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Augmentation of EGNOS Ionospheric Data with Locally Adapted Ionospheric Model

Global Navigation Satellite Systems (GNSS) provide positioning, navigation and timing solutions that can be used by users with different requirements. Performance of GNSS is affected by several error sources and the ionosphere is recognized as the major one. Ionospheric error varies depending on the time of day, season, position of the receiver, solar activity and the Earth’s geomagnetic field. The ionospheric error is quantified by the amount of Total Electron Content (TEC) on the path between satellite and receiver. This error is frequency dependent so it can be estimated using two frequencies in the satellite-receiver communication. Vast majority of GNSS receivers still uses only one frequency and they rely on global ionospheric models to calculate and mitigate ionospheric error. Such models can perform well in non-disturbed ionospheric conditions. However, in the time of high solar activity and sudden ionospheric changes, single frequency GNSS receivers’ performance degrades severely. To overcome this problem, Satellite-Based Augmentation Systems (SBAS) can be used. Using networks of dual-frequency GNSS receivers the current ionospheric error over some region is estimated. The calculated correction parameters are transmitted to the receivers by geostationary satellites. European Geostationary Navigation Overlay Service (EGNOS) provides Safety-of-Life (SoL) service with emphasis on integrity, primarily for usage in aviation, and Open Service (OS) with emphasis on accuracy, intended for non-SoL usage. EGNOS OS coverage area extends over most of the European Civil Aviation Conference (ECAC) region, but the eastern border area remains uncovered. In case of ionospheric disturbances, coverage is additionally disrupted because of lack of Ranging and Integrity Monitoring Stations (RIMS) in that area. This research identifies effects of different solar and geomagnetic conditions on EGNOS ionospheric correction performance in middle latitudes, especially on its eastern borders. The observed period is within the current solar cycle 24, as EGNOS OS became operational in 2009. The reference data are TEC obtained from dual-frequency GNSS stations’ RINEX observation files. Such TEC is affected by inter-frequency biases (IFB), produced by the receiver and the satellites, and has to be calibrated before being used as a reference value. After recognizing areas with permanent or ionospheric-condition-dependent lack of EGNOS ionospheric corrections, a solution with locally adapted ionospheric model is proposed. Even though there are no RIMS stations in the recognized area, several GNSS stations with publicly available data are situated there. Local reference TEC can be used to modify a global ionospheric model in a way to adapt the model to the local ionospheric conditions. The model of choice is NeQuick 2, a global model able to compute ionospheric density between any two given points, designed for simple and fast execution. The NeQuick 2 model can be locally adapted if the local ionization level is used as a model input instead of a solar flux index. Further research will determine the distance from reference data source where such a locally fitted model would provide the level of ionospheric correction similar to EGNOS, i.e. the area in which it could augment EGNOS OS.

GNSS; EGNOS; ionospheric model; local adaptation

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