# TEC and ROTI Measurements from a New GPS Receiver at BOWEN University, Nigeria
## 1 Introduction  
The ionosphere is regarded as the part of the atmosphere with the maximum concentration of ions and electrons that lies between 80 km and 300 km altitudes. It is then defined as the region in the Earth's atmosphere with the largest number of free electrons and ions that have the ability to affect radio waves. The ionosphere is highly variable in space and time (sunspot cycle, seasonal, and diurnal periods), also depending on geographical location (polar, aurora zones, mid-latitudes, and equatorial regions) and on certain solar-related ionospheric disturbances. Ionosphere research has attracted significant attention from the Global Positioning System (GPS) community because the ionosphere's range delay on GPS signals is a major error source in GPS positioning and navigation. The ionosphere has practical importance in GPS applications because it influences trans-ionospheric radio wave propagation. The ionosphere causes GPS signal delays to be proportional to thetotal electron content (TEC) along the path from the GPS satellite to a receiver. The TEC is defined by the integral of electron density in a 1 m squared column along the signal transmission path. The TEC is a key parameter in the mitigation of ionospheric effects on the radio system. The TEC measurements obtained from dual-frequency GPS receivers are one of the most important methods for investigating the Earth's ionosphere [1]. In addition, with the random fluctuations in the amplitude and phase of trans-ionospheric radio signals, ionospheric plasma irregularities have become a continuous and major concern for satellite communication systems. Previous studies have shown that ionospheric scintillations are most likely to occur near the magnetic equator in the post-sunset period [2; 3; 4; 5; 6]. These irregularities are caused by plasma instability typically formed at the bottom of the F-region during the post-sunset period due to the Rayleigh-Taylor (R-T) instability mechanism. Post-sunset, the absence of solar radiation causes the lower ionosphere to rapidly decay, thereby increasing the bottom side of the F-layer. This is synonymous with a heavy fluid being situated above a light fluid [7]. Such a contrast in density leads to the mixing of the two layers by forming bubbles and spikes. As the bubbles develop, there is a rapid local variation in the refractive index of the Earth's ionosphere through which radio signals traverse. The effect of these density abnormalities manifests as signal amplitude and/or phase fluctuations called ionospheric scintillation [3]. These fluctuations cause significant degradation of trans-ionospheric signals, resulting in signal fading below the fade margin of the receiver, and could lead to signal loss and cycle slips [4; 8]. However, these effects have been investigated using GPS receivers for many years [4; 5; 9; 10; 11]. For instance, [2] investigated the irregularities in Lagos state, Nigeria, using GPS observations during the 2009-2011 period. They reported that the equinoctial months recorded the highest occurrence of irregularities, while the lowest was recorded during the June solstices. In addition, [12] studied the trend of the ionospheric irregularities in Africa using ROTI derived from Global Navigation Satellite System (GNSS) data. They found that irregularities occurred from March to November, with a minimum observed around June and very low activity around January. Despite the various studies conducted in Africa, particularly in the equatorial region where Nigeria is located, no GPS receivers have been operational in Nigeria since the year 2018. As a result of this, the characteristics of the varying ionosphere over Nigeria are not known. This led to the installation of a Connected Autonomous Space Environment Sensor (CASES) GPS receiver at Bowen University in Iwo, Osun State, Nigeria.