Book excerpt: "Knowledge of your current location is now taken for granted by people worldwide. This state of affairs is largely due to the advent of satellite-based navigation systems, particularly the Global Positioning System (GPS). These Global Navigation Satellite Systems (GNSS) are still rapidly evolving with more capability and even greater robustness. The fundamental purpose of all of them is determining location in four dimensions--3 of geographical positions plus time. A user is indifferent to the source of location knowledge--any technique will do if it is reliable. While much of this book is devoted to satellite-based navigation systems, or SatNavs, we intend to give full explanations of virtually all modern sources of position, navigation, and time (PNT)"--