1,721,045 research outputs found
Five-Degree-of-Freedom Pose Estimation from an Imaged Ellipsoid of Revolution
Full text linkIn this Note, I present an analytical solution to 5 degrees of freedom pose estimation from imaged ellipsoids of revolution. A theory is developed which shows that the range, the latitude of observation, and three attitude angles can be retrieved from the image of an ellipsoid of revolution (the longitude being unobservable due to the axial symmetry of the target). Starting from an image of the ellipsoid, the limb is extracted and fitted to an ellipse. Then, by making use of some analytical results from perspective geometry, the range and the latitude of observation are computed as the solution of a system of equations involving the eigenvalues of the fitted ellipse. Finally, the attitude matrix is computed by solving a modified orthogonal Procrustes problem. The algorithm is verified through numerical simulations with synthetically generated images of a spheroid and both synthetic and real images of Ceres taken by Dawn spacecraft. Results for the spheroid indicate the very good performance of the method in retrieving the 5 pose DOF. When applied to a target departing from an ideal spheroid as Ceres, performance degrades slightly for the range and attitude angles; the latitude of observation is instead estimated with poor accuracy. The method developed can be exploited for coarse OPNAV, when the relative attitude between the spacecraft and the target is poorly constrained or, conversely, for relative attitude determination when the position between the spacecraft and the target is not well known
Attitude Determination from Ellipsoid Observations: A Modified Orthogonal Procrustes Problem
Full text linkThe determination of the attitude from a set of vector
observations is a recurrent problem for spacecraft and aerial
vehicles in general. As such, it has been extensively studied for many
decades, leading to several solution methods. Perhaps the most
popular formulation for the static attitude determination problem is
the one in the form of a least-squares minimization, due to Grace
Wahba. Finding the attitude matrix minimizing Wahba’s loss
function has been shown to be equivalent to an orthogonal Procrustes
problem, which can be solved through the singular value
decomposition method.
In this Note, we address a related problem, which is the one of
attitude determination from imaged ellipsoids
A solution to spacecraft position and attitude determination from an imaged ellipsoid
No full textWhen imaged through a camera, an illuminated ellipsoid gives rise, in general, to two ellipses on the image plane, namely the limb ellipse and the terminator ellipse. If the directum of illumination is known in the ellipsoid frame. then the full six degrees of freedom pose of the camera can be retrieved, apart from ambiguities which anse due to symmetry, by solving a quadnc-to-comc plus conic-to-conic correspondence problem. A solution to such task is proposed in this manuscript. By fittmg an ellipse to the imaged planetary limb arc, another ellipse to the terminator points, and exploiting analytical results available from projective geometry equations for computing the entire pose are formulated. The position is expressed as the least squares solution of an over-determined system of four equations m three unknowns, while the attitude is estimated tough a modified orthogonal Procrustes problem, whose solution involves the spectral decomposition of symmetric 3x3 matrices. Preliminar^ validation using synthetically generated images of illuminated ellipsoids suggests that the proposed algorithm can achieve position relative accuracy up to 10-3, and attitude error withm the milli-rad
Analytic solution for perturbed Keplerian motion under small acceleration using averaging theory
No full textA novel approach is developed for analytic orbit propagation based on asymptotic expansion with respect to a small perturbative acceleration. The method improves upon existing first order asymptotic expansions by leveraging on linear systems and averaging theories. The solution starts with the linearization of Gauss planetary equations with respect to both the small perturbation and the six orbital elements. Then, an approximate solution is obtained in terms of secular and short period components. The method is tested on a low-thrust maneuver scenario consisting of a Keplerian orbit perturbed by a constant tangential acceleration, for which a solution can be obtained in terms of elliptic integrals. Results show that the positional propagation error is about one order of magnitude smaller with respect to state-of-the-art methods. The position accuracy for a LEO orbit, apart from pathological cases, is typically in the range of tens of meters for a tangential acceleration of 10−7 km/s2 after 5 orbital periods propagation
Planet–sun sensor revisited
Full text linkSince the seminal work of Daniele Mortari (“Moon-Sun Attitude Sensor,” Journal of Spacecraft and Rockets, Vol. 34, No. 3, 1997, pp. 360–364), the concept of an attitude sensor using images of illuminated celestial bodies has been pushed forward through the years. The basic idea consists of extracting two independent directions from the image of a celestial body, namely, the camera-to-planet and the planet-to-sun directions. The former is estimated from the center of an ellipse fitted to the imaged limb points and the latter from the symmetry axis of the illuminated region. These assumptions, however, only hold for far-distant spherical targets. In this work, the problem is reformulated in the framework of projective camera transformations of quadrics and conics, and an algorithm estimating the line of sight to the planet and the illumination direction from the limb and terminator ellipses, respectively, is presented. The method is applicable to any ellipsoidlike celestial body having known orientation. The algorithm is first validated on synthetically generated images and then tested using real pictures of Dione and Enceladus satellites gathered from Cassini spacecraft. Results show that the sensor concept returns rms errors in the order of the angular width of a pixel in computing the nadir direction, and subdegree accuracy in computing the sun direction
