Large ionospheric delays and scintillation effects can degrade the performance of Global Positioning System (GPS)-based navigation systems. In low latitudes the ionosphere is known to be much more intense than in mid latitudes due to active geomagnetic effect, and has various phenomena such as Equatorial Plasma Bubble (EPB), equatorial spread F, and Equatorial Ionospheric Anomaly (EIA). Considering that GPS signals are significantly affected by the ionosphere, investigation of the low latitude ionospheric anomalies is necessary to design monitoring systems and mitigation strategies for the operation of GPS-based navigation systems. This paper presents the case studies on anomalous equatorial ionospheric behaviors regarding the temporal and spatial variations of ionospheric delay. Using GPS dual-frequency measurements observed throughout Brazil, a simplified truth processing method is carried out to obtain ionospheric delay estimates. Rate of Total Electron Content (TEC) Index (ROTI) is calculated to identify ionospheric irregularities. The ionospheric spatial gradients are computed using the station-par method. Dual frequency ionospheric measurements are then compared with L1-cmc measurements to validate the ionospheric anomaly. The results demonstrate that large spatial gradients and ionospheric scintillations occur more frequently in low latitudes than in mid latitudes. Accordingly, we should cope with the equatorial ionospheric effects for the deployment of GPS-based navigation systems in low latitudes.

Keywords: equatorial plasma bubble, ionospheric spatial decorrelation, ionospheric irregularities, GPS-based navigation systems