Global Positioning System (GPS) is an effective tool for monitoring the Earth’s ionosphere. This paper concerns with temporal and spatial variations of ionospheric total electron content (TEC) at RAMO, Israel (geographic coordinates: 30.597^oN, 34.76^oE; geomagnetic coordinates: 27.17^oN, 112.40^oE), and ZAMB, Zambia (geographic coordinates: 15.42^oS, 28.31^oE; geomagnetic coordinates: 16.98^oS, 98.67^oE) for the descending phase of solar cycle-24. The VTEC estimated from GPS measurements and VTEC values modeled from the IRI-2016 model are obtained over both the GPS stations, i.e., RAMO station, in Northern Hemisphere (NH) and ZAMB station in Southern Hemisphere (SH). The diurnal, seasonal, annual, and solar cycle variations in TEC are investigated during 2016–2018. Also, a comparative study is performed between VTEC derived from GPS observations and International Reference Ionosphere-2016 (IRI-2016) model using the statistical analysis. It has been observed that the observed and modeled maximum VTEC decreases with the declining phase of solar cycle-24 over both the stations. The semiannual patterns are noticed in VTEC values of both the IRI-2016 model and GPS observations for all the years, i.e., 2016–2018. At RAMO station, seasonal analysis depicted a year-wise decrease in maximum TEC as follows March Equinox (Mar-Equ), September Equinox (Sep-Equ), December Solstice (Dec-Sol), and June Solstice (Jun-Sol). It is observed from the monthly average estimations of the IRI-2016 model that it has relatively more overestimations of VTEC values over RAMO station in NH than over ZAMB in SH during 2016–2018. However, the IRI-2016 model has underestimated the GPS-VTEC values from June–September 2018 over NH, RAMO station. The root-mean-square error (RMSE) values of the IRI-2016 model delineate that the model has more RMSE during March Equinox than September Equinox, whereas these RMSEs are recorded high over NH (RAMO) than SH (ZAMB). At RAMO, the IRI-2016 model has shown high RMSE values during the June solstice compared to the December solstice. On the other hand, at ZAMB, the highest RMSE values are observed during the December solstice than June solstice. Ionolab-TEC and GIM-TEC also considered over both the stations for the analysis. The IRI-2016 model predictions are in good agreement with GPS-VTEC values over SH (ZAMB) compared to NH (RAMO).

全球定位系统 (GPS) 是监测地球电离层的有效工具。本文涉及以色列 RAMO（地理坐标：30.597 ^o N，34.76 ^o E；地磁坐标：27.17 ^o N，112.40 ^o E）和赞比亚ZAMB（地理坐标）的电离层总电子含量 (TEC) 的时空变化。坐标：15.42 ^o S, 28.31 ^o E；地磁坐标：16.98 ^o S, 98.67 ^oE) 用于太阳活动周期-24 的下降阶段。从 GPS 测量估计的 VTEC 和从 IRI-2016 模型建模的 VTEC 值是通过两个 GPS 站获得的，即北半球 (NH) 的 RAMO 站和南半球 (SH) 的 ZAMB 站。在 2016-2018 年期间调查了 TEC 的昼夜、季节、年度和太阳周期变化。此外，还使用统计分析在从 GPS 观测得出的 VTEC 与国际参考电离层-2016 (IRI-2016) 模型之间进行了比较研究。已经观察到，观测和模拟的最大 VTEC 随两个站的太阳周期 24 的下降阶段而降低。在 IRI-2016 模型和 GPS 观测的所有年份（即 2016-2018）的 VTEC 值中都注意到了半年模式。在 RAMO 站，季节性分析描述了最大 TEC 的逐年下降，如下所示：三月春分 (Mar-Equ)、九月春分 (Sep-Equ)、十二月至日 (Dec-Sol) 和六月至日 (Jun-Sol)。从IRI-2016模型的月平均估计可以看出，在2016-2018年期间，NH的RAMO站的VTEC值高于SH的ZAMB的VTEC值相对更多。然而，IRI-2016 模型低估了 2018 年 6 月至 9 月 NH、RAMO 站的 GPS-VTEC 值。IRI-2016 模型的均方根误差 (RMSE) 值表明模型在三月春分期间的 RMSE 比九月春分多，而这些 RMSE 在 NH (RAMO) 上的记录高于 SH (ZAMB)。在 RAMO，与十二月至日相比，IRI-2016 模型在六月至日期间显示出较高的 RMSE 值。另一方面，在ZAMB，最高的 RMSE 值出现在 12 月至 6 月至 12 月期间。Ionolab-TEC 和 GIM-TEC 还考虑了两个站点的分析。与 NH (RAMO) 相比，IRI-2016 模型预测与 SH (ZAMB) 上的 GPS-VTEC 值非常一致。