Browsing by Author "Akala, A. O."
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Item Open Access Comparison of equatorial GPS-TEC observations over an African station and an American station during the minimum and ascending phases of solar cycle 24(European Geosciences Union (EGU), 2013-11) Akala, A. O.; Seemala, G. K.; Doherty, P. H.; Valladares, C. E.; Carrano, C. S.; Espinoza, Jhan Carlo; Oluyo, S.GPS-TEC data were observed at the same local time at two equatorial stations on both longitudes: Lagos (6.52° N, 3.4° E, 3.04° S magnetic latitude), Nigeria; and Pucallpa (8.38° S, 74.57° W, 4.25° N magnetic latitude), Peru during the minimum (2009, 2010) and ascending (2011) phases of solar cycle 24. These data were grouped into daily, seasonal and solar activity sets. The day-to-day variations in vertical TEC (VTEC) recorded the maximum during 14:00-16:00 LT and minimum during 04:00-06:00 LT at both longitudes. Seasonally, during solar minimum, maximum VTEC values were observed during March equinox and minimum during solstices. However, during the ascending phase of the solar activity, the maximum values were recorded during the December solstice and minimum during the June solstice. VTEC also increased with solar activity at both longitudes. On longitude by longitude comparison, the African GPS station generally recorded higher VTEC values than the American GPS station. Furthermore, harmonic analysis technique was used to extract the annual and semi-annual components of the amplitudes of the TEC series at both stations. The semi-annual variations dominated the TEC series over the African equatorial station, while the annual variations dominated those over the American equatorial station. The GPS-TEC-derived averages for non-storm days were compared with the corresponding values derived by the IRI-2007 with the NeQuick topside option. The NeQuick option of IRI-2007 showed better performance at the American sector than the African sector, but generally underestimating TEC during the early morning hours at both longitudes.Item Restricted Impacts of ionospheric scintillations on GPS receivers intended for equatorial aviation applications(American Geophysical Union, 2012-07-25) Akala, A. O.; Doherty, P. H.; Carrano, C. S.; Valladares, C. E.; Groves, K. M.This study examines the impacts of ionospheric scintillations on GPS receivers that are intended for equatorial or transequatorial aviation applications. We analyzed GPS data that were acquired at Ascension Island during the Air Force Research Laboratory (AFRL) campaign of the solar maximum year of 2002. Strong scintillations impacted the receiver‐satellite geometry, leading to poor dilution of precisions and positioning accuracy. In addition, deep signal fades (>20 dB‐Hz), leading to navigation outages were observed during most of the nights of the campaign. Under quiescent conditions, the C/No of satellites fluctuated slowly between 50 dB‐Hz and 35 dB‐Hz baselines for both L1 (1.5754 GHz) and L2 (1.2276 GHz) signals, depending on the satellite's elevation angle. The satellite's elevation angle and the effective scan velocity of the satellite's ionospheric penetration point (IPP) with respect to the magnetic field and plasma drift influenced the rate of fading of satellite signals.