## 1 Introduction  
The ionospheric electron density variation in the Earth's upper atmosphere acts as a potential threat to the modern space-based technological systems by either introducingdelays due to the total electron content (TEC) or the fading/loss of lock due to scintillations in the satellite radio and navigation signals, such as in global navigation satellite systems (GNSSs). In adverse conditions, this variation can cause severe damage to the highly dynamic and sensitive technological systems that rely on the ionosphere for their functionality [1; 2]. When space weather conditions are insignificant, standard global positioning system (GPS) receivers can compensate for the average or quiet-time ionosphere delays through integrated models or combined solutions for positioning and navigation that generally correspond to the first-order ionospheric effects. However, when the ionosphere is disturbed by space weather events (SWEs), the models are no longer accurate and the receivers are unable to effectively provide positioning and navigation accuracy [3]. The ionospheric effects on modern space-based technologies are highly dynamic and complicated, especially near the geomagnetic equator, where they tend to be subjected to some anomalies such as the seasonal anomaly, the semi-annual anomaly, the equinoctial anomaly, spread F, noon bite-out, and equatorial electrojet (EEJ) current [4; 5; 6; 7]. A majority of these anomalies can be predicted and mitigated up to some level using certain engineering design solutions. It has been known that the total delay suffered by satellite radio signals propagating through the ionosphere depends on both the frequency of the radio signal and the TEC [8; 9]. However, the TEC is known to suffer spatiotemporal variations, and neglecting these changes can introduce a few centimeters to several meters of error in position calculations [7; 10; 11; 12; 13; 14; 15]. The legacy GPS constellation provides input parameters associated with a simplistic empirical model of the ionosphere known as the Klobuchar model [16], which determines and mitigates the delay error in single-frequency positioning to some extent [17]. When conditions deviate from those predicted by the Klobuchar model, GPS/GNSS systems may have larger positioning errors, so the use of dual-frequency (L1 (f1 = 1575.42 MHz) and L2 (f2 = 1227.60 MHz)) GPS receivers becomes more accurate. The exploitation of GNSS radio and navigation systems, such as GPS, has tremendously increased in the last decade in modern technological infrastructure, as well as in the defining and modeling of the atmospheric and ionospheric variations and their effects [18; 19]. GPS receivers are in-built in almost every cell phone, as well as in most automobiles, airplanes, trucks, ships, and other modern mobile equipment that demand a precise positioning and navigation solution today. The high precision of dual-frequency GPS is used in farming, construction, exploration, surveying, snow removal, and many other applications that are useful to modern society but are limited in cost-effectiveness. Hence, the implementation of a relatively low-cost robust ionospheric error mitigation method through the modeling of the day-to-day ionospheric effects is a challenging task for the scientific community, as it needs a thorough investigation of latitudinal and longitudinal uncertainty from global and regional perspectives. Apart from exploring GNSS observables for monitoring the ionospheric TEC, various models developed by different research groups also help in understanding the climatological variability of the TEC and its associated physical mechanisms. Two representatives of such models that have been popularized among researchers are the global ionospheric maps (GIMs) [1], derived from a large dataset of GNSS observations across the globe, and the global empirical International Reference Ionosphere (IRI) [20], both of which have been undergoing routine improvements over the years.