TABLE OF CONTENTS Low-cost autonomous navigation, onboard maneuver planning, and autonomous constellation control become feasible when a GNSS is employed, leading to a substantial reduction in mission operation costs, mostly related to the labor intensive overwhelming also in terms of ground personnel requirements planning and control of a single spacecraft or a constellation from the ground [109]. Installing a GNSS receiver onboard a spacecraft provides the opportunity to use a single lightweight, low-cost sensor for a multitude of functions: position, velocity, attitude, attitude rate, and time, resulting in reduced cost, power mass, and complexity of the spacecraft. In addition, the replacement of many different sensor devices and their interfaces with a single one would yield a noticeable improvement in system reliability.
The nonspherical shape of the Earth and the nonhomogeneous distribution of its inner mass determine precession and nutation of the satellite orbital plane. These effects can be counterbalanced through periodical corrections on the motions around three satellite axes. As a consequence, the knowledge of the satellite attitude, which is determined by the angles between the satellite body mechanical axes and three reference axes (roll, pitch, and yaw), becomes of paramount importance. All spacecraft but the simplest ones adopt some means of active attitude control, using actuators such as control momentum gyros, reaction wheels, offset thrusters, and magnetic torque rods. Attitude control is mostly performed by an onboard closed-loop system, in particular in case of LEO spacecraft where daily ground contacts are very restricted in time. Only unusual events, such as...
