Controllo d'assetto a tre assi di un simulatore per il volo in formazione - Software design e hardware test

Il fine del lavoro svolto è lo sviluppo di un controllo d'assetto a tre assi di un simulatore per il volo in formazione. Si è sviluppata una strategia di controllo PID basata sui quaternioni e un simulatore software rappresentativo della dinamica d'assetto reale. Le caratteristiche inerziali sono determinate mediante un sistema a pendolo torsionale. L'esecuzione di manovre d'assetto permette di validare le strategie adottate, estendendo le capacità dell'apparato meccanico di simulazioneope

Design and testing of a vision based navigation system for a spacecraft formation flying simulator

In the last decades, Spacecraft Formation Flight (SFF) and proximity operations have aroused a great deal of interest within the international space community for the wide range of advantages generated with respect to traditional mission architectures. The idea of distributing the capabilities of space systems over multiple satellites, in general, results in lower development costs, redundancy to failure, flexibility and improved measurement performances of the payloads. Moreover, in sight of the extension of the operational lifetimes of orbiting spacecraft, performing on orbit inspections, maintenance and repair provides significant advantages from an economical point of view. Nonetheless, all these advantages comes at a cost as the execution of coordinated maneuvers between two or more spacecraft impose requirements not only on the absolute state, but also on the relative dynamic state. This reason pushes researchers and institutions to advance, from a technical and technological point of view, the development of accurate and robust Guidance, Navigation and Control (GN&C) strategies to enable the autonomous execution of relative navigation tasks on orbit. Prior to be accepted for the flight, each strategy has to be validated in a relevant and representative environment: one popular and effective way to reach this objective is means of ground-based testbeds. The current work focuses on the development of vision based estimation strategies for relative navigation tasks in the context of Spacecraft Formation Flight and proximity operations. In order to validate the developed GN&C strategies in a relevant environment, part of my activities are focused on the development and realization of the cooperating spacecraft testbed for autonomous proximity operations experiments, called SPARTANS, whose design and realization started in 2010 at the Center of Studies and Activities for Space (CISAS) "G. Colombo" of the University of Padova. SPARTANS is a ground-based hardware simulator that features robotic units that allow to reproduce the relative motion of two ore more spacecraft flying in formation or in close proximity, allowing therefore to validate advanced relative GN&C strategies developed within this context. The first activity of my Ph.D. regards the characterization of a monocular vision system for relative pose estimation in the context of cooperative spacecraft. An original custom-made set of fiducial markers is proposed along with the required image analysis procedure to extract relevant points from the acquired images. The estimation of the relative pose is obtained by solving the perspective-from-three points problem followed by a subsequent pose refinement procedure. The metrological performances of the proposed approach are assessed following an experimental approach: known displacements are imposed to a satellite mock-up and, by comparing the estimated poses with the imposed ones, the measurement error is characterized. Finally an uncertainty analysis, validated by a series of tests, was performed yielding an average accuracy of about 1 cm and 1 deg on the relative position and attitude, respectively. This activity is followed by the implementation of new features for the SPARTANS testbed. At the beginning of my Ph.D., the testbed featured only one spacecraft module capable of performing 3 degrees of freedom attitude maneuvers exploiting the measurements coming from incremental quadrature encoders and actuating control actions thanks to a cold gas thrusters system. A small contribution is initially given in the development of the Translational Module (TM), that enables low friction planar displacements to the robotic platforms, and in the design and development of the laboratory test table that provides the testbed with a planar surface on top of which the spacecraft simulators can translate. Subsequently, I was involved in the realization, from a practical point of view, of the laboratory setup: in particular, the second main activity I have been working on regards the development, the implementation and the characterization of a global navigation system that allows to estimate the position and the orientation of each spacecraft module within the laboratory operative volume. The first step of this activity consists in the development of a cost-effective prototype based on a monocular system and a set of three Optical Flow Sensors to obtain planar pose estimations of the TM. The estimation scheme relies on two sets of measurements that are conveniently merged by means of an Extended Kalman Filter. The second development phase consists in the extension of the proposed estimation approach by adapting it to the laboratory characteristics: a Motion Capture system consisting of six infrared cameras is employed to this purpose. A pose estimation strategy is proposed in this context and a proper calibration procedure is developed. The results of the characterization of the Motion Capture system shows sub-degree and sub-millimeter accuracy respectively on the estimated orientations and positions, allowing to obtain reference measurements to assess the performances of the GN&C algorithms tested with the spacecraft simulator. The last part of my activity is focused on the development of a relative navigation strategy based on an Extended Kalman Filter that exploits only visual data provided by a stereoscopic vision system. The proposed estimation scheme relies on the a priori knowledge of the geometry of the observed object (target) and it is implemented in C++ in sight of a foreseen implementation on embedded systems. The filter capabilities are assessed by means of numerical simulations in two different scenarios. In the first scenario, the relative dynamic state of two spacecraft orbiting in a leader-follower formation at short range is simulated numerically by means of a software orbital propagator. In the second scenario, the operative conditions of the laboratory setup are reproduced by means of a 3D simulated environment. Finally, the proposed navigation strategy is validated by means of laboratory testing exploiting the robotic spacecraft units. The proposed navigation algorithm proves to be capable of estimating the relative dynamic state of an observed target with centimeter-level and degree-level accuracy. In its current configuration, the SPARTANS testbed features a first complete spacecraft unit, a 3 x 2 m flat test table, a global navigation system consisting of 6 infrared cameras and an external control station. Once the construction of the second spacecraft unit will be completed, the testbed will allow to perform coordinated maneuvers between two (or more) spacecraft units for the study and validation of strategies related to SFF, automated rendez-vous and docking and, in general, proximity maneuvers

