Charging strategies for electrostatic control of spacecraft formations

Formation control by means of electrostatic forces, generating attractive or repulsive actions by charging the satellites’ surfaces, has been recently proposed for high altitude orbits to precisely maintain the configuration without risk of plume impingement. This paper focuses on electrostatic control and switching strategies for charge distribution in spacecraft formations, taking into account the limits on the power requirements. Two nonlinear global control approaches are presented and applied to two and three satellites’ formations. Then, an optimized charge distribution process among the satellites is discussed and applied to the three spacecraft formation case. Numerical simulations are performed in order to evaluate the advantages and drawbacks of this configuration control technique

Attitude stability and altitude control of a variable-geometry Earth-orbiting solar sail

A variable-geometry solar sail for on-orbit altitude control is investigated. It is shown that, by adjusting the effective area of the sail at favorable times, it is possible to influence the length of the semi-major axis over an extended period of time. This solution can be implemented by adopting a spinning quasi-rhombic pyramidal solar sail which provides the heliostability needed to maintain a passive “sun-pointing” attitude and the freedom to modify the shape of the sail at any time. In particular, this paper investigates the variable-geometry concept through both theoretical and numerical analyses. Stability bounds on the sail design are calculated by means of a first-order analysis, producing conditions on the opening angles of the sail, while gravity gradient torques and solar eclipses are introduced to test the robustness of the concept. The concept targets equatorial orbits above approximately 5,000 km. Numerical results characterize the expected performance, leading to (for example) an increase of 2,200 km per year for a small device at GEO

Attitude and Orbital Dynamics of a Variable-Geometry, Spinning Solar Sail in Earth Orbit

At the ISSS 2013, a novel concept of variable-geometry solar sail was introduced: deployed in the shape of a three-dimensional quasi-rhombic pyramid (QRP), the sail exploited its shape and shift between center of mass and center of pressure to naturally achieve heliostability (stable sun-pointing) throughout the mission. In addition, mechanisms allowed to vary the flare angle of the four booms in opposite pairs, thus allowing to control the area exposed to the sun without the need of slew maneuvers. Using these adjustments in favorable orbital positions, it is possible to build a regular pattern of acceleration to achieve orbit raising or lowering without the need of propulsion system or attitude control. Subsequent more detailed investigations revealed that eclipses, even if lasting only a fraction of the orbit, have a substantial (and negative) impact on the heliostability effect: and even a small residual angular velocity, or disturbance torque, are enough to cause the spacecraft to tumble. In this work, we present a novel and improved concept which allows the sail to preserve its attitude not only with eclipses, but also in presence of disturbance torques such as the gravity gradient. The solution we propose is to add a moderate spin to the solar sail, combined with ring dampers. The gyroscopic stiffness due to the spin guarantees stability during the transient periods of the eclipses, while the heliostability effect, combined with the dampers, cancels any residual unwanted oscillation during the parts of the orbit exposed to the sun, and at the same time guarantees continuous sun-pointing as the apparent direction of the sun rotates throughout the year. Both theoretical and numerical analyses are performed. First, stability bounds on the sail design are calculated, obtaining conditions on the flare angles of the sail, in the different orbital regimes, to test the robustness of the concept. Then, a numerical analysis is performed to validate the study in a simulated scenario where all perturbations are considered, over extended amount of time. The concept targets equatorial orbits above approximately 5,000 km. Results show that an increase of 2,200 km per year for a small device at GEO can be achieved with a CubeSat-sized sail

Image-Based Visual Servoing Control for Spacecraft Formation Flying

This paper proposes an image-based visual-servoing algorithm that allows for optimal formation control. The proposed distributed controller utilizes visual features of other team members, retrieved from images captured by onboard cameras, to autonomously plan and perform formation acquisition, keeping or reconfiguration maneuvers. The problems of minimization of the control effort is analyzed and the paper proposes an optimal framework for developing controllers that address the issue. The viability of such a technique is explored through numerical simulations

