The Lab Book Pages

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Dr. Andrew Greensted
Last modified: 18th November 2009

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GPS Module

Gumstix make a GPS enabled add-on board, this page give some details in using it.

The GPS Module

The GPSStix expansion includes a GPS module.

Gumstix GPS and Audio Module

Gumstix GPS and Audio Module

The GPS Module uses the Standard UART (STUART). This can be accessed via /dev/ttyS2.

> stty -F /dev/ttyS0 9600 -icrnl
> cat /dev/ttyS2

GPS Codes

The data output from the GPS modules is tagged with codes defined by the NMEA standard. The table below gives a description of some of these. There is a lot more information available from

Code Description
GPGGA Global Positioning System Fix Data (This is probably the most interesting)
GPGLL Geographic Position, Latitude / Longitude and time
GPGSA GPS DOP (Dilution of Precision) and active satellites
GPGSV GPS Satellites in view
GPRMC Recommended minimum specific GPS/TRANSIT data
GPVTG Track Made Good and Ground Speed
GPZDA UTC Date / Time and Local Time Zone Offset

GPS Data Reading Program

The data sent by the GPS module has a well defined structure so reading it in isn't too complicated. Below are three c files to demonstrate one approach. gps.h and gps.c give a set of handy functions for parsing the GPGGA data. gpsTest.c shows how to read from the GPS data from the serial port.

A simple make file is included to help you get the code compiled.

File: gps.h
typedef struct {
   int degrees;
   int mins;
   int minFrac;
   char quadrasphere;
} DMData;

typedef struct {
   DMData latDM;
   DMData longDM;
   int quality;
   int numSats;
   int checkSum;
} GPSData;

void printGPSData(GPSData *gpsData);
int hexStr2Int(char* str, int sPos, int numChars);
int extractNum(char* str, int sPos, int ePos, int *valPtr);
int decodeGPSString(char *str, GPSData *gpsData);
File: gps.c
#include <stdio.h>
#include "gps.h"

// $GPGGA,,ddmm.mmm,a,dddmm.mmm,b,q,xx,p.p,a.b,M,c.d,M,x.x,nnnn

//   UTC of position
// ddmm.mmm    Latitude of position
// a           N or S, latitutde hemisphere
// dddmm.mmm   Longitude of position
// b           E or W, longitude hemisphere
// q           GPS Quality indicator (0=No fix, 1=Non-differential GPS fix, 2=Differential GPS fix, 6=Estimated fix)
// xx          Number of satellites in use
// p.p         Horizontal dilution of precision
// a.b         Antenna altitude above mean-sea-level
// M           Units of antenna altitude, meters
// c.d         Geoidal height
// M           Units of geoidal height, meters
// x.x         Age of Differential GPS data (seconds since last valid RTCM transmission)
// nnnn        Differential reference station ID, 0000 to 1023

void printGPSData(GPSData *gpsData)
   printf("Lat: %dd %d.%d' %c, Long: %dd %d.%d' %c, Sats: %d, Checksum: %02X\n",
      gpsData->latDM.degrees, gpsData->latDM.mins, gpsData->latDM.minFrac, gpsData->latDM.quadrasphere,
      gpsData->longDM.degrees, gpsData->longDM.mins, gpsData->longDM.minFrac, gpsData->longDM.quadrasphere,
      gpsData->numSats, gpsData->checkSum);

int hexStr2Int(char* str, int sPos, int numChars)
   int val = 0;
   while (numChars > 0)   {
      int d = str[sPos] - 48;
      if (d>9) d -= 7;
      val = (val << 4) + (d & 0xF);

   return val;

int extractNum(char* str, int sPos, int ePos, int *valPtr)
   int val = 0;
   for ( ; sPos<=ePos ; sPos++)
      if (str[sPos] > 57 || str[sPos] < 48) return 1;

      val = (val*10) + str[sPos] - 48;

   *valPtr = val;
   return 0;

int decodeGPSString(char *str, GPSData *gpsData)
   const char *code = "$GPGGA";
   int i;

   // Check for the correct code
   for (i=0 ; i<6 ; i++) if (str[i] != code[i]) return 1;

   int pos = 1;
   int commaPos[14];
   int commaCount = 0;
   int starPos = 0;
   int nlPos = 0;
   char checkSum = 0;

   // Iterate through string characters
   while (pos < 100) {

      // Check for end of line
      if (str[pos] == '\n') {
         nlPos = pos;

      // Check for star and update checksum
      if (starPos == 0) {
         if (str[pos] == '*') starPos = pos;
         else checkSum ^= str[pos];

      // Check for comma
      if (str[pos] == ',') {
         if (commaCount < 14) commaPos[commaCount] = pos;
         commaCount ++;

      pos ++;

   // Check for sensible format
   if (starPos==0 || nlPos==0 || commaCount!=14 || (nlPos-starPos)!=4) return 2;

   // Compare Checksums
   gpsData->checkSum = hexStr2Int(str, starPos+1, 2);
   if (checkSum != gpsData->checkSum) return 3;

   // Extract Latitude
   int err = 0;
   err |= extractNum(str, commaPos[1]+1, commaPos[1]+2, &gpsData->latDM.degrees);
   err |= extractNum(str, commaPos[1]+3, commaPos[1]+4, &gpsData->latDM.mins);
   err |= extractNum(str, commaPos[1]+6, commaPos[1]+10, &gpsData->latDM.minFrac);
   gpsData->latDM.quadrasphere = str[commaPos[2]+1];
   if (err) return 3;

   // Extract Longtude
   err = 0;
   err |= extractNum(str, commaPos[3]+1, commaPos[3]+3, &gpsData->longDM.degrees);
   err |= extractNum(str, commaPos[3]+4, commaPos[3]+5, &gpsData->longDM.mins);
   err |= extractNum(str, commaPos[3]+7, commaPos[3]+11, &gpsData->longDM.minFrac);
   gpsData->longDM.quadrasphere = str[commaPos[4]+1];
   if (err) return 4;

   // Extract Quality
   err = extractNum(str, commaPos[6]+1, commaPos[7]-1, &gpsData->quality);
   if (err) return 5;

   // Extract Number of Satelites
   err = extractNum(str, commaPos[6]+1, commaPos[7]-1, &gpsData->numSats);
   if (err) return 6;

   return 0;
File: gpsTest.c
#include <stdio.h>
#include "gps.h"

int main(void)
   char buffer[200];
   GPSData gpsData;

   FILE *fp;
   fp = fopen("/dev/ttyS2", "r");

   while (1)
      if (fgets(buffer, 100, fp) == NULL) {
         printf("Got NULL\n");

      if (ferror(fp)) {
         printf ("Error Reading\n");

      int result = decodeString(buffer, &gpsData);
      if (result == 0) printGPSData(&gpsData);


   return 0;
File: Makefile
# Gumstix GPS Makefile

CC       = /files/gumstix/gumstix-oe/tmp/cross/bin/arm-angstrom-linux-gnueabi-gcc
CFLAGS   += -Wall

OBJS     = gps.o

gpsTest: gpsTest.c $(OBJS)
   $(CC) $(CFLAGS) $(INCLUDE) $(OBJS) -o gpsTest gpsTest.c

gps.o: gps.c gps.h
   $(CC) $(CFLAGS) $(INCLUDE) -c gps.c

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