4,486 research outputs found
Autonomous attitude using potential function method under control input saturation
The potential function method has been used extensively in nonlinear control for the development of feedback laws which result in global asymptotic stability for a certain prescribed operating point of the closed-loop system. It is a variation of the Lyapunov direct method in the sense that here the Lyapunov function, also called potential function, is constructed in such a way that the undesired points of the system state space are avoided. The method has been considered for the space applications where the systems involved are usually composed of the cascaded subsystems of kinematics and dynamics and the kinematic states are mapped onto an appropriate potential function which is augmented for the overall system by the use of the method of integrator backstepping. The conventional backstepping controls, however, may result in an excessive control effort that may be beyond the saturation bound of the actuators. The present paper, while remaining within the framework of conventional backstepping control design, proposes analytical formulation for the control torque bound being a function of the tracking error and the control gains. The said formulation can be used to tune to the control gains to bound the control torque to a prescribed saturation bound of the control actuators
Discontinuous Galerkin methods for general-relativistic hydrodynamics: formulation and application to spherically symmetric spacetimes
We have developed the formalism necessary to employ the
discontinuous-Galerkin approach in general-relativistic hydrodynamics. The
formalism is firstly presented in a general 4-dimensional setting and then
specialized to the case of spherical symmetry within a 3+1 splitting of
spacetime. As a direct application, we have constructed a one-dimensional code,
EDGES, which has been used to asses the viability of these methods via a series
of tests involving highly relativistic flows in strong gravity. Our results
show that discontinuous Galerkin methods are able not only to handle strong
relativistic shock waves but, at the same time, to attain very high orders of
accuracy and exponential convergence rates in smooth regions of the flow. Given
these promising prospects and their affinity with a pseudospectral solution of
the Einstein equations, discontinuous Galerkin methods could represent a new
paradigm for the accurate numerical modelling in relativistic astrophysics.Comment: 24 pages, 19 figures. Small changes; matches version to appear in PR
Activities of the Space Advanced Research Team at the University of Glasgow
A wide range of technologies and methodologies for space systems engineering are currently being developed at the University of Glasgow. Much of the work is centred on mission analysis and trajectory optimisation, complemented by research activities in autonomous and multi-agent systems. This paper will summarise these activities to provide a broad overview of the current research interests of the Space Advanced Research Team (SpaceART). It will be seen that although much of the work is mission driven and focussed on possible future applications, some activities represent basic research in space systems engineering
Multimessenger Parameter Estimation of GW170817
We combine gravitational wave (GW) and electromagnetic (EM) data to perform a
Bayesian parameter estimation of the binary neutron star (NS) merger GW170817.
The EM likelihood is constructed from a fit to a large number of numerical
relativity simulations which we combine with a lower bound on the mass of the
remnant's accretion disk inferred from the modeling of the EM light curve. In
comparison with previous works, our analysis yields a more precise
determination of the tidal deformability of the binary, for which the EM data
provide a lower bound, and of the mass ratio of the binary, with the EM data
favoring a smaller mass asymmetry. The 90\% credible interval for the areal
radius of a NS is found to be (statistical and systematic uncertainties).Comment: 7 pages, 3 figures, accepted to the EPJA Topical Issue: The first
Neutron Star Merger Observation - Implications for Nuclear Physic
Universality and intermittency in relativistic turbulent flows of a hot plasma
With the aim of determining the statistical properties of relativistic
turbulence and unveiling novel and non-classical features, we resent the
results of direct numerical simulations of driven turbulence in an
ultrarelativistic hot plasma using high-order numerical schemes. We study the
statistical properties of flows with average Mach number ranging from to and with average Lorentz factors up to . We find
that flow quantities, such as the energy density or the local Lorentz factor,
show large spatial variance even in the subsonic case as compressibility is
enhanced by relativistic effects. The velocity field is highly intermittent,
but its power-spectrum is found to be in good agreement with the predictions of
the classical theory of Kolmogorov. Overall, our results indicate that
relativistic effects are able to significantly enhance the intermittency of the
flow and affect the high-order statistics of the velocity field, while leaving
unchanged the low-order statistics, which instead appear to be universal and in
good agreement with the classical Kolmogorov theory. To the best of our
knowledge, these are the most accurate simulations of driven relativistic
turbulence to date.Comment: 5 pages, 4 figures. Minor changes to match the version accepted on
ApJ
Universality and intermittency in relativistic turbulent flows of a hot gas
With the aim of determining the statistical properties of relativistic
turbulence and unveiling novel and non-classical features, we present the
results of direct numerical simulations of driven turbulence in an
ultrarelativistic hot plasma using high-order numerical schemes. We study the
statistical properties of flows with average Mach number ranging from to and with average Lorentz factors up to . We find
that flow quantities, such as the energy density or the local Lorentz factor,
show large spatial variance even in the subsonic case as compressibility is
enhanced by relativistic effects. The velocity field is highly intermittent,
but its power-spectrum is found to be in good agreement with the predictions of
the classical theory of Kolmogorov.Comment: Talk given at the ASTRONUM2012 conference on the 25th of June 201
Collision and evaporation avoidance for spacecraft formation
<p>Formation flying is an extremely promising approach to space operations with the potential to enable new types of missions and providing substantial increase in the performance of future space science and Earth observation applications. To successfully validate formation flying however requires the development of specific technologies and methodologies, which are beyond current state-of-the art in a wide range of diverse fields such as metrology and spacecraft guidance, navigation and control. A number of missions are currently under different stages of development to implement some of these stringent requirements.</p>
<p>The paper develops and compares collision avoidance algorithms, demonstrating them within a 6 degrees of freedom, multi-spacecraft environment. At first a number of different collision avoidance scenarios will be identified alongside the triggers that will cause the algorithms to be activated. Once activated the collision avoidance algorithm must ensure corrective action to avoid catastrophic consequences to the mission.</p>
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