Contents lists available at ScienceDirect Journal of Atmospheric and Solar–Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp Analysis of a grid ionospheric vertical delay and its bounding errors over West African sub-Saharan region O.E. Abe a,b, ⁎ , X. Otero Villamide a , C. Paparini a , S.M. Radicella a , B. Nava a a The Abdus Salam International Centre for theoretical Physics (ICTP), Trieste, Italy b Department of Physics, Federal University Oye-Ekiti, Oye-Ekiti, Ekiti State, Nigeria ARTICLE INFO Keywords: GIVD GIVE IGP EIA Ionosphere SBAS Plasma irregularities West African sub-Saharan region ABSTRACT Investigating the effects of the Equatorial Ionization Anomaly (EIA) ionosphere and space weather on Global Navigation Satellite Systems (GNSS) is very crucial, and a key to successful implementation of a GNSS augmentation system (SBAS) over the equatorial and low-latitude regions. A possible ionospheric vertical delay (GIVD, Grid Ionospheric Vertical Delay) broadcast at a Ionospheric Grid Point (IGP) and its confidence bounds errors (GIVE, Grid Ionospheric Vertical Error) are analyzed and compared with the ionospheric vertical delay estimated at a nearby user location over the West African Sub-Saharan region. Since African sub-Saharan ionosphere falls within the EIA region, which is always characterized by a disturbance in form of irregularities after sunset, and the disturbance is even more during the geomagnetically quiet conditions unlike middle latitudes, the need to have a reliable ionospheric threat model to cater for the nighttime ionospheric plasma irregularities for the future SBAS user is essential. The study was done during the most quiet and disturbed geomagnetic conditions on October 2013. A specific low latitude EGNOS-like algorithm, based on single thin layer model, was engaged to simulate SBAS message in the study. Our preliminary results indicate that, the estimated GIVE detects and protects a potential SBAS user against sampled ionospheric plasma irregularities over the region with a steep increment in GIVE to non-monitored after local sunset to post midnight. This corresponds to the onset of the usual ionospheric plasma irregularities in the region. The results further confirm that the effects of the geomagnetic storms on the ionosphere are not consistent in affecting GNSS applications over the region. Finally, this paper suggests further work to be investigated in order to improve the threat integrity model activity, and thereby enhance the availability of the future SBAS over African sub-Saharan region. 1. Introduction Ionosphere and its variability remain a crucial issue for single- frequency Global Navigation Satellite Systems (GNSS) users worldwide for a critical application like air and maritime navigation. The situation could be worse at the equatorial and low-latitude regions (Prasad and Sarma, 2004) where the daytime ionization of electrons distribution is modified by the fountain effect, develops a crest at around to ± 15 ± 20 o o magnetic equator depending on solar activity and season, and a trough at the magnetic equator during the late local noon. ICAO (International Civil Aviation Organization) Ionospheric Studies Task Force (ISTF/1)/1 (2012) edition of their white paper reported that GNSS implementation programs should take into account the potential limitations and disruptions of ionospheric effects to GNSS services. The white paper also stressed the fact of the existence of a peculiar ionosphere over the equatorial and low latitude regions, which could severely limit the availability of GNSS and its augmentation based Approach with Vertical guidance (APV) service in that region. Stoneback and Heelis, (2014) reported that ionosphere plasma density irregularities after sunset, and sometimes to post midnight at the equatorial region could be responsible for significant disruption to radio communication and navigation systems. In order to support and enhance the satellite-based positioning navigation system with GNSS, American and European developed GNSS augmentation systems (SBAS, Satellite-Based Augmentation System) to cater for their regions using a single-shell ionospheric grid models. These allow the GNSS single-frequency users to correct errors due to ionospheric delay over Continental United State (CONUS) and European Civil Aviation Conference (ECAC) regions. The American and European SBAS are known as WAAS (Wide Area Augmentation System) and EGNOS (European Geostationary Navigation Overlay System) respectively. These systems were designed and built to http://dx.doi.org/10.1016/j.jastp.2016.12.015 Received 20 August 2016; Received in revised form 24 December 2016; Accepted 28 December 2016 ⁎ Corresponding author at: Department of Physics, Federal University Oye-Ekiti, Oye-Ekiti, Ekiti State, Nigeria. E-mail addresses: oabe@ictp.it, oladipo.abe@fuoye.edu.ng (O.E. Abe). Journal of Atmospheric and Solar–Terrestrial Physics 154 (2017) 67–74 Available online 29 December 2016 1364-6826/ © 2016 Elsevier Ltd. All rights reserved. MARK