The reduction of differential ionospheric delay for GPS carrier phase ambiguity resolution
To utilize the high precision of the GPS carrier phase observable it is necessary to solve for the unknown number of integer cycles between satellite and receiver. On differencing observations between pairs of satellites and receivers (the double differencing operation), any common errors will cancel. As the separation between receivers increase, so the errors induced by the charged region of the upper atmosphere (the ionosphere) on line of sight observations to the same satellite become less correlated. Thus the double differencing operation leaves a significant residual effect and ambiguity resolution may become difficult, if not impossible at station separations of greater than 15 km; a value dependent on the level of solar activity. It is then perhaps apparent that modelling of this spatial change in ionospheric delay will allow for mitigation of this residual error. Such a model was produced in the course of this thesis for the real time estimation and reduction of differential ionospheric delay. Three coefficients describe a polynomial surface in latitude and longitude differences from the reference station, and the delays thus estimated are applied to carrier phase observations at the reference and rover receivers. The success of this approach was quantified by attempting the resolution of ambiguities at successively longer baselines along the flight path used. Results show a significant increase in the distances over which ambiguities may apparently be reliably resolved. Data corrected via this model allowed the resolution of ambiguities at distance of up to 40km.