Most of our knowledge of the ionosphere comes from remote sensing by radio waves. One of the earliest ground-based high frequency radar devices is the ionosonde, which yields vertical electron density profiles up to but not above the altitude of the highest electron density (NmF2). Although incoherent scatter radars have the ability to make measurements up to the topside ionosphere, they are limited in number (10). To be able to specify and forecast ionospheric phenomenon very accurately, it is mandatory to have continuous and reliable observations simultaneously in thermospheric, ionospheric, mesospheric regions of the Earth’s Atmosphere. By virtue of technological advancements, it is now become possible to quickly yield 3D global coverage of Earth’s atmosphere from the surface of the Earth to 800 km with higher vertical resolutions using the GPS radio occultation (GPS RO) technique. This book focuses on global studies of ionosphere irregularities, for the first time, by using COSMIC GPS RO technique provided S4-index and discuss how ionosphere responds during quiet and geo-magnetic storm times by analyzing data provided by ground and satellite-based remote sensing techniques.