ALBERT

Beschreibung: We investigated total electron content (TEC) at Ilorin (8.50∘ N 4.65∘ E, dip lat. 2.95) for the year 2010, a year of low solar activity in 2010 with Rz=15.8. The investigation involved the use of TEC derived from GPS, estimated TEC from digisonde portable sounder data (DPS), and the International Reference Ionosphere (IRI) and NeQuick 2 (NeQ) models. During the sunrise period, we found that the rate of increase in DPS TEC, IRI TEC, and NeQ TEC was higher compared with GPS TEC. One reason for this can be attributed to an overestimation of plasmaspheric electron content (PEC) contribution in modeled TEC and DPS TEC. A correction factor around the sunrise, where our finding showed a significant percentage deviation between the modeled TEC and GPS TEC, will correct the differences. Our finding revealed that during the daytime when PEC contribution is known to be absent or insignificant, GPS TEC and DPS TEC in April, September, and December predict TEC very well. The lowest discrepancies were observed in May, June, and July (June solstice) between the observed values and all the model values at all hours. There is an overestimation in DPS TEC that could be due to extrapolation error while integrating from the peak electron density of F2 (NmF2) to around ∼1000 km in the Ne profile. The underestimation observed in NeQ TEC must have come from the inadequate representation of contribution from PEC on the topside of the NeQ model profile, whereas the exaggeration of PEC contribution in IRI TEC amounts to overestimation in GPS TEC. The excess bite-out observed in DPS TEC and modeled TEC indicates over-prediction of the fountain effect in these models. Therefore, the daytime bite-out observed in these models requires a modifier that could moderate the perceived fountain effect morphology in the models accordingly. The daytime DPS TEC performs better than the daytime IRI TEC and NeQ TEC in all the months. However, the dusk period requires attention due to the highest percentage deviation recorded, especially for the models, in March, November, and December. Seasonally, we found that all the TECs maximize and minimize during the March equinox and June solstice, respectively. Therefore, GPS TEC and modeled TEC reveal the semiannual variations in TEC.

Beschreibung: Rate of change of TEC (ROT) and its index (ROTI) are considered a good proxy to characterize the occurrence of ionospheric plasma irregularities like those observed after sunset at low latitudes. SBASs (satellite-based augmentation systems) are civil aviation systems that provide wide-area or regional improvement to single-frequency satellite navigation using GNSS (Global Navigation Satellite System) constellations. Plasma irregularities in the path of the GNSS signal after sunset cause severe phase fluctuations and loss of locks of the signals in GNSS receiver at low-latitude regions. ROTI is used in this paper to characterize plasma density ionospheric irregularities in central–western Africa under nominal and disturbed conditions and identified some days of irregularity inhibition. A specific low-latitude algorithm is used to emulate potential possible SBAS message using real GNSS data in the western African low-latitude region. The performance of a possible SBAS operation in the region under different ionospheric conditions is analysed. These conditions include effects of geomagnetic disturbed periods when SBAS performance appears to be enhanced due to ionospheric irregularity inhibition. The results of this paper could contribute to a feasibility assessment of a European Geostationary Navigation Overlay System-based SBAS in the sub-Saharan African region.

Beschreibung: We investigated total electron content (TEC) at Ilorin (8.50°N 4.65°E, dip lat. 2.95) during a low solar activity 2010. The investigation involved the use of GPS derived TEC, TEC estimated from digisonde portable sounder data (DPS-TEC), the International Reference Ionosphere model (IRI-TEC) and NeQuick 2 model (NeQ-TEC). The five most quietest days of the months obtained from the international quiet days (IQD) from the website http://www.ga.gov.au/oracle/geomag/iqd_form.jsp were used for the investigation. During the sunrise period, we found that the rate of increases in DPS-TEC, IRI-TEC and NeQ-TEC were higher with respect to GPS-TEC. One reason for this can be alluded to an overestimation of plasmaspheric electron content (PEC) contribution in modeled TEC and DPS-TEC. A correction factor around the sunrise where a significant percentage difference of overestimations between the modeled TEC and GPS-TEC was obtained will correct the differences. Our finding revealed that during the daytime when PEC contribution is known to be absent or insignificant, GPS-TEC and DPS-TEC in April, September and December predicts TEC very well. The lowest discrepancies were observed in May, June and July (June solstice) between the observed and all the model values in all hours. There is an overestimation in DPS-TEC that could be due to extrapolation error while integrating from the peak electron density of F2 (NmF2) to around ~1000km in the Ne profile. The underestimation observed in NeQ-TEC must have come from the inadequate representation of contribution from PEC on the topside of NeQ model profile whereas the exaggeration of PEC contribution in IRI-TEC amount to overestimations of GPS-TEC. The excess bite-out observed in DPS-TEC and NeQ-TEC show the indication of overprediction of fountain effect in these models. Therefore, the daytime bite-out observed in these two models require a modifier that could moderate the perceived fountain effect morphology in the models accordingly. Seasonally, we found that all the TECs maximize and minimize during the March equinox and June solstice, respectively. Therefore, GPS-, DPS-, IRI- and NeQ-TEC reveal the semi-annual variations in TEC as reported in all regions. The daytime DPS-TEC performs better than the daytime IRI-TEC and NeQ-TEC in all the months, however, the dusk period requires attention due to highest percentage difference recorded especially for DPS-TEC and the models in March, and November and December for DPS-TEC.