nmea

nmea

Byte-fed parser for the three NMEA 0183 sentences that cover 99% of GPS / GNSS application needs: RMC (recommended minimum), GGA (fix data), and GSA (DOPs and 3D fix). Pure C99, link with -lm.

The parser is single-pass and byte-fed — it accepts characters straight from the UART RX path, one at a time or in bursts. When a complete sentence arrives and its XOR checksum verifies, it fires a caller-supplied event callback and updates the latest snapshot stored in st_nmea. Bad checksums and truncated lines are dropped silently; a noisy stream cannot poison the next sentence.

#include "nmea.h"

static void on_msg(nmea_msg_type t, void *cookie)
{
    st_nmea *n = cookie;
    if (t == NMEA_MSG_RMC && n->rmc.valid == 'A') {
        printf("fix at %f, %f\n", n->rmc.lat_deg, n->rmc.lon_deg);
    }
}

st_nmea n;
nmea_init(&n, on_msg, &n);

/* In the UART RX ISR (or polling loop): */
nmea_feed_byte(&n, byte_from_uart);

Functions

Function	Purpose
nmea_init(n, on_msg, cookie)	Reset all state and store the optional event callback.
nmea_feed_byte(n, b)	Push one received byte. Safe to call from a UART RX ISR.
nmea_feed(n, buf, len)	Push len bytes in one go.
nmea_shift_tz(date, time, tz_min, *out_date, *out_time)	Apply a signed timezone offset (in minutes — Bangkok = 420, India = 330, US Eastern = -300, Nepal = 345) to a UTC date / time pair. Both forward and backward day rollover are handled, including leap-year February.

Decoded fields

All angles are in decimal degrees, signed (south and west are negative). All times are UTC unless you have already applied nmea_shift_tz.

typedef struct {                /* RMC -- recommended minimum */
    st_nmea_time time;
    st_nmea_date date;
    char    valid;              /* 'A' active, 'V' void */
    char    pos_mode;
    char    ns, ew;
    double  lat_deg, lon_deg;
    double  speed_knots;
    double  course_deg;
} st_nmea_rmc;

typedef struct {                /* GGA -- fix data */
    st_nmea_time time;
    char    ns, ew;
    char    fix_status;         /* '0' none .. '6' dead-reckoning */
    double  lat_deg, lon_deg;
    uint8_t sats_used;
    double  hdop;
    double  altitude_m;
} st_nmea_gga;

typedef struct {                /* GSA -- DOPs and 3D fix */
    char    mode;               /* 'M' manual, 'A' auto */
    char    fix_3d;             /* '1' none, '2' 2D, '3' 3D */
    double  pdop, hdop, vdop;
} st_nmea_gsa;

The talker prefix is ignored, so $GPRMC, $GLRMC, $GNRMC and so on are all accepted — useful for dual-constellation receivers (u-blox NEO-M8N etc.) that emit $GN… lines.

Sentences other than RMC / GGA / GSA are silently skipped. The parser does not allocate, log, or call back into your UART code; the only output path is the event callback you registered in nmea_init.

Build the self-test

make test

Builds example/check.c, runs it, prints nmea: All OK. on success and exits zero. The check covers:

• Happy-path decoding for RMC / GGA / GSA against reference numbers from u-blox docs (lat/lon at 1e-5 deg, speed / DOPs at 1e-3).
• Negative-hemisphere decoding (Sao Paulo, lat / lon < 0).
• A bad-XOR-checksum sentence is dropped (no callback fired).
• A buffer carrying two sentences back-to-back fires two callbacks.
• Junk bytes before the leading $ are ignored.
• nmea_shift_tz rollover: forward across leap-year Feb 29, forward across non-leap Feb 28 → March 1, backward across the new-year boundary, and minute-precision offsets (India +05:30).

Use in another project

Drop nmea.h and nmea.c into your sources, link with -lm. No other module dependencies.

License

MIT.