Real Time Precise GPS Constellation Orbits and Clocks Estimation using Zero-difference Integer Ambiguity Fixing

A method to solve the GPS zero-difference measurement equations with integer ambiguities has been recently introduced [ 1]. When the method is applied to data from a global network of GPS receivers it provides a consistent set of satellite orbits and clocks, which have an 'integer' property: phase residuals for any receiver computed using these orbits and clocks easily reveal integer ambiguities. The paper focuses on the application of this novel approach to the computation of real-time orbits and clocks for the GPS constellation. The benefit of using these products for real-time Precise Point Positioning (PPP) of user receivers (with integer ambiguity fixing) is demonstrated. In this method real-time corrections to extrapolated IGS IGU orbits are estimated at the same time as all other relevant parameters by a Kalman which processes measurements from a world-wide network of 50 IGS stations. The filter performs zero-difference ambiguity fixing in real-time. One month of data was processed (July 2007) to estimate the achieved precision. Relative to IGS final orbits, the 3-D precision of the real-time orbits is about 3 cm RMS. When these constellation orbits and clocks are used to perform real-time PPP for receivers outside of the reference network, the precision obtained using zero-difference integer ambiguity fixing is close to 2 cm RMS. This is about five times better than standard solutions which rely upon floating ambiguity fixing.