Giovanni B. Palmerini

Visual techniques applied to the ATV/1SS rendezvous monitoring

The proposed paper deals with an automatic trajectory monitoring system designed for the rendezvous between the automatic transfer vehicle (ATV) and the Iautononternational Space Station (ISS). During final approach phase, a TV camera on the ISS currently provides images of ATV visual targets to be used by ISS crew for visual monitoring. The proposed monitoring system, based on a dedicated image-processing algorithm applied to these TV images of the approach, is intended to autonomously and automatically determine relative ATV-ISS position and attitude. Artificial intelligence techniques for edge detection, Hough transform and pattern matching are used to implement a recognition algorithm, able to fast and accurately indicate ATV visual targets position and orientation with respect to ISS. Those values are then processed in order to calculate the full set of relative ATV navigation data. Finally, an estimation of the relative trajectory evolution via a classical astrodynamic tool as the Euler-Hill relative motion equations follows, enabling the crew and the control centre to verify that trajectory corridors and attitude constraints are respected. According to ATV mission constraints, which require using existing sensors (i.e. ATV visual cues and ISS TV camera), the performances and the accuracy of the monitoring system are evaluated for significant approach cases. Overall, computation time and hardware requirements of the proposed system easily fall into the limits foreseen for a real-time on-board application. Both the ATV Control Centre (real-time GNC data check against telemetry) and the ISS (automatic monitoring tool for ISS crew) could take advantages of the output data provided by this system. Moreover, obtained results clearly show that the monitoring system is a suitable candidate for further developments and improvements aimed to provide a complete support tool for ATV mission operations during final approach phase.

Design and tests of a frictionless 2D platform for studying space navigation and control subsystems

The design of a low cost free floating platform for ground test of on orbit complex operations is presented. The authors describe the bus design and the test of the different components, such as the pressured air tanks feeding the pads - needed for low friction planar operations-, the thrusters, the software and hardware architecture. A major focus is set on the navigation system. The selection process of the inertial sensors is illustrated, with relevant test campaign, performed for assessing the gyroscope and accelerometers characteristics (white noise, bias, random walk). The accelerometers, however, do not prove to be accurate enough to permit even time limited controlled maneuvers. At the scope we add an optical sensor, and include its measurements in the Kalman filter realized to estimate the platform state. Eventually, the performance of the guide, navigation and control system applied to the assembled platform is tested by means of a series of maneuvers characterized by growing complexity: rectilinear path tracking, attitude slewing maneuver, and coordinated translation and rotational control along curvilinear trajectories.

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.

Analysis and tests of visual based techniques for orbital rendezvous operations

Advanced visual based techniques are proposed in this paper for the dual task of identifying a target and evaluating the relative position and attitude between a chaser and the target during a space rendezvous. The algorithm is first tested on a given image, and then implemented onboard a free floating platform. The experimental results of this rendezvous performed with the testbed in the lab are reported, showing remarkable performance that offers the basis for future developments of close proximity control strategies.

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.

Tracking a ballistic target by multiple model approach

Radar tracking of a ballistic object flying in the Earths atmosphere is a very complex issue to cope with, due to the need of (suboptimal) nonlinear filtering techniques. When the characteristics of the target are poorly known, and an identification problem is added, a solution is represented by a multiple model approach. This paper investigates the problem by evaluating a number of parameters which affect the solution. The multi modal approach is compared with a generic extended Kalman filter. A theoretical limit for the performance is computed by means of the posterior Cramér-Rao lower bound.

Pose and Shape Reconstruction of a Noncooperative Spacecraft Using Camera and Range Measurements

Recent interest in on-orbit proximity operations has pushed towards the development of autonomous GNC strategies. In this sense, optical navigation enables a wide variety of possibilities as it can provide information not only about the kinematic state but also about the shape of the observed object. Various mission architectures have been either tested in space or studied on Earth. The present study deals with on-orbit relative pose and shape estimation with the use of a monocular camera and a distance sensor. The goal is to develop a filter which estimates an observed satellite’s relative position, velocity, attitude, and angular velocity, along with its shape, with the measurements obtained by a camera and a distance sensor mounted on board a chaser which is on a relative trajectory around the target. The filter’s efficiency is proved with a simulation on a virtual target object. The results of the simulation, even though relevant to a simplified scenario, show that the estimation process is successful and can be considered a promising strategy for a correct and safe docking maneuver.

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.

Design of the radiation shielding for a microsatellite

Abstract This paper aims to provide a detailed description of the problems concerning the radiation environment faced while designing a microsatellite at the University of Rome. Although main features of the microsatellite, as well as the environment characteristics expected in candidate orbits are detailed, emphasis is given to expose a generally appropriate procedure for this class of spacecraft. The sector analysis is carried on, and a simple qualitative way to point out critical areas of shielding is shown. The risk concerning the specific devices is assessed, both for total ionization dose and single event upset. The effect of the spot shielding on the most sensitive devices is considered, in order to mitigate SEE occurrence.

A comparison among classical and SDRE techniques in formation flying orbital control

A key point in formation flying mission design is represented by the accuracy and the cost of maintaining the requested orbital configuration. In fact, the relative geometry among spacecraft should be kept within tight limits in order to accomplish payload missions. At the same time, this effort requires to accommodate onboard the relevant amount of propellant, which should be correctly evaluated. The quest for optimal control strategy faces the non linear nature of the orbital dynamics, furthermore affected by perturbations that can be only modeled and therefore not perfectly known. As a result, traditional optimal strategies as the Linear Quadratic Controller (LQR), which design can be achieved under the hypothesis of simplified (as an example linearized) dynamics, not always meet the objective. Innovative approaches, like the State Dependent Riccati Equation (SDRE) technique, allow to better take into account, at an increasing level of approximations, the real dynamics. The paper presents extensive results of the simulations carried out for two different problems in formation flying control: the maintaining of a desired relative geometry and the acquisition of a requested configuration. A relevant point, also with respect to currently available literature, is the fact that the considered reference orbits have an eccentricity different from zero.