Precise ionosphere modeling is crucial and remains as a challenge for GPS positioning and navigation as well as many other Earth Observation Systems. This research develops and analyzes a new ionospheric modeling system based on a multiple-layer tomographic technique using spherical harmonic functions and empirical orthogonal functions in combination with Kalman filter estimator to perform real-time ionospheric modeling and perform ionospheric TEC predictions. A close form expression that links the smoothed TEC measurements and the tomographic model has been developed, which allows the simultaneous execution of TEC smoothing and model estimation and ionospheric TEC prediction. This system is feasible for real-time implementation to generate TEC predictions to support real-time GPS positioning and other real-time applications. In order to assess the accuracies of the ionospheric TEC prediction data, three quantitative indicators are proposed to evaluate the prediction performance. The tomographic model proposed in this research is function-based, which is computationally more efficient than tomographic models that are based on voxel concept and overcomes the limitations associated with single-layer ionospheric models. Comprehensive data analyses have been conducted to assess the model using data from different types of GPS networks and acquired under both ionosphere quiet and disturbed conditions. The model performance has been assessed using different elevation angles and prediction intervals. The numerical results at the independent user station show that over a local area GPS network using an elevation cutoff of 15°, the vertical TEC data predicted at 5-min or 10-min interval have an accuracy of 3.5~4.3 TECU during ionospheric quiet time period. An accuracy of 5.9 TECU can be obtained using a 30-min prediction interval. Over a wide area GPS network using an elevation cutoff of 15°, the 5- min and 10-min VTEC predictions have an accuracy about 5.0~5.8 TECU and the 30- min predictions about 5.5~5.9 TECU during ionospheric quiet day. During ionospheric disturbed day, the 5-min and 10-min VTEC predictions have an accuracy about 5.2~6.1 TECU and the vertical TEC prediction accuracy is about 5.8~6.7 TECU using a 30-min prediction interval.
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