File content as of revision 0:9c001c4e7bf4:

#include "mbed.h"
#include <string>
#include <sstream>

Serial pc(USBTX,USBRX);
Serial xbee(p9,p10);//tx, rx
Serial gps(p28,p27);

typedef struct {
    int latitude;  //in .0001 minutes
    int longitude; //in .0001 minutes
    int altitude;  //in decimeters
    int time;      //in milliseconds
} GpsData;

GpsData gpsData;
GpsData otherGps;

void readSerial(Serial &s, char str[], int size){
    for (int i = 0; i < size; i++){
        str[i] = s.getc();   
    }
}

//sends: "$<command>*<checksum>\r\l"
void sendGpsCommand(string command){
    uint8_t checksum = 0;
    pc.printf("Sending command to gps: ");
    gps.putc('$');
    pc.putc('$');
    char c;
    for (int i = 0; i < command.length(); i++){
        c = command[i];
        checksum ^= c;
        gps.putc(c);
        pc.putc(c);
    }
    gps.putc('*');
    pc.putc('*');
    string checkSumString;
    while (checksum > 0){
        uint8_t checksumChar = checksum & 0x0F;
        if (checksumChar >= 10){
            checksumChar -= 10;
            checksumChar += 'A'; 
        } else {
            checksumChar += '0';   
        }
        checkSumString.push_back((char) checksumChar);
        checksum = checksum >> 4;    
    }

    for (int i = checkSumString.length() - 1; i >= 0; i--){
        gps.putc(checkSumString[i]);
        pc.putc(checkSumString[i]);   
    } 
    gps.putc('\r');
    pc.putc('\r');
    gps.putc('\n');
    pc.putc('\n');
}

long parseDec(string s){
    int mult = 1;
    int result = 0;
    for (int i = s.length() - 1; i >=0; i--){
        if (s[i] != '.'){
            result += (s[i] - '0') * mult;
            mult *= 10;
        }
    }
    return result;
}

//cs: little endian parsing
int nextInt(char *data, int i){
   i |= data[i];
   i |= (data[i + 1] << 8);
   i |= (data[i + 2] << 16);
   i |= (data[i + 3] << 24);
   return i;
}

void handleXbeeGps(){
    static bool reading = false;
    static char packet[16];
    static int i = 0;
    
    char c = xbee.getc();
    if (reading){
        packet[i] = c;
        i++;
        if (i == 16){
            i = 0;
            otherGps.latitude = nextInt(packet, 0);
            otherGps.longitude = nextInt(packet, 4);
            otherGps.altitude = nextInt(packet, 8);
            otherGps.time = nextInt(packet, 12);
            
            pc.printf("His GPS data: Lat: %d, Lon: %d, Alt: %d, Time:%d\r\n",
                otherGps.latitude, otherGps.longitude, otherGps.altitude, otherGps.time
            );
            reading = false;
        }
    } else if (c == 'X'){
        reading = true;    
    }
}

void handleGpsData(){
     static bool reading = false;
     static stringstream line;
     
     char c = gps.getc();
     
     if (reading){
        if (c == '*'){ //sentence buffer complete; we're ignoring the checksum
            string field;
            std::getline(line, field, ','); //GPGGA
            std::getline(line, field, ','); //time
            gpsData.time = parseDec(field);
            std::getline(line, field, ','); //latitude
            gpsData.latitude = parseDec(field);
            std::getline(line, field, ','); //N or S
            std::getline(line, field, ','); //longitude
            gpsData.longitude = parseDec(field);
            std::getline(line, field, ','); //E or W
            std::getline(line, field, ','); //skip  
            std::getline(line, field, ','); //skip
            std::getline(line, field, ','); //skip
            std::getline(line, field, ','); //altitude
            gpsData.altitude = parseDec(field);
            
            //update whatever needs updating when gps updates
            pc.printf("My GPS data: Lat: %d, Lon: %d, Alt: %d, Time:%d\r\n",
                gpsData.latitude, gpsData.longitude, gpsData.altitude, gpsData.time
            );
            
            line.str(string(""));
            reading = false;
            
        } else {
            line.put(c);    
        }
        
     } else if (c == '$') {
        reading = true;
     }
}

int main(){
    gps.baud(57600);
    xbee.baud(9600);
    pc.baud(57600);
    
//    sendGpsCommand("PMTK301,1");
//    while(true){pc.putc(gps.getc());}
    gps.attach(&handleGpsData, Serial::RxIrq);
//    xbee.attach(&handleXbeeGps, Serial::RxIrq);
}