kosma@kosma.pl> and Patryk Szymczak <patryk.szymczak@gmail.com> at Cloud Your Car, with bugs fixed by countless good people.