This book focuses on a specific space application of the satellite-based navigation Global Positioning System (GPS), namely its use for the precise relative positioning and navigation of formation flying satellites. Satellite formation flying is the concept that multiple satellites can work together in a group to accomplish the objective of one larger, usually more expensive, satellite. Coordinating smaller satellites has many benefits over single satellites including simpler designs, faster build times, cheaper replacement creating higher redundancy, unprecedented high resolution, and the ability to view research targets from multiple angles or at multiple times. Nowadays satellites may arrive at and maintain formations with fast response time and have the ability to change the formation for varied resolution of observations. This book describes the most recent GPS Data Processing Techniques and Algorithms used for the estimation of the initial carrier phase ambiguities with thehighest level of accuracy possible for the determination nearly in real-time of the relative baseline between two GPS Receivers installed on-board two Low Earth Orbit (LEO) satellites in formation flying.