Leonard Felicetti

Adaptive Thrust Vector Control during On-Orbit Servicing

On-orbit servicing missions often include a final propulsive phase where a spacecraft pushes the other one towards a different orbit. Specifically this is the case of the debris grasping mission where the chaser, after capturing the target by means of robotic arms, has to perform a de-orbit operation. The large thrust involved needs a perfect alignment with respect to the center of mass or the system composed by chaser and target, in order to avoid attitude changes. Such accurate alignment is quite difficult to achieve especially when the characteristics of the target are not perfectly known. A procedure is proposed in this paper, allowing a complete estimation of the center of mass position and of the moments of inertia of the system, starting from the data obtained by the gyros mounted on board of the spacecraft. The output is used to design a maneuver for correcting the target and chaser relative position by moving the robotic arms. Numerical simulations show the proficiency and the applicability of the estimation algorithm and of re-alignment maneuver to a selected mission scenario.

Evaluation of control strategies for spacecraft electrostatic formation keeping

The adoption of electrostatic (Coulomb) forces to acquire and maintain the relative configuration in a spacecraft formation is a topic of significant current research interest. Recent technological advances allow the independent charging of each platform enabling the control of their relative position by means of attractive and repulsive forces. This technique could offer high precision, high equivalent specific impulse, and long operational life time. In real space applications, the effectiveness of the electrostatic force should be limited by the plasma shielding effect. The commanded separation among the spacecraft is ruled by the Debye length parameter, which is larger at higher orbital altitudes. MEOs and GEOs are therefore the preferred scenarios for this control technique. Open-loop electrostatically controlled formations should be dynamically unstable, and a feedback control law is needed to stabilize their motion. Indeed, this paper proposes a comparison among possible different strategies to implement this technique. Due to the non-linearity of the governing equations of motion, the problem needs to be suitably formulated to allow the application of some traditional control laws. The classical proportional derivative technique, as well the optimal LQR approach are considered, together with a Lyapunov-based strategy. The more recent State Dependent Riccati Equation (SDRE) control approach, especially interesting for non-linear system, is also applied. The findings of numerical simulations relevant to a small spacecraft cluster in GEO are discussed in depth.

Attitude coordination strategies in satellite constellations and formation flying

The coordination of the attitude among different spacecraft belonging to a multiple platform system (formation or constellation) is a basic requirement in several missions, mainly the ones involving sensors like radars or optical interferometers. It is also an open topic in research, above all as it matches the characteristics of the current trend towards interoperability and federated systems. Different approaches are possible to define and chase such a coordinated attitude. The classic control strategy is the so-called leader-follower architecture, where all spacecraft depend on (follow) the behavior of a single master. Alternatively, the behavioral approach involves a continuous re-selection of the desired target configuration which is computed on the basis of the behavior of all the platforms. A third possibility is to define a virtual architecture, especially suitable with respect to the mission requirements, which is not dependent on the current kinematic state of the platforms. The paper proposes a unified treatment of these concepts by using some fundamental definitions of the consensus dynamics and cooperative control. The convergence to the targeted configuration is addressed both analytically, by using Lyapunov stability criteria, and numerically, by means of numerical simulations. The attitude requirements and constraints are highlighted and a solution for the control algorithm - involving continuous actuators on each platform - is developed. A comparative analysis of different optimal control strategies, the Linear Quadratic Regulation (LQR) and the State Dependent Riccati Equation (SDRE) - suitably modified to address the needs of coordination - is presented. The results show the general value of the proposed approach with respect to either linear or nonlinear models of the dynamics.

Coordinated Attitude Control for Multiple Heterogeneous Satellites Missions

The paper investigates cooperative control strategi es for spacecraft formations, also in case platforms are not homogeneous but differs in a ttitude control actuators. Specifically, either a common inertial or a time-varying pointing are considered as requirements for a formation of spacecraft, controlled either entirely by reaction wheels or partly by wheels and partly by thrusters. Two control strategies, namely the classical leader-follower or a more cooperative one, also labelled as behavioural based , where the kinematic state of each spacecraft is known to the others and enters in the ir command loop, are applied. In order to actually compute the actions, two controllers are c onsidered: a classical proportionalderivative (PD) and an optimal one using the variab le gain state dependent Riccati equation (SDRE). Numerical simulations to validate the approach are presented and, within this implementation, SDRE approach shows to succeed even in cases when PD fails.

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 semimajor axis over an extended period of time. This solution can be implemented by adopting a spinning quasi-rhombic pyramidal solar sail that 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 5000xa0km. Numerical results characterize the expected performance, leading to (for example) an increase of...

Modeling the formationkeeping control with multibody codes

Formation Flying control involves the computation of relative kinematics and dynamics among a number of orbiting platforms. Formations are not the only space application in which several components operate coordinately at the same time. Multibody is the scheme usually adopted to model the robotic arms of the space manipulators or large space platforms as the International Space station, and multibody can be also seen as a set of components orbiting together. A number of software codes have been developed during the years to represent and simulate this scheme, taking into account the differential forces acting on each member. This paper proposes to build on this effort to test a different way for evaluating the control of spacecraft formations. The formation spacecraft will be represented by the joints of the multibody. The links, represented as structural element with infinite stiffness, virtually reproduce the relative constraints in position and attitude among the platforms. The idea is to consider the orientation and the length of the links such that the joints (spacecraft) will actually assume the relative geometry which is the desired state at a given time. The forces and torques to be provided to the real spacecraft belonging to the formation are related to the reaction torques and forces which are provided at the joints in the corresponding multibody representation. These reactions can be easily computed by available multibody codes, and the values found can be applied to a standard orbital propagator to compute the dynamical behavior and to validate the approach. The advantage stays with the quick, easy computation of the inverse kinematics, which is routinely performed by multibody software. The solution should be useful to both the cases of keeping an already acquired configuration, like large distributed antennas virtually built by several spacecraft, as well as to the rigid reorientation of a formation, like in some astronomical missions.

Spacecraft formation for debris surveillance

This paper explores the viability and performance of a new algorithm for in-orbit space debris surveillance, which utilizes a network of distributed optical sensors carried onboard multiple spacecraft flying in formation. The resulting network of spacecraft is able to autonomously detect unknown debris, as well as track the existing ones, estimate their trajectories, and send the estimation results directly to the mission control centers for planning the required collision avoidance maneuvers. The proposed concept includes (a) an estimation algorithm that allows for sharing observations of common debris objects among spacecraft; (b) a coordination algorithm for the re-orientation of an ad hoc team of spacecraft to align their onboard optical sensors towards common targets; and (c) a control algorithm for the detection and tracking of the debris which uses vision-based attitude maneuvers.

Attitude Dynamics and Control of Drag-Balance CubeSats

The article examines the system stability, obtaining upper bounds for the mass unbalance to verify the applicability of magnetic control laws valid for rigid bodies to the drag-balance CubeSat two- ...

Space webs dynamics and configuration control by means of reaction wheels

Space webs are large, deployable systems composed by several spacecraft connected by tethers. Their dynamics is commanded by the directional effects of the gravity gradient on the specific configur ...