In this thesis, GPS-VTEC measurements are used to assess the performance of the most recent International Reference Ionosphere extended to the plasmasphere (IRI-Plas 2017) model. GPS-VTEC data is obtained from a dual-frequency GPS receiver located at Addis Ababa (geographic location: 9.04◦ N latitude, 38.77◦ E longitude) during the complete solar cycle 24 period (2009–2019). The GPS-VTEC variations are compared with those derived from the IRI-Plas 2017 model at the diurnal, monthly, seasonal, and solar cycle scales. The diurnal variation analysis indicated that the VTEC value sharply increased to its peak value between 08:00 and 16:00 UT and decreased between 17:00 and 07:00 UT both the GPS and the model. In monthly variations, February, March, April, Septem ber, October, November, and December, the maximum monthly mean value is observed. Based on seasonal variations, VTECs were at their highest during equinoctial months and their lowest during solstice months, especially in June. The statistical results indicate that the maximum RMSE is about 13.87 TECU and 13.12 TECU during March Equinox in the year 2014 and 2015, respectively. The minimum seasonal difference is registered in the year 2012 with an RMSE value of 4.68 TECU in June solstice. These day-to-day, sea sonal and monthly mean curves are consistent with the notion of the IRI-Plas 2017 model derived VTEC profile. VTEC variations are also examined during the intense storm pe riod (CME-driven storm Dst <-100 nT). The analysis indicates that during the selected intense storm periods, the IRI-Pals 2017 model underestimates the GPS-VTEC values