Metrological characterization of a vision-based system for relative pose measurements with fiducial marker mapping for spacecrafts

An improved approach for the measurement of the relative pose between a target and a chaser spacecraft is presented. The selected method is based on a single camera, which can be mounted on the chaser, and a plurality of fiducial markers, which can be mounted on the external surface of the target. The measurement procedure comprises of a closed-form solution of the Perspective from n Points (PnP) problem, a RANdom SAmple Consensus (RANSAC) procedure, a non-linear local optimization and a global Bundle Adjustment refinement of the marker map and relative poses. A metrological characterization of the measurement system is performed using an experimental set-up that can impose rotations combined with a linear translation and can measure them. The rotation and position measurement errors are calculated with reference instrumentations and their uncertainties are evaluated by the Monte Carlo method. The experimental laboratory tests highlight the significant improvements provided by the Bundle Adjustment refinement. Moreover, a set of possible influencing physical parameters are defined and their correlations with the rotation and position errors and uncertainties are analyzed. Using both numerical quantitative correlation coefficients and qualitative graphical representations, the most significant parameters for the final measurement errors and uncertainties are determined. The obtained results give clear indications and advice for the design of future measurement systems and for the selection of the marker positioning on a satellite surface

X-ray emission from the Sombrero galaxy: discrete sources

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30 kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray binaries (LMXBs). We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-low indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC LF at fainter luminosities down to 1E35 erg/s. The derived index rules out a faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.Comment: 11 figures, 5 tables, ApJ in pres

Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV

A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7 TeV is presented. The data were collected at the LHC, with the CMS detector, and correspond to an integrated luminosity of 4.6 inverse femtobarns. No significant excess is observed above the background expectation, and upper limits are set on the Higgs boson production cross section. The presence of the standard model Higgs boson with a mass in the 270-440 GeV range is excluded at 95% confidence level.Comment: Submitted to JHE

Measurement of the Z/gamma* + b-jet cross section in pp collisions at 7 TeV

The production of b jets in association with a Z/gamma* boson is studied using proton-proton collisions delivered by the LHC at a centre-of-mass energy of 7 TeV and recorded by the CMS detector. The inclusive cross section for Z/gamma* + b-jet production is measured in a sample corresponding to an integrated luminosity of 2.2 inverse femtobarns. The Z/gamma* + b-jet cross section with Z/gamma* to ll (where ll = ee or mu mu) for events with the invariant mass 60 < M(ll) < 120 GeV, at least one b jet at the hadron level with pT > 25 GeV and abs(eta) < 2.1, and a separation between the leptons and the jets of Delta R > 0.5 is found to be 5.84 +/- 0.08 (stat.) +/- 0.72 (syst.) +(0.25)/-(0.55) (theory) pb. The kinematic properties of the events are also studied and found to be in agreement with the predictions made by the MadGraph event generator with the parton shower and the hadronisation performed by PYTHIA.Comment: Submitted to the Journal of High Energy Physic

Search for New Physics with Jets and Missing Transverse Momentum in pp collisions at sqrt(s) = 7 TeV

A search for new physics is presented based on an event signature of at least three jets accompanied by large missing transverse momentum, using a data sample corresponding to an integrated luminosity of 36 inverse picobarns collected in proton--proton collisions at sqrt(s)=7 TeV with the CMS detector at the LHC. No excess of events is observed above the expected standard model backgrounds, which are all estimated from the data. Exclusion limits are presented for the constrained minimal supersymmetric extension of the standard model. Cross section limits are also presented using simplified models with new particles decaying to an undetected particle and one or two jets

Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS

We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final version to appear in Physics Letters

Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables, revised author list, matches European Journal of Physics C versio

Observation of a new chi_b state in radiative transitions to Upsilon(1S) and Upsilon(2S) at ATLAS

The chi_b(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider (LHC) at sqrt(s) = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb^-1, these states are reconstructed through their radiative decays to Upsilon(1S,2S) with Upsilon->mu+mu-. In addition to the mass peaks corresponding to the decay modes chi_b(1P,2P)->Upsilon(1S)gamma, a new structure centered at a mass of 10.530+/-0.005 (stat.)+/-0.009 (syst.) GeV is also observed, in both the Upsilon(1S)gamma and Upsilon(2S)gamma decay modes. This is interpreted as the chi_b(3P) system.Comment: 5 pages plus author list (18 pages total), 2 figures, 1 table, corrected author list, matches final version in Physical Review Letter