Dinamica, guida e controllo di sistemi spaziali multipiattaforma

La tesi è organizzata in tre parti, che rappresentano essenzialmente i tre campi in cui si è suddivisa l’attività, portando il candidato ad affrontare problemi di natura diversa pur mantenendo un filo logico nella ricerca. La prima parte descrive e studia la dinamica dei sistemi multibody propriamente detti, cioè vere e proprie catene cinematiche che presentino un grado di vincolo “rigido” che si trovano ad operare in un ambiente non usuale come quello spaziale. Questo è tipicamente il caso di manipolatori spaziali montati a bordo di piattaforme orbitanti atti ad effettuare manovre di grasping di altri oggetti in orbita, ma anche, ad esempio, dei sistemi di dispiegamento dei pannelli solari sui satelliti. Successivamente si è rilassato il grado di vincolo riducendolo da rigido ad elastico e studiando la dinamica delle “Space Webs”, cioè sistemi formati da più piattaforme poste a grande distanza reciproca ma allo stesso tempo interconnesse tra loro mediante l’utilizzo di cavi. Infine nella terza parte si giunge alla completa eliminazione del vincolo materiale tra le piattaforme e quindi allo studio delle formazioni di satelliti, siano essi considerati come punti materiali (ed analizzandone la sola posizione) o come corpi di dimensioni finite (e considerandone quindi anche l’assetto relativo). La ricerca in questo caso ha riguardato lo studio di leggi di guida e controllo atte proprio a ridefinire un vincolo di tipo virtuale tra le piattaforme stesse, eseguendo manovre di riconfigurazione della posizione relativa e riorientamento dell’assetto come se appartenessero ad un unico sistema multibody

A ROS/Gazebo-based framework for simulation and control of on-orbit robotic systems

The use of simulation tools such as ROS/Gazebo is currently common practice for testing and developing control algorithms for typical ground-based robotic systems but still is not commonly accepted within the space community. Numerous studies in this field use ad-hoc built, but not standardized, not open-source, and, sometimes, not verified tools that complicate, rather than promote, the development and realization of versatile robotic systems and algorithms for space robotics. This paper proposes an open-source solution for space robotics simulations called OnOrbitROS. This paper presents a description of the architecture, the different software modules, and the simulation possibilities of OnOrbitROS. It shows the key features of the developed tool, with a particular focus on the customization of the simulations and eventual possibilities of further expansion of the tool. In order to show these capabilities, a computed torque-based controller for the guidance of a free-floating manipulator is proposed and simulated using the ROS/Gazebo-based framework described in the paper

Atmospheric effects on testing and calibrating star tracking algorithms

Star trackers are usually considered to be the most accurate sensors, able to achieve a sub-arcminute precision. Star tracker algorithms are often tested and validated with simulated space views. Testing the algorithms with real space images is expensive as it requires implementing them on existing in-space star trackers, or launching new satellites. This study shows that those algorithms are usually performing poorly with groundbased sky pictures and that some adaptations are necessary to take into account the atmospheric effects. In order to tackle this issue, this study will start by implementing and testing two published Lost-In-Space algorithms with a simulated sensor to compare their performance against various noise sources. After comparing the space-based generated views with groundbased images, an adaptation for the aforementioned algorithms is proposed. In order to counter the effect of atmospheric extinction, the number of stars visible in the image is increased by modifying the field-of-view of the camera, the exposure time and estimating the experimental inter-star angular distance error. The idea is to match the star density used in the state-of-the-art algorithms in the experimental pictures. The modified algorithms are tested with the experimental images, and the adaptation process is validated with a good success rate

A mission architecture and systems level design of navigation, robotics and grappling hardware for an on-orbit servicing spacecraft

