Patent US 6,963,304

A system for providing integrated communications, navigation and surveillance capabilities. A space segment comprises a plurality of time synchronized satellites broadcasting navigation signals and have two-way communication capabilities. A terrestrial segment comprises processing apparatus that communicates with the satellites. A user segment comprises at least one mobile user device capable of two way communication with the plurality of satellites and of using the navigation signals to derive its position. The user device is operable to inform the space segment that the user device is not receiving a navigation signal that the user device is capable of receiving.