DETERMINATION OF IONOSPHERIC TOTAL ELECTRON CONTENT DERIVED FROM GNSS MEASUREMENTS

Abstract:

Global Navigation Satellite System (GNSS) has been used in numerous fields especially related to satellite- based radio navigation system for a long time. Ionosphere, one of the upper atmosphere layers ranges from 60 km to 1500 km, is a dispersive medium and it includes a number of free electrons and ions. The ionization is mainly subject to the sun and its activity. Ionospheric activity depends also on seasonal, diurnal variations and geographical location.

Total Electron Content (TEC), which is also called Slant Total Electron Content (STEC), is a parameter that changes according to ionospheric conditions and has highly variable structure. Furthermore, Vertical TEC (VTEC) can be explained as TEC value in the direction of zenith. Thanks to VTEC, TEC values can be modelled. TEC is measured in units of TECU and 1TECU= 10¹⁶ electrons/m².

Ionospheric modelling has a great importance for improving the accuracies of positioning and understanding the ionosphere. Thus, various models have been developed to detect TEC value in the last years. Single Layer Model (SLM) which provides determining TEC value and GPS positioning in the ionosphere accurately is one of the most commonly used models. SLM assumes that all free electrons are concentrated in a shell of infinitesimal thickness.

In this paper SLM model was used to derive TEC values by means of Bernese 5.0 program developed by the University of Bern, Sweden. In this study, we have used regional ionosphere model to derive TEC value. First of all, GPS data have been collected from 10 stations in Turkey and 13 IGS stations for 7 days from 06.03.2010 to 12.03.2010. Then, Regional Ionosphere Model (RIM) is created with the reference of the GPS data. At the end of the process, the result files are stored as IONEX format. TEC results for those days are obtained with two hours interval. TEC variation related to the research area ranges from nearly 6 TECU to approximately 20 TECU. The obtained results show that TEC values start increasing until mid-days and reach peak value at 12:00 UT. After 12:00 UT it begins decreasing gradually towards night because of recombination of the ions. As a result, SLM is an effective model for mapping TEC values and determination of TEC variation can be used to identify many studies such as precursor of earthquakes, volcanic eruptions and launching site determination etc.