ALBERT

Description: The Earth’s upper atmosphere – a part of it, the ionosphere- is a dynamic partly ionized region with temporal and spatial variations under different phases of solar activity. The ionosphere being a dispersive medium causes signal strength fluctuation, propagation delay, signal attenuation, and signal degradation. These have constituted significant threats to both communication and navigation systems operating in microwave band which is due to the presence of high electron density and its irregularities. The key parameter of the ionosphere which is closely related to most of these delay effects on radio signals is the electron density and density gradients, in particular - its vertical integral, the Total Electron Content (TEC) which can be estimated from the Global Positioning System (GPS) data. The estimated TEC profiles, and TEC perturbation are studied to gain insights into the occurrence of irregular structures in the ionosphere and their distribution. One of the ionospheric irregularities located within the F region, and E region top side are Traveling ionospheric disturbances (TIDs). TIDs are propagating perturbations in the ionospheric electron density as a consequence of Atmospheric Gravity Waves (AGWs) passage. The AGWs originate in the troposphere or stratosphere, and exhibit neutral wind perturbations propagating to the F region heights (i.e. ionospheric heights), where the neutral wind perturbations interact with the plasma via collisions, carrying it along the magnetic field lines (i.e. ion-neutral collision). This entire process in the ionosphere is manifested as oscillations of the ionospheric electron density, resulting in a TID. However, TIDs vary in scale sizes ranging within a few hundred kilometers (km) to over one thousand km, and based on this, they are categorized as either medium-scale TIDs (MSTIDs) or large scale TIDs (LSTIDs). In this thesis, we focus only on MSTIDs as one of the major and frequent ionospheric irregularity phenomena which may degrade positioning systems and could cause a delay in GPS signal transmission between a satellite and the GPS receiver. Multiple studies of ionospheric irregularities with the main focus on MSTIDs over different regions and continents around the world have been carried out, but studies of MSTIDs over the African region have neither been carried out nor reported probably due to lack of GPS data set, and the question of what drives its occurrence in the region which is not yet documented. The objective of this thesis is to study and describe for the first time the occurrence of MSTIDs and its characteristics over the African region under quiet geomagnetic condition (Kp ≤ 3) during the years 2008 – 2016. In addition, this thesis presents novel results of the time series of MSTIDs percentage occurrence rate (POR) during daytime and nighttime, and seasonal occurrence. Ionneutral coupling processes like the connection between AGW and MSTIDs are also discussed in the study. Observational TEC data used in this thesis are obtained from ground-based GPS networks within the African region and nearby stations. Additionally, temperature data from COSMIC radio occultation and SABER satellite observations for some case studies were used to validate AGWs passage as a driving source of MSTIDs, especially during the daytime. Consequently, regional MSTIDs distribution maps have been generated to capture the latitudinal, seasonal, and local time extent of the MSTID occurrence. Investigation of regional ionospheric irregularities over Africa (IRIA) gives a novel result of a climatological view of MSTIDs over Northern and Southern hemispheres in the African region.

Description: The ionosphere model is essential to satellite-based systems to accurately correct the ionospheric error encountered by satellite signals en route. The Levenberg–Marquardt backpropagation (LMBP) algorithm in the artificial neural network (ANN) was used in this work to predict the total electron content (TEC) within the trough of equatorial ionization anomaly (EIA) over Nigeria. Two sets of data were used over the period of three consecutive years (2011–2013) of high solar activity. The first set was used as an input to the ANN model and the second set of data was used as a target. Seventy percent of the data sets were used to train the network, 15% of the data were used for validation, and 15% used for testing. The performance of the model was assessed during specific quiet and disturbed geomagnetic conditions. The regression analysis of the model output was optimized by minimizing a cost function of the mean square error (MSE). The results of the errors, regression, and comparative analyses have revealed that the ANN model is able to predict accurate and reliable TEC that compares well with the actual experimental data at any geophysical conditions. Hence, this model would be useful to forecast TEC over Nigeria to a reliable threshold.

Description: During the recent two decades ground- and satellite-based GNSS remote sensing (GNSS-RS) methods evolved into a ver-satile and powerful tool for Earth system research on dif-ferent spatio-temporal scales with operational applications. Large regional and global GNSS ground networks and numer-ous space-borne receivers provide unique neutral atmospher-ic/ionospheric data and observations of the Earth’s surface. One of the pioneering institutions for these developments is the German Research Centre for Geosciences GFZ at Potsdam. We briefly review recent GFZ developments in GNSS remote sensing.

Description: We present for the first time the climatology of medium-scale traveling ionospheric disturbances (MSTIDs) by using Global Positioning System (GPS) receiver networks on geomagnetically quiet days (Kp ≤ 3) over the North African region during 2008–2016. The ionospheric Total Electron Content (TEC) were estimated from the dual-frequency GPS measurements, and the TEC perturbations (dTEC) data were derived from the estimated TEC data. We focused on the TEC perturbations (dTEC) associated MSTIDs and statistically analyzed its characteristics, occurrence rate, diurnal and seasonal behavior as well as the interannual dependence. The results show that MSTID is a local and seasonal dependence. The result reveals that occurrence of MSTIDs increases with solar activity. It also shows that MSTIDs predominantly propagates towards the South (equatorward). The MSTIDs event period is (12 ≤ period ≤ 53 min), while the dominant peak-to-peak amplitude is (0.08 ≤ amp ≤  ~ 1.5 dTECU). The study also shows that the amplitude of MSTIDs is higher at the northwest (Lat: ~ 32° N to ~ 38° N, Long: ~ 2° W to ~ 15° W) when compared with northeast (Lat: ~ 28° N to ~ 38° N, Long: ~ 23° E to ~ 40° E), and the disturbance occurrence time is more frequent within the hours of (1200–1600 LT), and (1000—1400 LT) in December solstice at daytime for stations located in the northwest and northeast part of the African region, respectively. While at the nighttime, the MSTIDs also exhibits variability in disturbance occurrence time around (northwest: 2100–0200 LT) and (northeast: 1900-0200 LT) in June solstice, but get extended to March equinox during solar maximum (2014). The mean phase velocity in daytime MSTIDs is higher than the nighttime in every season, except during June solstice.

Description: This paper examines the accuracy of the Global Positioning System (GPS) within the African low-latitude sector. We evaluated the vertical and horizontal of the single-point positioning (SPP) accuracy of GPS and critically analysed the effects of the equatorial ionospheric anomaly (EIA) over the region of study. Our results imply that using single-frequency GPS for any application could give a positioning error up to 45.00 m vertically and ~25.00 m horizontally. The study revealed that 54% of GNSS positioning errors during the night-time could be linked to ionospheric plasma irregularities. Also, positioning errors are higher in western Africa than in the eastern region. The influence of geomagnetic activities is not consistent with GPS positioning accuracy. However, positioning errors are lower during geomagnetically disturbed conditions in comparison to quiet conditions. The unique phenomena of the EIA can severely limit GPS services at night-time for positioning in the study area.