1,710 research outputs found
Nonlinear Attitude Filtering: A Comparison Study
This paper contains a concise comparison of a number of nonlinear attitude
filtering methods that have attracted attention in the robotics and aviation
literature. With the help of previously published surveys and comparison
studies, the vast literature on the subject is narrowed down to a small pool of
competitive attitude filters. Amongst these filters is a second-order optimal
minimum-energy filter recently proposed by the authors. Easily comparable
discretized unit quaternion implementations of the selected filters are
provided. We conduct a simulation study and compare the transient behaviour and
asymptotic convergence of these filters in two scenarios with different
initialization and measurement errors inspired by applications in unmanned
aerial robotics and space flight. The second-order optimal minimum-energy
filter is shown to have the best performance of all filters, including the
industry standard multiplicative extended Kalman filter (MEKF)
Mapping the neutral atomic hydrogen gas outflow in the restarted radio galaxy 3C 236
The energetic feedback that is generated by radio jets in active galactic
nuclei (AGNs) has been suggested to be able to produce fast outflows of atomic
hydrogen (HI) gas that can be studied in absorption at high spatial resolution.
We have used the Very Large Array (VLA) and a global
very-long-baseline-interferometry (VLBI) array to locate and study in detail
the HI outflow discovered with the Westerbork Synthesis Radio Telescope (WSRT)
in the re-started radio galaxy 3C 236. We confirm, from the VLA data, the
presence of a blue-shifted wing of the HI with a width of
. This HI outflow is partially recovered by the
VLBI observation. In particular, we detect four clouds with masses of
with VLBI that do not follow the regular
rotation of most of the HI. Three of these clouds are located, in projection,
against the nuclear region on scales of , while the
fourth is co-spatial to the south-east lobe at a projected distance of
. Their velocities are between and
blue-shifted with respect to the velocity of the
disk-related HI. These findings suggest that the outflow is at least partly
formed by clouds, as predicted by some numerical simulations and originates
already in the inner (few tens of pc) region of the radio galaxy. Our results
indicate that all of the outflow could consist of many clouds with perhaps
comparable properties as the ones detected, distributed also at larger radii
from the nucleus where the lower brightness of the lobe does not allow us to
detect them. However, we cannot rule out the presence of a diffuse component of
the outflow. The fact that 3C 236 is a low excitation radio galaxy, makes it
less likely that the optical AGN is able to produce strong radiative winds
leaving the radio jet as the main driver for the HI outflow.Comment: 13 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
What triggers a radio AGN? The intriguing case of PKSB 1718-649
We present new Australia Telescope Compact Array (ATCA) observations of the
young (< 10^2 years) radio galaxy PKS B1718-649. We study the morphology and
the kinematics of the neutral hydrogen (HI) disk (M(HI) = 1.1x 10^10 M(sun),
radius ~ 30 kpc). In particular, we focus on the analysis of the cold gas in
relation to the triggering of the nuclear activity. The asymmetries at the
edges of the disk date the last interaction with a companion to more than 1 Gyr
ago. The tilted-ring model of the HI disk shows that this event may have formed
the disk as we see it now, but that it may have not been responsible for
triggering the AGN. The long timescales of the interaction are incompatible
with the short ones of the radio activity. In absorption, we identify two
clouds with radial motions which may represent a population that could be
involved in the triggering of the radio activity. We argue that PKS B1718-649
may belong to a family of young low-excitation radio AGN where, rather than
through a gas rich merger, the active nuclei (AGN) are triggered by local
mechanisms such as accretion of small gas clouds.Comment: 8 pages, 9 figures, Accepted to A&
Lumpable hidden Markov models - model reduction and reduced complexity filtering
Copyright © 2000 IEEEThis paper is concerned with filtering of hidden Markov processes (HMP) which possess (or approximately possess) the property of lumpability. This property is a generalization of the property of lumpability of a Markov chain which has been previously addressed by others. In essence, the property of lumpability means that there is a partition of the (atomic) states of the Markov chain into aggregated sets which act in a similar manner as far as the state dynamics and observation statistics are concerned. We prove necessary and sufficient conditions on the HMP for exact lumpability to hold. For a particular class of hidden Markov models (HMM), namely finite output alphabet models, conditions for lumpability of all HMP representable by a specified HMM are given. The corresponding optimal filter algorithms for the aggregated states are then derived. The paper also describes an approach to efficient suboptimal filtering for HMP which are approximately lumpable. By this we mean that the HMM generating the process may be approximated by a lumpable HMM. This approach involves directly finding a lumped HMM which approximates the original HMM well, in a matrix norm sense. An alternative approach for model reduction based on approximating a given HMM by an exactly lumpable HMM is also derived. This method is based on the alternating convex projections algorithm. Some simulation examples are presented which illustrate the performance of the suboptimal filtering algorithmsLangford B. White, Robert Mahony and Gary D. Brush
Wigner-Moyal description of free variable mass Klein-Gordon fields
A system of coupled kinetic transport equations for the Wigner distributions
of a free variable mass Klein-Gordon field is derived. This set of equations is
formally equivalent to the full wave equation for electromagnetic waves in
nonlinear dispersive media, thus allowing for the description of broadband
radiation-matter interactions and the associated instabilities. The standard
results for the classical wave action are recovered in the short wavelength
limit of the generalized Wigner-Moyal formalism for the wave equation.Comment: 9 pages, accepted for publication in Journal of Mathematical Physic
Estimating body-fixed frame velocity and attitude from inertial measurements for a quadrotor vehicle
© 2014 IEEE. A key requirement for effective control of quadrotor vehicles is estimation of both attitude and linear velocity. Recent work has demonstrated that it is possible to measure horizontal velocities of a quadrotor vehicle from strap-down ac-celerometers along with a system model. In this paper we extend this to full body-fixed-frame velocity measurement by exploiting recent work in aerodynamic modeling of rotor performance and measurements of mechanical power supplied to the rotor hub. We use these measurements in a combined attitude and velocity nonlinear observer design to jointly estimate attitude and body-fixed-frame linear velocity. Almost global asymptotic stability of the resulting system is demonstrated using Lyapunov analysis of the resulting error system. In the current work, we ignore bias and leave it for future work. The performance of the observer is verified by simulation results
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