Abstract
Ionospheric time delay (VΔt) variability using Global Positioning System (GPS) data over Akure (7.15°N, 5.12°E), Nigeria, has been studied. The observed variability of VΔt in comparison to older results of vertical total electron content (TEC) across similar regions has shown equivalent signatures. Higher monthly mean values of VΔt (MVΔt) were observed during daytime as compared to nighttime (pre- and post-midnight) hours in all months. The highest MVΔt observed in September during daytime hours range between ~6 and ~21 ns (~1.80 and ~6.30 m) and at post-midnight, they are in the range of ~1 to ~6 ns (~0.3 to ~1.80 m). The possible mechanisms responsible for this variability were discussed. Seasonal VΔt were investigated as well.
Highlights
Navigation systems have witnessed tremendous improvements in the past four decades
For clearer representation of monthly mean hourly values of vertical ǻt (Vǻt) (MVǻt) during daytime, pre, and post-midnight hours, September with highest magnitudes of mean values of Vǻt (MVǻt) was divided into daytime (07:00-18:00 local time (LT)), pre-midnight (19:00-24:00 LT), and post-midnight (00:01-06:00 LT) hours (Fig. 1)
The MVǻt magnitudes measured in nano seconds [ns] are plotted on the y axis, with local time (LT) on the x axis
Summary
Navigation systems have witnessed tremendous improvements in the past four decades. One of such areas of achievements is the invention of Global Positioning System (GPS) by the Department of Defence (DoD), United States of America (USA), in the 1990s. GPS signals traversing through ionospheric medium to the receiving point are expected to propagate along a straight and geometric line, but, in a refractive medium like ionosphere, they bend due to refraction In this scenario, the wave could be in phase (advance) and the refractive index will be smaller than 1; otherwise, if the wave is in group (delay), the refractive index is larger than 1. By comparing the phase and group propagation effects using the ionospheric first order, higher orders and total electron content (TEC), the GPS time delay observation can be made. The TEC estimates (Eq 3), which were originally aimed at correcting the ionospheric error due to electron density population from the GPS radio signal when travelling through the ionosphere could provide an extraordinary ionospheric research regarding ionospheric time delay. The variability of vertical time delay over the Nigeria ionosphere, its influences on terrestrial applications and possible mechanisms responsible for its variability will be investigated
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