To avoid spaghetti code I put the parsing of the NMEA string into a class. This is a quick hack but you don't see many examples like this so here you go.
Just for fun I've boxed this little project up all held together with hot glue and running from a 3.3V LiPo cell. The low price of the GPS along with the low price of an Arduino Nano Pro makes this a very attractive platform for building embedded computing devices.
/********************************************************************* Display info from a cheap GPS on an Adafruit OLD display *********************************************************************/ #include <SPI.h> #include <Wire.h> #include <Adafruit_SSD1306.h> #include <SoftwareSerial.h> SoftwareSerial GPS(2,3); // rx pin = 2, tx pin for GPS #define OLED_RESET 4 Adafruit_SSD1306 display(OLED_RESET); class NmeaRecord { public: NmeaRecord(char * line); // GPGGA fields enum { eLabel, eTimeStamp, eLat, eLatNS, eLon, eLonEW, eFix, eSatellites, eHorizDilution, eAltitude, eHeight, eEmpty1, eEmpty2, eChecksum }; // string lengths, +1 for null terminator enum { kLabelLen = 7, kTimeStampLen = 11, kLatLen = 9, kLatNSLen = 2, kLonLen = 10, kLonEWLen = 2, kFixLen = 2, kSatellitesLen = 3, kHorizontalDilutionLen = 4, kAltitudeLen = 7 }; char label[kLabelLen]; char timeStamp[kTimeStampLen]; char lat[kLatLen]; char latNS[kLatNSLen]; char lon[kLonLen]; char lonEW[kLonEWLen]; char fix[kFixLen]; char satellites[kSatellitesLen]; char horizontalDilution[kHorizontalDilutionLen]; char altitude[kAltitudeLen]; private: NmeaRecord::storeField(int index, char*field); }; void setup() { GPS.begin(9600); Serial.begin(115200); Serial.println("started"); // by default, we'll generate the high voltage from the 3.3v line internally! (neat!) display.begin(SSD1306_SWITCHCAPVCC, 0x3C); // initialize with the I2C addr 0x3C (for the 128x32) // init done display.clearDisplay(); display.setTextSize(1); display.setTextColor(WHITE); } void loop() { String line = GPS.readStringUntil('$'); Serial.println(line); NmeaRecord rec = NmeaRecord(line.c_str()); // GPGGA,054540.000,3346.7737,S,15113.2178,E,2,12,0.99,94.2,M,21.9 display.clearDisplay(); display.setCursor(0,0); display.print("UTC: "); display.print(formatTime(rec.timeStamp)); display.print(" sats="); display.println(rec.satellites); display.print("Lat: "); display.print(formatLat(rec.lat)); display.print(","); display.println(rec.latNS); display.print("Lng: "); display.print(formatLng(rec.lon)); display.print(","); display.println(rec.lonEW); display.print("Alt: "); display.print(rec.altitude); display.print("m fix="); display.print(rec.fix); display.display(); } NmeaRecord::NmeaRecord(char *line) { char *token; token = strtok(line, ","); if(strcmp(token, "GPGGA") != 0) { return; // don't parse } int fieldIndex = 0; while(token != NULL) { Serial.print("index = "); Serial.print(fieldIndex); Serial.print(", token = "); Serial.println(token); this->storeField(fieldIndex, token); fieldIndex++; token = strtok(NULL, ","); } } NmeaRecord::storeField(int index, char*field) { Serial.print("Storefield index = "); Serial.print(index); Serial.print(", field = "); Serial.println(field); switch(index) { case eLabel: strncpy(label, field, kLabelLen); break; case eTimeStamp: strncpy(timeStamp, field, kTimeStampLen); Serial.print("got timeStamp = "); Serial.println(this->timeStamp); break; case eLat: strncpy(lat, field, kLatLen); break; case eLatNS: strncpy(latNS, field, kLatNSLen); break; case eLon: strncpy(lon, field, kLonLen); break; case eLonEW: strncpy(lonEW, field, kLonEWLen); break; case eFix: strncpy(fix, field, kFixLen); break; case eSatellites: strncpy(satellites, field, kSatellitesLen); break; case eHorizDilution: strncpy(horizontalDilution, field, kHorizontalDilutionLen); break; case eAltitude: strncpy(altitude, field, kAltitudeLen); break; } } String formatTime(String timeString) { String newTime = timeString.substring(0,2); newTime += ":"; newTime += timeString.substring(2,4); newTime += ":"; newTime += timeString.substring(4,6); return newTime; } String formatLat(String raw) { // 4807.038,N Latitude 48 deg 07.038' N String result = raw.substring(0,2); result += " deg "; result += raw.substring(2,7); return result; } String formatLng(String raw) { // 01131.000,E Longitude 11 deg 31.000' E String result = raw.substring(0,3); result += " deg "; result += raw.substring(3,8); return result; }
