GPS Utilities / minmea GPS NMEA 0183 parser

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.
• Easily extendable to support new sentences.
• Complete with a test suite and static analysis.

Supported sentences

• RMC (Recommended Minimum: position, velocity, time)
• GGA (Fix Data)
• GSA (DOP and active satellites)
• GLL (Geographic Position: Latitude/Longitude)
• GST (Pseudorange Noise Statistics)
• GSV (Satellites in view)

Adding support for more sentences is trivial; see minmea.c source.

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 ±20cm LSB 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 hardware.
• The user might want to perform this conversion later on or retain the original values.

Example

char line[MINMEA_MAX_LENGTH];
while (fgets(line, sizeof(line), stdin) != NULL) {
    switch (minmea_sentence_id(line)) {
        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:

• Check Framework (http://check.sourceforge.net/).
• Clang Static Analyzer (http://clang-analyzer.llvm.org/).

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

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. Native Windows builds should use -Dtimegm=_mkgmtime instead which will work correctly in all timezones.

Bugs

There are plenty. Report them on GitHub, or - even better - open a pull request. Please write unit tests for any new functions you add - it's fun!

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@cloudyourcar.com> at Cloud Your Car.