Development of a Regional Ionosphere Model for Norway

A.B.O. Jensen, O. Ovstedal and G. Grinde

Abstract:	GNSS satellite signals are affected while being transmitted through the ionosphere and the troposphere, the parts of the Earth atmosphere extending from the surface of the Earth to an altitude of about 1000 km. In the ionosphere the effect is a function of the amount of free electrons along the signal path. The ionosphere is dispersive for radio waves, and the ionospheric effect is therefore frequency dependent. For GNSS positioning with dual frequency receivers, the first order effect of the ionosphere can be handled by linear combinations of the observations from the different frequencies. For single frequency users operating in real time the ionospheric effect is handled by the broadcast ionosphere model, and for single frequency users carrying out positioning in post mission the ionospheric effect is normally handled by post processed ionosphere models. During enhanced ionospheric activity, residual ionospheric effects can be present in the position solution no matter which approach is used for correction of the ionospheric effect. Residual ionospheric effects in the position and navigation solutions do affect the GNSS users, especially at high latitudes. Norway is located at high latitudes, extending from about 60 to 80 degrees North, geographic latitude. The Norwegian Mapping Authority has therefore initiated development of a regional ionosphere model based on the SATREF® network of permanent GNSS reference stations. This paper describes the development of the Norwegian ionosphere model. The SATREF® Ionosphere Model is based on estimation of the ionospheric delay from dual frequency GPS observations, using the geometry free linear combination of observations from the L1 and L2 frequencies. Also, the receiver differential code biases are estimated in a pre processing step. Satellite differential code biases are extracted from the IGS Global Ionosphere Models. The estimated vertical ionosphere delays at the ionosphere pierce points (IPP) are then used as the basis for a spatial ionosphere model. The spatial model is grid based, and interpolation of the vertical delay values from the ionosphere pierce points to the grid points is based on an interpolation scheme combining ordinary kriging and inverse distance dependent weighting. The paper presents the algorithms implemented for estimation of vertical total electron content (TEC) at the ionosphere pierce points, and the chosen interpolation algorithms are described. Tests carried out for verification of the SATREF® model are also described, supplemented with plots of the spatial variation of the results. The SATREF® Ionosphere Model is verified by comparison with the IGS Global Ionosphere Model. Test results are based on three days of GPS data from a number of SATREF® reference stations in Norway, and test scenarios with 16 and 90 grid points. The test data was selected from days with low, medium and high ionospheric activity respectively, and tests were carried out using both a small dense network of reference stations covering only the Southern part of Norway, and a larger but sparser network covering most of Norway including the Norwegian Sea between Norway and Iceland. In general, an overall compliance with the IGS GIM of about 30 cm is achieved with low and medium ionospheric activity. For the tests carried out with a very high level of ionospheric activity, the compliance with the IGS GIM is reduced as the noise level in the GPS data increases. Finally, in conclusion of the paper, a discussion on some implications for a real time operational setup of the model is provided, along with a discussion on possible improvements to the current SATREF® Ionosphere Model.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2880 - 2889
Cite this article:	Jensen, A.B.O., Ovstedal, O., Grinde, G., "Development of a Regional Ionosphere Model for Norway," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2880-2889.
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