How to get the current satellite long-lang from a mob device

Question

How to get the current satellite GPS position values from a mobil device (Android & IPAD & Win Mobile Tablet) for ASP.NET Web Application? I am going to develop a ASP.NET web application (MVC or ASP.NET or Silverlight project with WCF service) which will run on GPS integrated mobile devices. When a person close enough to a location in a specific matter of distance,he could be able to login to my application. When person far away from a location in a specific matter of distance, he could not be able to login my application.
It is assumed that the mobil device has built with GPS chip integrated itself, it will not be needed to get lang-long data from IP addresses. Accuracy is important.
Any resource, any idea will be very helpful.I am waiting for help.