Chapter 3 is dedicated to details about the space segment of a satellite navigation system. Starting from the basic Kepler's law of planetary motion, this chapter brings insight step by step into the dynamics of the satellite. This chapter explains the fundamentals of satellite dynamics and related physics to illustrate the origin and nature of different dynamic parameters. Then, it describes important orbital parameters and explains how to estimate satellite positions from them. Finally, a few important orbits are discussed and the rationale for the selection of parameters to design of a satellite navigation system is explained.