Youngho Sohn, Changdon Kee
International Journal of Aeronautical and Space Sicences, vol. 5, no. 1, pp.94-100, 2004

Abstract : Ionospheric time delay is the biggest error source for single-frequency DGPS applications, including time transfer and Wide Area Differential GPS (WADGPS). Currently, there are many attempts to develop real-time ionospheric time delay estimation techniques to reduce positioning error due to the ionospheric time delay. Klobuchar model is now widely used for ionospheric time delay calculation for single-frequency . users. It uses flat surface at night time and cosine surface at day time[1]. However, the model was developed for worldwide ionosphere fit, it is not adequate for local area single-frequency users who want to estimate ionospheric time delay accurately [2] Therefore, 3-D ionosphere model using tomographic estimation has been developed. 3-D tomographic inversion model shows better accuracy compared with prior algorithms[3]. But that existing 3-D model still has problem that it requires many coefficients and measurements for good accuracy. So, that algorithm has limitation with many coefficients in continuous estimation at the small region which is obliged to have fewer measurements. In this paper, we developed an modified 3-D ionospheric time delay model using tomography, which requires only fewer coefficients. Because the combinations of our base coefficients correspond to the full coefficients of the existing model, our model has equivalent accuracy to the existing. We confirmed our algorithm by simulations. The results proved that our modified algorithm can perform continuous estimation with fewer coefficients.

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