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MONITORING IONOSPHERIC VARIATION FOR A DEFINITE PERIOD TIME IN TURKEY
Paper ID : 1114-SMPR-FULL
Authors:
samed inyurt *1, Cetin Mekik2, Omer Yildirim3
1bulent ecevit university 67100 zonguldak
2Bulent Ecevit University
3Gaziosmanpasa University
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 includes a number of free electrons and ions. A signal coming from a satellite to a receiver is affected by this ionization and this is named as group delay (or phase advance) briefly. The ionization is not only subject to the sun and its activity but also to seasonal, diurnal variations and geographical location. Total Electron Content (TEC), also called Slant Total Electron Content (STEC), is a parameter that changes in relation to ionospheric conditions, and has a highly variable structure. Furthermore, Vertical TEC (VTEC) can be referred to as TEC value in the direction of zenith, and can be modelled by means of VTEC. TEC is measured in units of TECU and 1TECU= 〖10〗^16 electrons/m^2.
Ionospheric modelling has a great impact on improving the accuracies of positioning; thus, various models have been developed to detect TEC values in the last years. Single Layer Model (SLM), providing determination of TEC values and position of GPS signals in the ionosphere accurately, is one of the most commonly used models, and assumes that all free electrons are concentrated in a shell of infinitesimal thickness.
Ionosphere has been studied by a number of scientists in recent years. Since GPS observations cannot provide TEC value directly, it can be estimated from combination of observations. In this study TEC values derived from GPS observations were produced variations with two hours increments from the eighth day to fifteenth day in months just after beginning of each season, namely January, April, July and October in 2014 for ZONG TUSAGA-Aktif station and 41 other stations (TUSAGA-Aktif, EUREF, IGS). TEC values computed by the Bernese 5.0 software were compared with the Global Ionosphere Model (GIM) TEC values regularly published by Center for Orbit Determination in Europe (CODE) and International Reference Ionosphere (IRI-2012) TEC values. As a result of this study, with the comparison of GPS TEC values to GIM TEC values, it has become obvious that GPS TEC values are quite similar to the GIM TEC values. The differences of TEC values derived from GPS TEC and GIM TEC change from 0.91 TECU (January) to 1.88 TECU (October). On the other hand, GPS TEC values are also compared with IRI TEC values, and found out that there is a considerable difference between the two TEC values ranging from 6.30 TECU (January) to 15.15 TECU (April). Furthermore, the level of Ionospheric activity were also examined for these four months using KP, Dst, F10.7, R index values, and it is deduced that ionospheric activity is construed as calm except for some days. Moreover, TEC values derived from GPS measurements are attained similar to GIM TEC, but found to stray from IRI-2012 TEC values remarkably.
Keywords:
Ionosphere, Total Electron Content, GNSS
Status : Paper Accepted (Oral Presentation)