The Ionospheric Delay Effort For GPS Single-Frequency Users-Analysis Study For Simulation Purposes

Ashraf Farah

Abstract:	The ionospheric delay is still one of the largest sources of error that affects the positioning accuracy of any satellite positioning system. The ionospheric error is considered the major source of potential range delay for single frequency users after the Selective Availability (SA), the intentional degradation of the accuracy of the single- frequency GPS position by the American Department Of Defense, was turned off on 2 May, 2000. The effect may be minimized due to the dispersive nature of the ionosphere by combining simultaneous measurements of signals at two different frequencies, but it remains a problem for single-frequency users. Much effort has been made in establishing models for single-frequency users to make this effect as small as possible. These models vary in accuracy, input data and computational complexity, so the choice between the different models depends on the individual circumstances of the user. From the simulation point of view, the model should be accurate, with a global coverage and good description to the ionosphere’s variable nature with both time and location. The author has reviewed some of these established models, starting with the BENT model, the Klobuchar model and the IRI (International Reference Ionosphere) model. Klobuchar model is widely used model due to its simplicity. Any GPS user may access Klobuchar model coefficients in the broadcast navigation message. CODE, the Centre for Orbit Determination in Europe provides a new set of coefficients for the Klobuchar model, which give more accurate results for the ionospheric delay computation. The IGS (International GPS Service) products include global ionospheric maps in IONEX-format (IONosphere map EXchange format) which enable the computation of the ionospheric delay at any desired location and time. The study was undertaken from the GPS-data simulation point of view. The aim was to select a model for the simulation of GPS data that gives a good description of the ionosphere’s nature with a high degree of accuracy in computing the ionospheric delay that would yield more precisely simulated data. The author based on IGS Global Ionospheric Maps (GIMs) has developed a new model. A comparison study had been made involving the Klobuchar model with GPS broadcast coefficients, the Klobuchar model with CODE coefficients and the new established model. The study has proved that the Klobuchar model has shortcomings regarding its low accuracy and less detailed description for the ionosphere’s behaviour. Using the CODE Klobuchar- styled coefficients in the Klobuchar model will give a better accuracy in computing the delay but it will still give a low-resolution description of the ionosphere performance. The use of the global ionospheric maps, from one of the IGS centers, gives higher accuracy in computing the ionospheric delay and also very detailed description for the ionosphere nature, which is essential for the simulation purposes.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	1313 - 1320
Cite this article:	Farah, Ashraf, "The Ionospheric Delay Effort For GPS Single-Frequency Users-Analysis Study For Simulation Purposes," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1313-1320.
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