A method and apparatus for efficiently obtaining improved coverage in augmenting GPS position location with differential GPS integrity and correction messages using existing satellite systems, planned satellite systems, or a combination of both to broadcast these integrity and correction messages without requiring dedicated satellite resources. At least one existing satellite system offers full global coverage whereas the conventional approach of disseminating these integrity and correction messages using geostationary satellites necessarily omits Polar Regions. This existing satellite system also provides redundant coverage globally whereas the conventional approach using geostationary satellites must increase the number of geostationary satellites in direct proportion to the level of redundancy desired. At least one other existing satellite-based system offers higher availability (i.e., more geographically complete) coverage with these integrity and correction message by also employing ground-based transmitters to gain three way redundant coverage. Using existing or planned satellite systems to broadcast differential GPS integrity and correction messages also eliminates the expense and delays associated with building and launching either dedicated geostationary satellites or dedicated satellite payloads on host satellites. Using available satellite resources also enables cost efficient customization of disseminated integrity and correction messages. The apparatus comprises a set of ground reference stations with individual assigned geographic spaces, a master station that gathers integrity and correction messages from these ground reference stations and transmits them to a satellite-based system or systems that broadcast integrity and correction messages, possibly customized, and users with equipment capable of receiving signals from the satellite system.
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