This reference and handbook describes the functional software library of the Global Positioning System (GPS/Galileo). It is the original software source-code of many functions, models, algorithms and tools described in the textbook "GPS – Theory, Algorithms and Applications" written by the same author published by Springer. The software functions are parts of the KSGsoft and MFGsoft programs developed by the author at the GFZ in Potsdam. All the source-code are verified by real GPS data processing applications. The software uses the books "GPS – Theory, Algorithms and Applications" as its theoretical description. The functional library covers physical models, disturbance models, mathematical methods, adjustment and filtering algorithms, the transformation, interpolation, integration methods, and numerous tools.