# Intersatellite and Downlink Agile Attitude Maneuvers
## 1 Introduction  
The use of large satellite constellations, placed in low Earth orbits and tailored to enhancing global connectivity, has attracted strong interest since the 90s. Some programs (Teledesic, Skybridge, Celestri) were canceled before launch and only a few systems (e.g., Iridium and Globalstar) became operational in the 90s [1; 2; 3; 4]. [PERSON] [5] and [PERSON] [6] reviewed some of these constellation architectures, while [PERSON] [7] focused on a wider class of configurations, also involving geosynchronous and medium-altitude orbits. In recent years, a renewed interest is leading some private ventures to completing the deployment of large constellations, tailored to providing global broadband coverage for high-speed internet access, especially for rural and remote areas, all around the world. Most recently, the first satellites equipped with laser technology were launched. In a recent publication, [PERSON] et al. [8] provide a comprehensive analysis of three large satellite constellations, already launched (or being delivered) by SpaceX, OneWeb, and Telesat, while identifying some major challenges related to similar systems. Collision avoidance represents one such challenge, investigated by [PERSON] et al. [9], and imposes continuous monitoring and a high degree of automation. Efficient geographical data routing is another fundamental systemic aspect, and is investigated by [PERSON] et al. [10]. Information sharing in large constellations implies the capability of performing agile attitude maneuvers, in the context of two different operational scenarios: (a) pointing toward a specific location on Earth, and (b) pointing toward a companion satellite. In both cases, the attitude control system of the satellite of interest must be able to track atime-varying pointing direction, to perform either (a) data acquisition or downlink connection, or (b) an intersatellite connection for data sharing. The two preceding operational requirements can be both formulated as attitude tracking problems. The design, implementation, and testing of algorithms for slewing or tracking maneuvers is an active research area, with the final objective of identifying suitable feedback schemes, effective and accurate for autonomous attitude control. Recent contributions highlight the growing interest toward agile attitude maneuvers for remote sensing applications [11; 12; 13; 14]. In particular, [PERSON] et al. [13] investigate autonomous guidance for slewing maneuvers, while taking actuator limitations into account. [PERSON] et al. [15] consider the effects of model fidelity and parameter uncertainty on the performance of a feedback feedforward algorithm for attitude tracking of a satellite with flexible appendages. [PERSON] and [PERSON] [16] show that, contrary to common assumptions, the available angular momentum and torque, supplied by the actuation system, impose more restrictive limits on maneuverability than the dynamics of the spacecraft, modeled as a flexible system. Different representations for attitude kinematics are available. Unlike Euler angles, Euler parameters (quaternions) are suitable for large reorientation and attitude tracking maneuvers. Recently, [PERSON] and [PERSON] [17] proposed second-order quaternionic equations for attitude kinematics and dynamics. In the scientific literature, several contributions employed the Euler parameters as the kinematics variables [18; 19; 20]. The final goal was in identifying feedback control laws that enjoy quasi-global stability properties. Their main drawback is represented by the need of accurate knowledge regarding the spacecraft mass distribution, in particular its instantaneous inertia matrix. This information may be not sufficiently accurate, and this circumstance can compromise the pointing maneuver or reduce its precision. Recently, some inertia-free algorithms were proposed that do not require any accurate knowledge of the spacecraft mass distribution [21; 22]. In particular, [PERSON] et al.