This book presents the study of relationship between the Total Electron Content (TEC) derived from a Global Positioning System (GPS) receiver and the critical frequency of the F2-layer of the ionosphere in the Equatorial Ionization Anomaly (EIA) region. A practical problem of devising a method to use GPS TEC as a proxy of foF2 was considered for Vanimo, (Papua New Guinea). The study was carried out successfully for southern and northern hemisphere EIA stations. The International Reference Ionosphere (IRI) and electron density profiler NeQuick models were used and the observations were...