Non-linear relativistic perturbation theory with two parameters

An underlying fundamental assumption in relativistic perturbation theory is the existence of a parametric family of spacetimes that can be Taylor expanded around a background. Since the choice of the latter is crucial, sometimes it is convenient to have a perturbative formalism based on two (or more) parameters. A good example is the study of rotating stars, where generic perturbations are constructed on top of an axisymmetric configuration built by using the slow rotation approximation. Here, we discuss the gauge dependence of non-linear perturbations depending on two parameters and how to derive explicit higher order gauge transformation rules.Comment: 5 pages, LaTeX2e. Contribution to the Spanish Relativity Meeting (ERE 2002), Mao, Menorca, Spain, 22-24.September.200

Communication and Mutual Physical Position Estimation System for CubeSat

In recent years, constellations of CubeSats organized as a swarm are redefining the classical concept of space missions. The biggest challenge for the realization of an efficient swarm is to provide to the CubeSats the ability to interact and communicate each other. In this paper we present a system able to provide to the CubeSats belonging to a swarm the ability to establish an inter-satellite communication crosslink and to determine the mutual physical positioning. The basic idea is to provide every CubeSat with a system involving a smart-antenna array. By exploiting the array, CubeSats can transmit or receive signals to / from every element of the swarm so as to perform the inter-satellite communication. The smart-antenna is managed by a beamforming control strategy: during the transmission, the beamforming algorithm controls the smart-antenna in order to shape the beam and establish a reliable and directive communication link with other spacecraft and/or with the ground station. Hence, the beam shaping avoid to perform attitude maneuvers to optimize the transmission. Every CubeSat acquires signals transmitted from other elements of the swarm and estimate the Direction-of-Arrival (DOA) and the distance (Range) in order to calculate the mutual physical positioning. By an appropriate distribution of the antennas on the structure of the CubeSat it is possible to obtain a working range of 4π steradians. Every element of the smart-antenna is connected to a signal conditioning chain able to modify the phase and the amplitude of the signal transmitted / received. The beamforming algorithm manages this signal conditioning chain dynamically to maximize the performance of the system. Thanks to his small footprint, the system can be mounted on every CubeSat geometry and it is completely integrated with the bus so as not to occupy space dedicated for the payload as shown in figure [1]. Through the use of a deployable structure fully developed at Politecnico di Torino, we increase the external surface of CubeSats: this surface allows to gain the interspace between elements of the smart-antenna (figure [2]). As a consequence, the directivity and detection performance of the DOA system in terms of directivity and accuracy are improved. Moreover, the deployable structure offers a greater usable surface, so a larger number of solar panels can be used, e.g.: up to 40x30 cm2 for a 1U CubeSat. Hence, the communication distance increase because a power up to 6W is available for the transmit mode. This paper describes the physical implementation of the antenna array system on a 1U CubeSat using the deployable structure developed. In section I we describe how the subsystem has been designed, we analyze how the hardware works and we focus on the main blocks that realize the positioning measurement / network communication. In the section II we describe how the swarm subsystem can be hosted on every CubeSat structure (even 1U) by exploiting a deployable structure able to increase the useful surface of the CubeSat and the antenna baseline. This structure allows to gain the available power supply for the transmission (also for the other on board systems) and improves the precision of the mutual positioning estimator. In section III we describe how the subsystem establishes the communication between the CubeSats and how it measures the direction of arrival (DOA) and the distance (Range) of the received signals in order to establish the mutual physical position of every CubeSat composing the swarm

Robust multi-agent collision avoidance through scheduling

We propose a class of computationally efficient algorithms for conflict resolution in the presence of modeling and measurement uncertainties. Specifically, we address a scenario where a number of agents, whose dynamics are possibly nonlinear, must cross an intersection avoiding collisions. We obtain an exact solution and an approximate one with quantified error bound whose complexity scales polynomially with the number of agents.National Science Foundation (U.S.) (Award CNS 0930081)Roberto Rocca Foundatio

Questo lo chiami amore? Una chiave di lettura dell’umano

Intermezzo: "Questo lo chiami amore? Una chiave di lettura dell’umano

A commercial macroalgae extract in a plant-protein rich diet diminished saturated fatty acids of Oncorhynchus mykiss Walbaum fillets

Seaweeds are considered novel feed ingredients, nutraceutical compounds and source of pigments and proteins. They appear to possess bioactive properties, such as hypolipidemic, antioxidant and immune-stimulative actions; furthermore, their proteins are considered just as nutritious as terrestrial vegetables. In the present work, rainbow trout (Oncorhynchus mykiss Walbaum) was fed for 95 days with three diets: a fishmeal-based positive control diet (C+), a vegetable protein-based negative control diet (C−) and a diet similar to C − where 5% of soybean concentrate was replaced by a commercial blend of seaweeds (T). The monitored parameters were fish performance, physical and marketable characteristics, fillet oxidative status and fatty acid content. The estimated indices of enzyme activities involved in lipid metabolism were calculated. Fish performances were reduced in C − and T fish; C − and T fillet fatty acid (FA) profiles globally showed the same pattern and were distinct from C+, i.e. with a lower content of polyunsaturated FAs and a higher content of monounsaturated and polyunsaturated n6 FAs; contrarywise, saturated FAs were significantly lower in T group in comparison to the other two groups (p <.01). The estimated indices of enzyme activity highlighted differences between dietary groups; desaturase activities of C18 and n3 FAs were higher in T in comparison to C− (p <.001). A slight impoverishment of antioxidant activity was found in T compared to C + group. Dietary seaweeds seemed to be capable of influencing fillet FA composition as well as the activity of enzymes related to lipid metabolism. Further studies in this regard are encouraged.Highlights Seaweeds are novel feed ingredients with nutraceutical properties Saturated fatty acids of rainbow trout fillets were lowered by seaweed inclusion in the diet Estimated desaturase activities of C18 and n3 fatty acids were higher in seaweed-fed trout

Memòria Digital de Catalunya

Tít. i menció de responsabilitat obtinguts de l'epígraf del f. sign. [a]1Peu d'impr. obtingut dels repertoris (i descrit segons figura a l'OPAC). Segons ISTC "The book was on sale in Germany by 6 July 1471"Signatures: [ ]12, [ ]10, [ ]10, [ ]10, [ ]10, [ ]12. Segons GW: [a12, b-e10, f12] i qualificat erròniament de 2ºEspais en blanc per a les caplletre

Two-parameter non-linear spacetime perturbations: gauge transformations and gauge invariance

An implicit fundamental assumption in relativistic perturbation theory is that there exists a parametric family of spacetimes that can be Taylor expanded around a background. The choice of the latter is crucial to obtain a manageable theory, so that it is sometime convenient to construct a perturbative formalism based on two (or more) parameters. The study of perturbations of rotating stars is a good example: in this case one can treat the stationary axisymmetric star using a slow rotation approximation (expansion in the angular velocity Omega), so that the background is spherical. Generic perturbations of the rotating star (say parametrized by lambda) are then built on top of the axisymmetric perturbations in Omega. Clearly, any interesting physics requires non-linear perturbations, as at least terms lambda Omega need to be considered. In this paper we analyse the gauge dependence of non-linear perturbations depending on two parameters, derive explicit higher order gauge transformation rules, and define gauge invariance. The formalism is completely general and can be used in different applications of general relativity or any other spacetime theory.Comment: 22 pages, 3 figures. Minor changes to match the version appeared in Classical and Quantum Gravit