## 5 Conclusions  
In the present study, we performed a comparative analysis of the diurnal, seasonal, solar cycle, and LT trends with respect to universal time effects at the equatorial and low-latitude ionosphere over the American, African, and Asian longitude sectors. The observed GPS-TEC data were taken from two well-separated locations at each longitude sector. The corresponding TEC values extracted from CODE-GIMs and IRI-2016 were also analyzed to underpin the longitudinal discrepancies in the ionospheric parameter. The diurnal variations in TEC from GPS, GIM and IRI-2016 showed similar features across the longitudes, except for the African longitude, where the diurnal peaks of the GPS-TEC, GIM and IRI-2016 models were observed during the post-sunset hour at the station away from the dip equator (CPVG). The post-sunset peak during some seasons at CPVG was referredto as an anomaly in the African equatorial ionosphere, where the peak of ionospheric parameters such as TEC was found to occur at noon to post-noon periods. The values of the diurnal variations in the GPS-TEC, GIM and IRI-2016 models were observed to be higher in the African longitude than the other two longitudes and during the descending phase than in the minimum phase. Regarding the assessment of the GIM and IRI-2016 models on GPS-TEC, both models showed either overestimation or underestimation, though the GIM seemed relatively more suitable for the estimation of values, especially in terms of its similarity in shape to GPS-TEC. However, the IRI-2016 model mostly showed a noon bite-out curve at all stations at the three longitudes, except at CPVG in the African longitude, where it was able to reveal the true shape of GPS-TEC. At CPVG, IRI-2016 showed an overestimation of GPS-TEC, especially during the minimum phase of the solar cycle. The overestimation characteristic of the GIM was more pronounced during the March equinox of the descending phase year of 2016 at the three longitudes. The local time effect of the longitudinal variations in TEC was evident, showing peak shifting to different hours of the day in the universal time frame at the three longitudes. The seasonal and semi-annual variations observed in this study showed some anomalies that were different from previous results, especially during the solar minimum years at the three longitudes. Finally, our observations suggest that there is an insignificant effect of longitudinal variations on equinoctial asymmetry. The important outcomes from this study could augment the past efforts of scientists to understand the seasonal effects of the longitudinal variations in TEC, which may be useful for the statutory protection and baseline positioning accuracy of satellite positioning. Furthermore, the GIM and IRI-2016 model groups and research scientists may find results from this research useful to generate additional improvements of their respective models. As an extension of the present work, we intend to apply IRI-2020 in a future analysis and also for solar cycle 25 to verify the improvements in the embedded submodels of the latest edition and other planned updates. In particular, improvements of the diurnal, seasonal and geomagnetic disturbance time representations in the latest version will be emphasized, as there is still rooms for improvements of the same in the equatorial and low latitudes. Conceptualization, A.O. and S.K.P. ; methodology, A.O., O.O. and S.K.P. ; software, A.O., O.J.O. and S.K.P. ; validation, A.O., S.K.P., A.O.A., M.A.A., P.J. and O.O. ; formal analysis, A.O., O.O., M.A.A., P.J. and S.K.P. ; investigation, A.O., O.J.O., A.O. and S.K.P. ; resources, A.O., O.O. and S.K.P. ; data curation, A.O., O.O. and S.K.P. ; writing--original draft preparation, A.O. and O.O. ; writing--review and editing, A.O., O.J.O., A.O.A., M.A.A., P.J. and S.K.P. ; visualization, A.O., O.J.O., A.O.A., P.J. and S.K.P. All authors have read and agreed to the published version of the manuscript.