engleski

GPS Positioning Accuracy in Severe Space Weather Conditions in Croatia: A 2003 Halloween Event

Satellite positioning systems can be found in a wide range of modern technical systems. Therefore, the positioning performance affects the quality of systems and services considerably. Thus, maintaining the quality of positioning service becomes the task of the utmost importance. The American Global Positioning System (GPS) is only fully operational satellite navigation system at present, with the extension of the Russian GLONASS system and the development of the European Galileo system being under way. Since all satellite navigation systems deploy the same principle for position determination, they share the same set of positioning error causes. The ionospheric delay of satellite signals is the most important of them, due to its un-predictability and the largest amount of positioning error introduced. The ionospheric delay of the satellite signal is caused by numerous processes both in the ionosphere and within the Sun-Earth system. Entitled the space weather, those processes can cause abrupt and severe modifications of the vertical ionospheric profile, thus affecting the radio signal propagation characteristics and causing the ionospheric delay. Modelling the ionospheric delay for satellite navigation signals is a very complex problem, far from being completely solved. So far, the GPS ionospheric delay can be reasonably modelled during quiet or mildly disturbed space weather by utilisation of the standard GPS ionospheric model, commonly named after its inventor as the Klobuchar model. Although Klobuchar model can resolve up to 70% of positioning error caused by ionosphere and space weather, its performance deteriorates considerably in severe space weather conditions and during the ionospheric storms. Finally, the model responses very slowly to fast developing space weather and ionospheric disturbances, which affects the overall GPS positioning performance considerably. This paper addresses the GPS positioning performance during severe space weather and ionospheric conditions, as seen from the end-users' point of view. A particular severe space weather condition event during October and November 2003 has been chosen as a case study. The archived GPS data sets collected at the reference stations Dubrovnik, Croatia and Osijek, Croatia have been used in generation of the time series of positioning samples. Positioning samples are presented in relation to known true position of the reference station, with the overall positioning error presented through its two components: northing and easting. This approach allows the analysis of not only the absolute value of the positioning error, but also the orientation of the error values in the two-dimensional space. The time series of GPS positioning samples have been correlated with the time series of parameters of space weather (sunspot number, solar flux, planetary Ap index) in order to measure the impact of the space weather on the actual GPS positioning performance in Croatia. A huge vertical positioning error response to the ionospheric storm around Halloween 2003 has been observed along with the marked increase in northing GPS positioning error. At the same time, horizontal GPS positioning error components were affected only for a brief period of time. In addition, the actual GPS ionospheric delay extracted from raw GPS pseudoranges observed during the Halloween 2003 event were compared with the Klobuchar-modelled GPS ionospheric delay for the same period in order to evaluate the impact of the extreme space weather on GPS ionospheric model, and thus on the GPS ionospheric error. Causes of disturbed time series dynamics of vertical and horizontal GPS positioning errors have been identified in both the impact of the space weather and the ionospheric storm, and the drawbacks of Klobuchar model application in severe space weather conditions.

Space weather; GPS positioning error; GPS ionospheric delay

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