I'm using the following GPX file in Xcode 4.2 to simulate a location change. It works well, but I can't control the speed of the location change. stamp seems to be not working. Does anyone have a solution for this?

```
<?xml version="1.0"?>
<gpx version="1.1" creator="Xcode"> 
    <wpt lat="37.331705" lon="-122.030237"></wpt>
    <wpt lat="37.331705" lon="-122.030337"></wpt>
    <wpt lat="37.331705" lon="-122.030437"></wpt>
    <wpt lat="37.331705" lon="-122.030537"></wpt>
</gpx>

```