Patent ID: 7110883

Claim:
A method, comprising: receiving Space Based Augmentation System (SBAS) correction messages from a selected SBAS satellite, wherein SBAS Includes a plurality of precisely-surveyed ground reference stations to receive Global Positioning System (GPS) signals from a plurality of GPS satellites and determine GPS signal errors for the plurality of GPS satellites, and the SBAS further includes at least two SBAS satellites to broadcast correction messages to GPS receivers based on the determined GPS signal errors using a frequency used by the plurality of GPS satellites to send GPS signals to the GPS receivers; determining whether at least one criterion is satisfied for using the selected SBAS satellite as a correction source including; determining whether the selected SBAS satellite is sending SBAS correction messages, and determining whether a differential position can be created from the received SBAS correction messages; and upon determining that the at least one criterion is not satisfied for using the selected SBAS satellite as a correction source, selecting a second SBAS satellite to be used as a correction source from which to receive SBAS correction messages.