This book constitutes an excellent source for understanding the basic operation of the Global Positioning Systems (GPS). It also allows the reader to take the first steps towards understanding how the Kalman Filter is implemented in the estimation of drifting parameters of the GPS. The "Kalman Filter Basics" chapter is preceded by a descr iption of the different errors that affect the GPS signals. Furthermore, this book includes sections on the Wide-Area Augmentation System, Inertial Navigation, and Differential GPS, which enables the understanding of these concepts and thus, provides the reader an overall perspective of the Global Positioning System.