GPS Xbee on a Seeeduino Stalker v2

GpsXbeeStalker.c

/*
Ported from SD card datalogger example 
Write XBEE GPS output to the SD card of a Seeduino Stalker
 */

#include <SD.h>
#include <Streaming.h>
#include <TinyGPS.h>
TinyGPS gps;
// On the Ethernet Shield, CS is pin 4. Note that even if it's not
// used as the CS pin, the hardware CS pin (10 on most Arduino boards,
// 53 on the Mega) must be left as an output or the SD library
// functions will not work.
const int chipSelect = 10;

//Set to zero to turn off the debug writes
#define DEBUG 1

void setup()
{
  Serial.begin(9600);
  Serial.print("Initializing SD card...");
  // make sure that the default chip select pin is set to
  // output, even if you don't use it:
  pinMode(10, OUTPUT);
  
  //Led - On when 
  pinMode(8, OUTPUT);
  
  // see if the card is present and can be initialized:
  if (!SD.begin(chipSelect)) {
    Serial.println("Card failed, or not present");
    // don't do anything more:
    return;
  }
  Serial.println("card initialized.");
}

void loop()
{
  // make a string for assembling the data to log:
  String dataString;
  while(Serial.available())
  {
    int input;
    input = (char)Serial.read();
    
    if(gps.encode(input))
    {
  
      // process new gps info here
      long lat, lon;
      unsigned long fix_age;

      // retrieves +/- lat/long in 100000ths of a degree
      gps.get_position(&lat, &lon, &fix_age);

      dataString += "$GPS #lat:"; 
      dataString += lat;
      dataString += " #lon:"; 
      dataString += lon; 
      dataString += " #fix_age:"; 
      dataString += fix_age;
      dataString += "\n";
      
      if (DEBUG)
      {
        if (fix_age == TinyGPS::GPS_INVALID_AGE)
          dataString += "$Debug #fix_age_state:No fix\n";
        else if (fix_age > 5000)
          dataString += "$Debug #fix_age_state:Stale\n";
        else
          dataString += "$Debug #fix_age_state:Fixed\n";
      }
    }
  }
  
  if (DEBUG)
  {
    unsigned long chars;
    unsigned short sentences, failed_checksum;
    gps.stats(&chars, &sentences, &failed_checksum); 

    dataString += "$DEBUG #Char: ";
    dataString += chars;
    dataString += " #Sentences: ";
    dataString += sentences;
    dataString += " #Failed:";
    dataString += failed_checksum;
    dataString += "\n";
  }
  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  File dataFile = SD.open("datalog.txt", FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile) {
    dataFile.println(dataString);
    dataFile.close();
    // print to the serial port too:
    Serial.println(dataString);
  }  
  // if the file isn't open, pop up an error:
  else {
    Serial.println("error opening datalog.txt");
  } 
}