Wide area differential GPS reference system and method

A GPS reference system for providing ephemeris and/or clock corrections for GPS satellites to users at measurement epochs of the GPS reference system, the GPS reference system comprising:

• a network of reference stations including a primary reference station and secondary reference stations, each of the reference stations receiving GPS signals from those of the GPS satellites it observes and computing in response at each of the measurement epochs a pseudo-range residual for each of the GPS satellites it observes at the measurement epoch, each of the secondary reference stations positioned with respect to the primary reference station such that at least one of the GPS satellites it observes at each of the measurement epochs is simultaneously observed by the primary reference station;
• and
• a master station in communication with the reference stations to receive the computed pseudo-range residuals at each of the measurement epochs, the master station comprising:
• a pseudo-range residual synchronizer remove at each of the measurement epochs the clock differences from the received pseudo-range residuals at the measurement epoch so as to synchronize them in time in response to the received pseudo-range residuals at the measurement for each of the GPS satellites that is simultaneously observed at the measurement epoch by the primary reference station and at least one of the secondary reference stations;
• for each of the GPS satellites observed at each of the measurements epoch by the primary reference station and at least one of the secondary reference stations, a corresponding, independent ephemeris and clock correction estimator to compute at the measurement epoch an ephemeris correction and a clock correction for the corresponding GPS satellite in response to each of the synchronized pseudo-range residuals at the measurement epoch for the corresponding GPS satellite and independent of the synchronized pseudo-range residuals at the measurement epoch for the other GPS satellites observed by the reference stations;
• and
• a transmitter to transmit to users at each of the measurement epochs the computed ephemeris and/or clock corrections for each of the GPS satellites observed at the measurement epoch by the primary reference station and at least one of the secondary reference stations;
• wherein, for at least a subset of the measurement epochs, at least a subset of the GPS satellites for which the ephemeris and/or clock corrections are transmitted are observed by the primary reference station and at least one but no more than two of the secondary reference stations such that the transmitted ephemeris and/or clock corrections for each of the at least subset of GPS satellites at each of the at least subset of measurement epochs are computed in response to at least two but no more than three synchronized pseudo-range residuals for the GPS satellite at the measurement epoch.

A method of providing ephemeris and/or clock corrections for GPS satellites to users at measurement epochs of the method utilizing a network of reference stations and a master station, the method comprising the steps of:

• with each of the reference stations:
• receiving GPS signals from those of the GPS satellites it observes and in response computing at each of the measurement epochs a pseudo-range residual for each of the GPS satellites it observes at the measurement epoch;
• providing the computed pseudo-range residuals at each of the measurement epochs to the master station;
• with the master station:
• removing at each of the measurement epochs clock differences from the provided pseudo-range residuals so as to synchronize them in response to the provided pseudo-range residuals at the measurement epoch;
• for each of the GPS satellites observed at each of the measurement epochs by the reference stations, computing at the measurement epoch an ephemeris correction and a clock correction for the corresponding GPS satellite in response to the synchronized pseudo-range residuals at the measurement epoch for the corresponding GPS satellite and independent of the synchronized pseudo-range residuals at the measurement epoch for the other GPS satellites observed by the reference stations;
• and
• transmitting at each of the measurement epochs the ephemeris and/or clock corrections for each of the GPS satellites observed at the measurement epoch by at least one of the reference stations;
• wherein, for at least a subset of the measurement epochs, at least a subset of the GPS satellites for which the ephemeris and/or clock corrections are transmitted are observed by at least one but no more than three of the reference stations such that the transmitted ephemeris and/or clock corrections for each of the at least subset of GPS satellites at each of the at least subset of measurement epochs are computed in response to at least one but no more than three synchronized pseudo-range residuals for the GPS satellite at the measurement epoch.