This comprehensive volume  navigates through the complexities of Earth's ionosphere and its intricate relationship with GNSS spread-spectrum signals. The book delineates the nuanced structures and dynamics of Earth's atmosphere, offering profound insights into signal propagation, including a meticulous analysis of various ionospheric models.   Delving into the instruments of choice, it encapsulates the facets of the GNSS space segment, its theoretical background, and practical applications. The ground and user segments discussed provide readers with a deep understanding of satellite dynamics and navigation data.   Further chapters explore the intricacies of GNSS Software Defined Radio (SDR), offering a detailed examination of signal acquisition and tracking, and presenting innovative approaches to baseband data application without data decoding. A significant portion is devoted to manufacturing front-end hardware, providing guidance from specifications to testing, enabling readers to navigate the world of hardware design with ease.   Lastly, the book ventures into the applications of GNSS receivers for ionospheric study, highlighting the construction of advanced observables and their practical applications in navigation processors. This section also elucidates the role of multi-station observables in extracting critical ionospheric information.   A pivotal resource for researchers and professionals alike, this book stands as a testament to comprehensive research and innovation in the field of GNSS and SDR, fostering a deeper understanding of the Earth's ionosphere.   This book serves as a natural progression from Dr.Petrocski's prior contributions, offering readers actionable insights grounded in theoretical understanding.