engleski

GPS positioning accuracy in Croatia during the extreme space weather conditions in September 2005

Space weather is the most influential subject affecting wireless communication and navigation systems. The complexity of processes within the Sun-Earth system have been studied almost since the discovery of the existence of radio waves. Different theories have been developed in order to describe the impacts of energy and particles ejected during increased activities of our native star on the behaviour of the upper (ionised) layers of the atmosphere. With the rising amount of measurement sensors (mostly satellite based) today, it looks as the task of development of the understanding of the space weather processes and its impact on technical systems is near completion. However, this is still far from truth. Satellite navigation systems are considerably affected by the space weather processes. After cessation of the selective availability, ionospheric delay remains the largest single source of GPS positioning error. GPS ionospheric delay is proportional to the total amount of charged particles the GPS radio signal encounters along the propagation path between GPS satellite transmitting and user receiving aerial. The total amount of charged particles is expressed as the total electron content (TEC) measured in the number of charged particles per area unit on the GPS signal path. The impact of the ionosphere is particularly emphasised in the periods of increased solar activity, which appears in rough intervals of about 11 years. Intensive changes in the space weather create significant disturbances in the ionosphere causing unexpected and unpredictable changes in the total electron content (TEC), which causes the GPS ionospheric delay dynamics with the similar unpredictability. After decades of careful study of the behaviour of the ionosphere and the impact of space weather on the ionospheric processes, a number of forecasting models has been proposed in order to tackle the impact of the space weather on GPS positioning accuracy. One of them (so-called Klobuchar model, after its inventor) is even accepted as the standard, providing successful global correction of up to 60% of the positioning error caused by the ionosphere. However, this model is successful in periods of reasonably quite space weather or ring its minor disturbances. Unfortunately, the Klobuchar model does not take into account the local irregularities and extreme disturbances in the ionosphere. In order to better understand the impact of the space weather on the ionosphere and overcome the shortcomings of the Klobuchar model, a number of studies have been and will be performed. In this article, we contribute to the understanding of GPS positioning accuracy behaviour during the extreme space weather conditions by reporting the results of the analysis of data sets collected in Dubrovnik, Croatia during recent extreme space weather conditions in September 2005. First, the detailed description of space weather changes in observed period of time is presented, based on the analysis of the archived space weather-related parameters data (sunspot number, solar flux, global Kp and A indices). The characteristic patterns in the extreme space weather development were identified and correlated with the positioning results obtained from the archived Dubrovnik RINEX observation and navigation files. The results of the analysis show a marked relationship between changes in positioning accuracy and related phases in extreme space weather development. The analysis of GPS data sets collected during the September 2005 extreme space weather adds to better understanding of the processes in the extreme space weather development and its impact on satellite navigation. Since this is one of the rare examination of the GPS data collected in Croatia, this analysis will contribute to the better understanding of the local ionospheric behaviour, helping to develop a local GPS ionospheric error model for the Mediterranean area.

GPS ; ionosphere ; space weather

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