The ability to determine a user's position at any time requires accurate information about the positions of GPS satellites at the moments of signal transmission. As shown in Chapter 2, the coordinates of the GPS satellites as a function of time (or ephemeris) are transmitted as part of the navigation message. In fact, satellite coordinates are represented in the broadcast ephemeris in the form of predicted orbital parameters, which can be used to estimate future positions of GPS satellites. The ability to accurately predict the satellite orbit requires comprehensive knowledge of orbital mechanics, which is presented in a simple form in this chapter. First, the motion of space objects is briefly described, followed by a brief description of the common satellite orbits. The ideal satellite orbit (so-called Keplerian orbit) is described in Section 3.3. Actual (or perturbed) satellite orbits and their parameters as included in the broadcast ephemeris are then presented. A description of the GPS almanac and its use in mission planning is presented in Section 3.6. This chapter ends with a discussion on satellite visibility and the determination of visible number of satellites using the satellite almanac.