XMLHttpRequest leak

Below is the javascript code snippet. It doesn't work as expected, please help me on this.

<script type="text/javascript">

   function getCurrentLocation() {
     console.log("inside location");
     navigator.geolocation.getCurrentPosition(function(position) {
       insert_coord(new google.maps.LatLng(position.coords.latitude,position.coords.longitude)); 
       });
   }

   function insert_coord(loc) {
     var request = new XMLHttpRequest();
     request.open("POST","start.php",true);
     request.onreadystatechange = function() {
                                     callback(request);
                                  };
     request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
     request.send("lat=" + encodeURIComponent(loc.lat()) + "&lng=" + encodeURIComponent(loc.lng()));

     return request;
   }

   function callback(req) {
     console.log("inside callback");
     if(req.readyState == 4)
       if(req.status == 200) {
         document.getElementById("scratch").innerHTML = "callback success";
         //window.setTimeout("getCurrentLocation()",5000);
         setTimeout(getCurrentLocation,5000);
       }
   }

getCurrentLocation(); //called on body load
</script>

      

        
        
        
      

    

What I am trying to achieve is to send my current location to the php page every 5 seconds or so. I can see multiple coordinates in my database, but after a while it gets weird. Firebug is showing very strange logs like simultaneous POST at irregular intervals.

Here is a screenshot of firebug:

Is there a leak in the program. please help.

EDIT: The expected output in the firebug console should look like this: -

internal location
POST ....
inside callback

/ * 5 seconds later * /

internal location
POST ... inside callback

/ * repeat * /