STUDY ON IONOSPHERIC TOTAL ELECTRON CONTENT (TEC) VARIATION OVER SRI LANKA BASED ON CONTINUOUS GNSS OBSERVATIONS

Abstract

Solar activities produce highly dynamic weather conditions in outer space and create challenging environment for satellites and space data/signal propagation especially through the ionosphere. For instance, the propagation delay effecting the trans-ionospheric radio signals could significantly diminish the positional accuracy of Global Navigation Satellite System (GNSS). One of the key parameters of understanding the state of the ionosphere is the measure of Total Electron Content (TEC). There are several methods of measuring this vigorously changing TEC. Out of which, method based on dual frequency GNSS observations is common in practice due to the higher availability of data through the Continuously Operating Reference Station (CORS) networks established all over the world. In 2016, Survey Department of Sri Lanka also established the national CORS network with five stations located in the western part of the country. This initiation has unlocked the possibility of investigating the behavior of equatorial ionosphere over Sri Lanka. Therefore, this study aims to investigate the equatorial ionospheric TEC with respect to its diurnal, seasonal and latitudinal variations for the first time over Sri Lanka based on CORS data. In addition, the influence of solar activities on ionospheric TEC variation is also investigated by inducing two common solar indices, Sunspot Number (SSN) and 10.7 cm solar radio flux (F10.7). The analysis was done by using simultaneous GNSS observations recorded at the CORS receivers for a period of 12 months starting from October 2017. TEC calibrations were done using Ciraolo methodology by applying carrier phase measurements of GPS and GLONASS systems. Diurnal variation of TEC shows almost same trend throughout the study period showing high randomness during daytime around 11:00 to 14:00 hours. Equinox months have shown higher TEC values where solstice period has shown lower values. Higher TEC has been experienced at Kalutara (6.58◦ N) and a comparatively lower value was observed at Kegalle (7.25◦ N), which is located more towards the magnetic equator than Kalutara. A weak but positive relationship of 0.38 with VTEC and F10.7 has been observed. However, the same correlation with SSN has shown a very weak relationship of 0.19.