corelocation api issue

Hi,
I am working with core location api, and its working good. I am trying to loop my code and get locations after (say) 30 seconds .... and keep logging it. It works well but after some time when iphone goes to standby mode ( screen saver) ... location api stop working!!!!

any one have any idea how to over come this.... i want to run my application all the time but not when user close my application. But it should work even iphone screen is black.....

any suggestions or idea?