Phonegap watchPosition测量距离和触发事件

I'm developing a simple jQuery / Phonegap app and I'm using watchPosition to constantly calculate between the distance current location and some remote place. This works just fine, but this is my issue. I would like to trigger an event (function) when the distance is less than a mile from that location. The way I have it right now it would continue triggering the event as long as the distance is less than a mile. What can I do to "tell" watchPosition to trigger the event only once I'm developing a simple jQuery / Phonegap app a