# TEC and ROTI Measurements from a New GPS Receiver at BOWEN University, Nigeria
## 3 Methodology  
Data received from GPS satellites have been very useful in TEC estimation across the Earth. In an effort to understand ionospheric irregularities at the equatorial region, a new Connected Autonomous Space Environment Sensor (CASES) GPS scintillation monitoring receiver was installed at Bowen University, Iwo, Osun State, Nigeria, in June 2019. This is in collaboration with the Center for Space Science and Engineering Research (Space@VT) at Virginia Polytechnic Institute and State University, Blacksburg, VA, USA. The objective of the collaboration involves a detailed investigation of the characteristics of Nigeria's equatorial ionosphere. With these initiatives, more GPS receivers have been made available for space weather observations in lieu of the very expensive commercial brands. The CASES GPS receiver is set to operate at L1 (1575.42 MHz) and L2 (1227.60 MHz) band frequencies. It can measure the ionospheric irregularities or electron density fluctuations in the ionosphere with varying sampling rate schemes every three minutes. Figure 3 shows the data processing chain using the CASES GPS receiver. The Grid client software enables the data that are in binary digits to be logged into the local machine. For data post-processing, the method described by [16] is used to remove the receiver clock effects and eliminate low-frequency effects, including tropospheric effects, satellite motion, and the multipath passed through a high pass filter. To eliminate the effect of multipath, a minimum elevation angle of 20\({}^{\circ}\) was used. The VTEC data estimated are then subjected to a two-sigma (2 s) iteration, which is a measure of GPS point positioning accuracy (95% confidence level). This 95% confidence level corresponds to 1.96 standard deviations, which is then approximate to two standard deviations or 2 s, and the resulting values are the average of VTEC over all pseudorandom numbers (PRNs) on a day. Therefore, the data used for this investigation were collected from the newly installed Connected Autonomous Space Environment Sensor (CASES) GPS receiver for the period of January-December 2021 with the following coordinates: Latitude: 7.62\({}^{\circ}\) N, Longitude: 4.14\({}^{\circ}\) E, Diplat: 2.36\({}^{\circ}\) S. In addition, the GPS-TEC data for station BJCO in Cotonou, Benin Republic (Latitude: 6.38\({}^{\circ}\) N, Longitude: 2.45\({}^{\circ}\) E, Diplat: 3.08\({}^{\circ}\) S) were also retrieved for the period of January-December 2010 from [[https://www.sonel.org/](https://www.sonel.org/)]([https://www.sonel.org/](https://www.sonel.org/)) (accessed on 22 January 2023). There are no data for BJCO in the year 2021. Hence, the year 2010, a low-solar-activity period with almost the same solar flux, 80 fsu compared to -82 fsu in 2021, was chosen. The 3 min interval logged binary digits data from the CASES GPS receiver were converted to rinex file (Receiver Independent Exchange) and log file data formats using Binflate exe from [[http://cases.astraspace.net/software/](http://cases.astraspace.net/software/)]([http://cases.astraspace.net/software/](http://cases.astraspace.net/software/)) (accessed on 22 January 2023). However, the rinex files from both the CASES GPS receiver and from the GNSS station were converted to vertical TEC (VTEC) after removing the biases from the slant TEC (STEC), as shown below. This was made possible by using the GPS TEC software developed by [PERSON] (Global Positioning System total electron content analysis application user's manual, 2009, Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA), which read the raw data from the rinex file and the navigation file of the observation data and satellite biases from the International GNSS Service (IGS) to compute the VTEC. The output from the software is the average of VTEC over all pseudorandom numbers (PRNs) on a day, which is used to analyze the diurnal and monthly TEC variations.