On-orbit servicing (OOS) includes a range of servicing types that increase the lifetime of a satellite and its performance, as well as ensuring that it does not contribute to the growing issue of space debris. The avoidance of satellites becoming derelict is particularly important given the rise of ‘mega-constellations’. With the first cases of it in the 1970s, OOS has been achieved many times using crewed missions and robots controlled from the ground or by astronauts, for example during repairs and upgrades to the Hubble Space Telescope (HST) and on the International Space Station (ISS). This has allowed various space agencies and other organisations to mature processes and tools for several OOS mission types. The Northrop Grumman Mission Extension Vehicle-1’s (MEV-1) success servicing Intelsat 901 in early 2020 demonstrated that OOS is now viable from a commercial as well as technical standpoint. However, due to low technology maturity, autonomous rendezvous and proximity operations (RPO) and servicing remain challenging, despite autonomous rendezvous and docking with space stations having been demonstrated many times. This report will investigate the current state of the art in OOS and which technologies require further development to enable widespread adoption of OOS. A mission architecture to support OOS of satellites in the highest populated orbits will be described. Using this architecture, the report will focus on the selection of hardware required for guidance, navigation and control (GNC), for relative navigation towards and docking with the target satellite and of robotics to service the target. The report will use the design of the OneWeb satellites as a baseline for the target spacecraft but will also show how the servicing spacecraft’s services could be applied to a range of orbits and target spacecraf

Calibration and testing strategies to correct atmospheric effects on star tracking algorithms

Star trackers are usually considered to be the most accurate sensors, able to achieve a sub-arcminute precision. Star tracker algorithms are often tested and validated with simulated space views. Testing the algorithms with real space images is expensive as it requires implementing them on existing in-space star trackers, or to launch new satellites. This study shows that those algorithms are usually performing poorly with ground-based sky pictures and that some adaptations are necessary to take into account the atmospheric effects. The adaptation of star tracking algorithms to ground pictures could ease the prototyping phase for new star trackers, or for ground-based and air-borne star trackers, without the need to buy specific testing simulators. In order to tackle this issue, this study will start by implementing and testing two published Lost-In-Space algorithms with a simulated sensor to compare their performance against various noise sources. After comparing the space-based generated views with ground-based images, an adaptation for the aforementioned algorithms is proposed. In order to counter the effect of atmospheric extinction, the number of stars visible in the image is increased by modifying the field-of-view of the camera, the exposure time and estimating the experimental inter-star angular distance error. The idea is to match the star density used in the state-of-the-art algorithms in the experimental pictures. The modified algorithms are tested with the experimental images, and the adaptation process is validated with a good success rate

Design of an optical system for a Multi-CubeSats debris surveillance mission

The detection and observation of space debris in Low Earth Orbit is generally carried out through the use of ground based radars and telescopes. These instruments allow for a precise reconstruction of the space debris trajectories, and therefore represent a key asset for planning avoidance maneuvers when threats of collisions are predicted. The recent deployment of mega-constellations, with the consequent increase of the number of satellites, imposes new challenges in terms of simultaneous tracking capability and readiness of the current space situational awareness systems. This adds to the current need to track small and dull objects to further mitigate the probability of triggering cascade collisions. However, ground based observations are limited due to their intrinsic sensibility to atmospheric refraction, their diurnal inoperability and their dependence on meteorological hazards. This paper proposes to study the feasibility and the benefits of a potential deployment of a constellation of CubeSats in Low Earth Orbit, to acquire optical observations of space debris with enhanced accuracy, as part of the ORCA mission: Orbit Refinement for Collision Avoidance. Here, the focus is on the optical design of the payload instrument to be integrated onboard of the orbiting platforms. The study trades-off the current state-of-art of optical detection technologies, by assessing their performance against a set of specific requirements: (a) the minimization of the uncertainty associated to the image resolution; (b) a field of view that maximizes the extent of the monitored area; (c) an optimal exposure time to avoid under or overexposure of the image; (d) minimization of the effects of light diffraction and above all, (e) the maximization of the signal to noise ratio to detect the smallest and dullest objects possible. Several configurations of optical systems are then chosen as suitable for the ORCA constellation, also considering the system design implications of its integration into a CubeSat, such as size requirements. Commercial Off-The-Shelf hardware are explored and performances of the optical system are evaluated through numerical simulations in order to estimate the detectable sizes of space debris while taking into account their potential distances from the sensor. This paper concludes with estimates of the impact of the ORCA mission on space situational awareness for decades to come