Ionospheric irregularities are a significant source of error in GNSS positioning and navigation as they cause scintillations in trans-ionospheric signals. The elliptical orbit and the high-rate GPS and in-situ irregularity measurements of the Swarm Echo satellite provide a unique opportunity to examine characteristics and distribution of scintillation-causing irregularities. To obtain the results presented in this research, the satellite was re-oriented in 293 sets of novel experiments to make the GAP-O antenna point in the zenith direction. The surface current measurements of the IRM instrument were employed as a measure for in-situ irregularities. In the present thesis, I carried out three studies to investigate distribution, morphology, and scintillation impacts of irregularities that form in the equatorial topside ionosphere during post-sunset hours. The results of the first study show that large-scale ionospheric structures (larger than 80 km) form predominantly below 500 km between 18 and 21 MLT, whereas small-scale irregularities (as small as 160 m) occur at all examined altitudes (330-1280 km) and post-sunset hours (18-24 MLT). The second study presents a novel approach to generate regional maps of small-scale scintillation-producing irregularities using single-satellite observations. I demonstrate that these maps, which are produced using different ionospheric GPS indices, including S4, TEC, and ROTI, can determine the horizontal geo-locations of small-scale irregularities, albeit with large uncertainties in the cross-track direction. The third study presents the spectral characteristics of simultaneous in-situ irregularities and GPS signal intensities. The PSDs of in-situ irregularities show one-component power law behavior, with the most common irregularity spectral index values between 1 and 2. The spectral index for stronger irregularities approaches 1.7. The GPS signal intensity PSDs also obey a power law, with spectral index values within the range of 1.8 and 2.2. Furthermore, the roll-off frequencies obtained from the intensity PSDs are between 0.4 and 2.5 Hz, Which is considerably higher than Fresnel frequencies estimated from ground-based GPS measurements at low latitudes, which fall between 0.2 and 0.45 Hz.