Book excerpt: This book highlights ionospheric delay investigation and forecasting using GPS-TEC measurements in the equatorial region. The ionospheric delay error is the main source of error and a major concern for GPS applications as it corrupts the positioning and time transfer results. In the equatorial region, the variations of the total electron content (TEC) and the ionospheric delay are recognized to be high when compared with other regions such as mid-latitude and high latitude. The investigation and forecasting of trans-ionospheric propagation errors are essential for precise measurement and further contribute valuable information to satellite and space probe navigation, space geodesy, radio astronomy and other applications.