Satellite Orbits and Attitude

This chapter discusses fundamentals of orbital dynamics and provides a description of key perturbations affecting global navigation satellite system (GNSS) satellites along with their impact on the orbits. Models for perturbing accelerations including Earth gravity, third body perturbations, surface forces, and relativistic corrections are described with emphasis on empirical and semiempirical solar radiation pressuremodels. Long-termevolution of GNSS orbits and orbit keeping maneuvers are discussed. The concepts of broadcast orbit models such as almanac models, analytical ephemeris models and numerical ephemeris models used by current GNSS systems are presented along with cook book algorithms and a summary of their performance. Complementary to the discussion of GNSS satellite orbits, the chapter introduces the basic concepts of GNSS satellite attitude, which are, for example, required to describe the antenna location relative to the center-of-mass

Orbit and Clock Product Generation

Many sophisticated Global Navigation Satellite System (GNSS) applications require high-precision satellite orbit and clock products. The GNSS orbits and clocks are usually derived from the analysis of tracking data collected by a globally distributed GNSS receiver network. The estimation process adjusts parameters for the satellite orbits, transmitter and receiver clocks, station positions, tropospheric delays, Earth orientation, intersystem and interfrequency biases, and carrier-phase ambiguities. The estimation requires detailed modeling of geophysical processes, atmospheric and relativistic effects, receiver tracking modes, antenna phase centers, spacecraft properties, and attitude control algorithms. This chapter describes precise orbit and clock determination of the GNSS constellations as performed by the analysis centers of the International GNSS Service, including models, estimation strategies, products, and the combination of orbit and clock solutions