 
   // 13. Combine time, distance, and temperature into a single string.
   // -----------------------------------------------------------------
-  String dataString = String(unixTime) + " " + String(roundedDistance) + " " + String(roundedTemperature);
+  totalReadings += 1;
+  String dataString = String(totalReadings) + " " +
+    String(unixTime) + " " +
+    String(roundedDistance) + " " +
+    String(roundedTemperature);
 
   // Cache distance and time in global array variable
   sensorReadings[numCachedReadings].distance = roundedDistance;

    // 20. Turn off GSM.
    // -----------------

    // Turn off GSM power pin.
    if (sim900.ensureOffline() == false)
    {
      // TODO(richard-to): What would be best way to handle this issue?


// Global Variables
int numCachedReadings = 0;
int totalReadings = 0;
SensorReading sensorReadings[SEND_DATA_AFTER_X_READINGS];
UASensors_Sim900 sim900;
UASensors_RTC rtc(RTC_CS_PIN);
