One of the major limitations to the accuracy attainable using single frequency Global Positioning Systems receivers is the propagation delay of the signals as they pass through the ionosphere, especially during times of high solar activity. Errors of several parts per million can be encountered on baselines where only one frequency has been observed. This thesis presents an approach for modelling the ionospheric delay, using phase measurements from dual frequency receivers to estimate corrections for single frequency users operating within the same region. A surface is used to approximate the spatial distribution of the delay, and temporal changes are also taken into account by the estimation of a new surface at every epoch. To test the validity of the model, data were obtained from an experiment conducted near Ottawa, Canada, in October of 1990 by the Canada Centre for Remote Sensing and the Canada Centre for Surveying. Three dual frequency receivers on the ground are used to estimate ionospheric delay variations and to correct the observations from an airborne single frequency receiver moving in the vicinity of the other three receivers. It is shown that after the model has been applied, differences between three separate solutions for the position of the aircraft, computer with respect to different monitor stations, are at a level of one part per million (ppm). Before correction these differences were at two to three ppm, with periods of up to 50 ppm. It is felt that the model although fairly simple in design, is effective in reducing the ionospheric bias sufficiently well for a broad range of applications, including remote sensing, for which the test data was obtained.
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