a differential navigation system applicable to mobile users and covering a wide range geographic area , including remote regions . a reference receiver of known location tracks a navigation information service , computes differential data with respect to that information and communicates the data to a transmitting unit . the transmitting unit transmits differential data via a commercial geosynchronous earth satellite relay to a mobile user located no more than 500 miles from the reference receiver , but possibly located in a remote region . the mobile user receives the relayed signal with a non - directional , circularly polarized , non - stabilized antenna . reception at a sufficient data rate with sufficient reliability is made possible by formatting the differential data using spread spectrum techniques .