This reference and handbook describes Global Positioning System (GPS) theory, algorithms and applications. It is primarily based upon source-code descriptions of the KSGSoft program developed by author at the GFZ in Potsdam. The theory and algorithms are revised and extended for a new development of a multiple functional GPS software. New concepts such as the unified GPS data processing method and ambiguity-ionospheric algorithm, as well as general ambiguity search criteria, are reported for the first time. Mathematically rigorous, the book begins with the basics of coordinate and time systems and satellite orbits, as well as GPS observables, and deals with topics such as physical influences, observation equations, adjustment and filtering, ambiguity resolution, data processing, kinematic positioning, and the determination of perturbed orbits.