Automatic mass balancing system for a dynamic CubeSat attitude simulator: development and experimental validation
No full textThis paper describes the automatic balancing and inertia identification system for three degrees of freedom CubeSat attitude simulator testbed. For a reliable verification of the attitude determination and control subsystem, the on-orbit environment shall be simulated within the testbed, minimizing the external disturbances acting on the satellite mock-up. The gravity torque is expected to be the largest among the disturbances, and an automatic balancing procedure can largely reduce the time necessary for tuning the platform and minimize the residual torque. The automatic balancing system adopted in this work employs three sliding masses independently actuated by three electric motors using a two-step procedure. In the first step, a feedback control is employed for a plane balancing. The inertia parameters and the remaining offset component are then estimated by collecting free oscillating platform data. This two-step procedure is iterated towards increasingly finer balancing until no further improvement is obtained. For the planar balancing, a control law based on linearized equations and a newly developed nonlinear feedback law is implemented and compared, showing the superior performance of the latter. The unbalance offset vector component along the local vertical and inertia tensor are estimated by a constrained batch least squares filter. Experimental results show the effectiveness of the implemented approach, which leads to a residual disturbance torque acting on the balanced platform smaller than 5 × 10–5 Nm
Artificial Intelligence-Based Challenges as an Educational Tool in Aerospace Engineering: the u3S Laboratory Experience
Full text linkThis manuscript reports on the positive impact that Artificial Intelligence based challenges are having on the educational and research activities of the u3S laboratory at the University of Bologna, within the framework of degree courses in aerospace engineering. Three examples are discussed from the past academic years, highlighting the benefits along with the challenges, of using AI based competitions as the source of hand-on education to carry out final projects
Hybrid controller for global, robust, attitude stabilization of a magnetically actuated spacecraft
No full textA novel approach to the three-axis attitude control of a magnetically actuated spacecraft is proposed, based on hybrid systems theory. Due to the actuators type, the system model is instantaneously underactuated and time-varying, so that low pointing errors and robustness are difficult to achieve at the same time. In this work, a uniting control design is developed, which combines a local H-inf regulator, with guaranteed performance, and a global nonlinear controller for ensuring global stability and robustness. Hybrid control theory is employed to develop a mixed continuous-discrete controller able to switch between different feedbacks. Controllers’ domains are designed according to appropriate input–output functions and to the magnitude of disturbances affecting the system. As a result, global attitude stability is ensured, while achieving local optimality and robustness against bounded disturbances, both matched and unmatched by the control action, and measurement noise. Analytical results are verified by means of realistic numerical simulations: the state errors comply with the computed bounds and stability is guaranteed for conservative assumptions on the magnitude of the unmatched disturbances
Nanosatellite-class dynamic attitude simulator for hands-on aerospace control education
Full text linkDue to their low size, mass, development cost and time, nanosatellites have become an increasingly popular tool at universities for providing students with hands-on experience in aerospace education. Among spacecraft subsystems, the attitude determination and control one surely represents a fruitful resource for practicing aerospace control applications. To enable on-ground verification of spacecraft attitude control hardware and software, however, the biggest challenge to overcome is that of providing a representative testing environment. Towards this end, at the μ3S laboratory at the University of Bologna a dynamic hardware in the loop facility has been developed, which allows for testing attitude control subsystems of nanosatellites in the range of 1U to 3U, according to the CubeSat form factor. This paper describes the educational impact that the facility has been having, during both its development and commissioning phases, as well as its early use as a testbed for CubeSats attitude control, which is currently focused on magnetic-based actuation
Large constellations of small satellites: A survey of near future challenges and missions
Full text linkConstellations of satellites are being proposed in large numbers; most of them are expected to be in orbit within the next decade. They will provide communication to unserved and underserved communities, enable global monitoring of Earth and enhance space observation. Mostly enabled by technology miniaturization, satellite constellations require a coordinated effort to face the technological limits in spacecraft operations and space traffic. At the moment in fact, no cost-effective infrastructure is available to withstand coordinated flight of large fleets of satellites. In order for large constellations to be sustainable, there is the need to efficiently integrate and use them in the current space framework. This review paper provides an overview of the available experience in constellation operations and statistical trends about upcoming constellations at the moment of writing. It highlights also the tools most often proposed in the analyzed works to overcome constellation management issues, such as applications of machine learning/artificial intelligence and resource/infrastructure sharing. As such, it is intended to be a useful resource for both identifying emerging trends in satellite constellations, and enabling technologies still requiring substantial development efforts
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