## 3 Results
### Diurnal Variations
GPS-TEC increased from minimum values of about 1-3 TECU between 08:00 and 10:00 UT, corresponding to 05:00-07:00 LT at SALU and 04:00-06:00 LT at POVE, to maximum values of 10-14 TECU during post-noon hours with respect to UT at both stations before reducing to the minimum value afterwards. The diurnal variations of the GIM and IRI-2016 models also showed very similar diurnal trends to that of GPS-TEC, except that the IRI-2016 model values were lower than the GPS-TEC values at both stations and the GPS-TEC and GIM values were almost equal. In the African longitude (Figure 4c,d), GPS-TEC commenced rising from \(\sim\)2-3 TECU between 02:00 and 04:00 UT (05:00-07:00 LT) at ADIS and between 07:00 and 08:00 UT (05:30-06:30 LT) at CPVG to maximum values between 10:00 and 16:00 UT, corresponding to 13:00-19:00 LT at ADIS (\(\sim\)28 TECU; Figure 4d) and 08:30-14:30 LT at CPVG (\(\sim\)18 TECU; Figure 4c), before further reducing to minimum values, as shown in Figure 4c,d. The GIM showed a similar diurnal pattern to GPS-TEC, though with slightly lower values (23 TECU) at ADIS and slightly higher values (15 TECU) at CPVG. The IRI-2016 model values, which were observed to fade away at ADIS, were in contrasting, slightly higher than those of both GPS-TEC and GIM at CPVG. In the Asian longitude (Figure 4e,f), there were no GPS-TEC observation data at HUTB (Figure 4e), but at PPPC, GPS-TEC data were available for the majority of days. From the available data, the GPS-TEC, GIM and IRI-2016 models showed similar diurnal patterns, increasing from the minimum value of 0-2 TECU at 22:00 UT (06:00 LT) to the maximum value of 10-16 TECU during the pre-sunrise hour with respect to UT before reducing to the minimum value during the post-sunset hour with respect to UT (Figure 4f). We decided to be silent on the diurnal variations of the GIM and IRI-2016 at HUTB (Figure 4e) because there were no GPS-TEC observation data throughout the year. Finally, for the minimum phase year of 2019 (Figure 5a-e)) at the three longitudes used in this research, the GPS-TEC, GIM and IRI-2016 model values were seen to further reduce than the previous minimum phase year (i.e., 2018). The diurnal variations in the GPS-TEC, GIM and IRI-2016 models were very similar to the diurnal variations of the minimum phase year of 2018, though with slightly lower values. Unfortunately, there were no GPS-TEC observation values at SALU in the American longitude and HUTB in the Asian longitude, which made it difficult to evaluate the assessment of the GIM and IRI-2016 models on GPS-TEC. At POVE (Figure 5b) in the American longitude, the GPS-TEC, GIM and IRI-2016 models showed similar diurnal profiles, but the GIM showed slightly higher maximum values (\(\sim\)18-21 TECU) than GPS-TEC (\(\sim\)14-16 TECU) and IRI-2016 (11-13 TECU). In the African longitude (Figure 5c,d), the GPS-TEC, GIM and IRI-2016 models showed similar diurnal trends, but at CPVG (Figure 5c), the IRI-2016 model was observed to show slightly higher maximum values (\(\sim\)22-28 TECU) than GPS-TEC values (\(\sim\)18 TECU) and GIM (20 TECU). At ADIS (Figure 5d), GPS-TEC was observed to show the highest magnitude, followed by GIM, while the IRI-2016 model showed the least magnitude. Lastly, in the Asian longitude, GPS-TEC and GIM at PPPC (Figure 5f) were observed to show similar diurnal patterns that were slightly different than that of the IRI-2016 model. At PPPC, the diurnal magnitudes of the GPS-TEC, GIM and IRI-2016 models were less than 18 TECU from pre-sunrise to noon-time with respect to UT. However, at HUTB (Figure 5e), the values of the GIM and IRI-2016 models were shown to potentially be suitable to estimate the values of GPS-TEC, especially during the minimum phase, according to the consistent trend observed in this study.