Katarzyna Stepniak, Pawel Wielgosz, Jacek Paziewski

Accuracy analysis of the Klobuchar ionosphere model transmitted by the GPS system

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Abstract

The ionospheric delay has a distinctive impact on the GNSS observations. Single frequency GPS receivers use the Klobuchar model to correct the ionospheric delay of the L1 signals. The Klobuchar model is a single-layer model and represents the zenith delay as a constant value at nighttime and a half-cosine function in daytime. The GPS satellites broadcast eight model coefficients in their navigation message. It is expected that the model can reduce the RMS error due to uncompensated ionospheric delay by about 50%. Therefore its accuracy is often unsatisfactory even for the absolute positioning, this may be especially true in case of the increased solar activity.
In this study, the accuracy of the Klobuchar model for various geographical latitudes and ionospheric activity is analyzed. The accuracy of the model was estimated by comparing to Global Ionosphere Maps (GIM) provided by the International GNSS Service (IGS). In order to produce IGS GIMs representing the ionospheric total electron content (TEC), data from ~200 permanently tracking GNSS stations are used. This global model offers 2.5 by 5.0 degrees spatial, and 2-hour temporal resolutions.
The comparison results show that the differences between the Klobuchar model and the IGS GIMs may reach as much as 5 m of L1 delay. In general, the Klobuchar model underestimates the ionospheric delay for low latitude regions and overestimates the delay for high latitudes.

Keywords: Global Positioning System; Klobuchar model; Global Ionosphere Maps; ionospheric delay.