Ionospheric modelling for high precision GPS real time kinematic surveying over long distances

Over the past decade real time kinematic (RTK) GPS has evolved into an important practical tool for a wide range of positioning applications. For short baselines the current accuracy is limited only by multipathing problems but as the distance increases so other errors begin to decorrelate. In particular errors due to the ionosphere play an increasingly important role and typically RTK cannot be used beyond about 10km. Many practical applications require RTK to operate over longer distances and this thesis is concerned with the ionospheric modelling necessary for this to be possible. At present global and regional ionosphere maps (broadcast on web) based on global ionosphere models (GIM) and regional ionosphere models (RIM) are available for worldwide DGPS (WWDGPS) and wide area DGPS (WDGPS). Because of the limited accuracy of these models, a local ionosphere model (LIM) has been developed to see whether it would be possible to enhance the performance of RTK GPS. Several LIMs based on different mapping functions, and using data from one station only, have been constructed. These have been used to predict the double difference ionospheric corrections for a pair of stations and compared with the "true" corrections based on the data actually collected at the two stations (and knowing the exact coordinates of the two stations). In this way the accuracy of the different LIMs has been compared. These (LEM-based) corrections have also been used to correct raw GPS phase data before use in RTK software in order to see whether or not integer ambiguity determination can be improved - especially over distances greater than those currently used in practical RTK GPS. It has been found that significant improvements can result from the use of these corrections over distances of up to 20km when using clean GPS data in relatively benign multipathing.