ABSTRACT
In the last decades, knowledge about the ionosphere has grown considerably thanks to the use of GNSS measurements, and in turn the GNSS have highly benefited from this improved knowledge. This Dissertation studied the TEC value for the Nigerian GNSS Permanent Reference Network (NIGNET) over a period of four and a half years (January 2010 to June 2014). The TEC over the NIGNET stations were estimated using two prominent data sources viz. NIGNET CORS observed data in RINEX format, and Global Ionospheric Maps (GIMs) in IONEX format. Annual TEC time series were generated using TEC values from both data sources. The NIGNET CORS-derived TEC results were validated with the GIM-derived TEC (GTEC). Various effects on TEC values were studied such as (diurnal, season, spatial, magnetic storms and solar eclipse, and solar activity). In general the TEC reveals higher values in day time with peak occurring between 13:00 and 14:00 hours. The TEC is lower in the early hours of the day and at night times with minimum diurnal TEC occurring between the hours of 03:00 and 04:00. The TEC increase gradually in the March equinox from a mean value of 23 TECU in January to 27 TECU in February and then 33 TECU in March. The TEC series attains a maximum in the March equinox between March and April with a 43-45% increment from its mean value in January and 5-7% increment from its value at February. There is less than 2% rise in the mean TEC value between the month of March and April, as the TEC remains at its peak within these months. The TEC gradually declines after the month of April and attaining a mean minimum value of 20-22 TECU within the June solstice, corresponding to 33-35% decrease from the TEC peak that occurred in the March equinox. After June solstice, the TEC once again begin a gradual increment in August, attaining another maximum value of 30-32 TECU between the end of September equinox and the beginning of December solstice (between October and November) after which it once again declines gradually at the end of the December solstice. The TEC varies spatially with both longitudes and latitudes, having higher values for the southern stations which are closer to the equator compared to the northern stations farther away from the equator. It was found that the effect of geomagnetic storms on TEC results in increased values during days of storms and the TEC values are proportional to the strength of geomagnetic storm type. A comparison of the TEC values estimated from the NIGNET CORS data and GIM over each NIGNET station for these flares and solar indices show evidence of geomagnetic and solar activity dependence of ionospheric GNSS-derived TEC. Improvement to the study will be to consider the TEC estimation in near or real time and for a longer time period of 11 years corresponding to the solar cycle in order to ascertain the long time correlation of TEC with the solar cycle variation.