56 research outputs found
TERRAIN-BASED NAVIGATION: A TOOL TO IMPROVE NAVIGATION AND FEATURE EXTRACTION PERFORMANCE OF MOBILE MAPPING SYSTEMS
Terrain-referenced navigation (TRN) techniques are of increasing interest in the research community, as they can provide alternative navigation tools when GPS is not available or the GPS signals are jammed. Some form of augmentation to cope with the lack of GPS signals is typically required in mobile mapping applications in urban canyons and is of interest for military applications. TRN could provide alternative position and attitude fixes to support an inertial navigation system, since such systems inevitably drift over time if not calibrated by GPS or other methodologies. With improving imaging sensor performance as well as growing worldwide availability of terrain high-resolution data and city models, terrain-based navigation is becoming a viable option to support navigation in GPS-denied environments. Furthermore, the feedback from the imaging sensors can be used even during GPS availability, which increases the redundancy of the measurement update step of the navigation filter, enabling more reliable integrity monitoring at this stage. The relevance of TRN to mobile mapping applications is twofold: (1) the process of obtaining real-time position and attitude fixes for the navigation filter is based on feature extraction, and, in particular, on the capability to separate the static and dynamic objects from the image data, and (2) the use of already available terrain data, including surface models (DSM), raster or vector data in CAD/GIS environments, such as city models, can effectively support the extraction processes. These two tasks could overlap, although the separation of the static and dynamic objects should work without any terrain data, and in fact, this is, to a large extent, the idea behind the removal of vehicles (moving objects) from imagery. The overall TRN concept, where LiDAR and optical imagery are matched with the existing terrain data is discussed and initial performance results are reported
Parity properties of an advection-dominated solar \alpha^2\Om-dynamo
We have developed a high-precision code which solves the kinematic dynamo
problem both for given rotation law and meridional flow in the case of a low
eddy diffusivity of the order of cm/s known from the sunspot
decay. All our models work with an \alf-effect which is positive (negative) in
the northern (southern) hemisphere. It is concentrated in radial layers located
either at the top or at the bottom of the convection zone. We have also
considered an \alf-effect uniformly distributed in all the convection zone. In
the present paper the main attention is focused on i) the parity of the
solution, ii) the form of the butterfly diagram and iii) the phase relation of
the resulting field components. If the helioseismologically derived internal
solar rotation law is considered, a model without meridional flow of high
magnetic Reynolds number (corresponding to low eddy diffusivity) fails in all
the three issues in comparison with the observations. However, a meridional
flow with equatorial drift at the bottom of the convection zone of few meters
by second can indeed enforce the equatorward migration of the toroidal magnetic
field belts similar to the observed butterfly diagram but, the solution has
only a dipolar parity if the (positive) \alf-effect is located at the base of
the convection zone rather than at the top. We can, therefore, confirm the main
results of a similar study by Dikpati & Gilman (2001).Comment: 9 pages, 16 figures, to appear on Astronomy and Astrophysic
The effect of a meridional flow on Parker's interface dynamo
Parker's interface dynamo is generalized to the case when a homogeneous flow
is present in the high-diffusivity (upper) layer in the lateral direction (i.e.
perpendicular to the shear flow in the lower layer). This is probably a
realistic first representation of the situation near the bottom of the solar
convective zone, as the strongly subadiabatic stratification of the tachocline
(lower layer in the interface dynamo) imposes a strong upper limit on the speed
of any meridional flow there.
Analytic solutions to the eigenvalue problem are presented for the cases of
vanishing diffusivity contrast and infinite diffusivity contrast, respectively.
Unlike the trivial case of a homogeneous system, the ability of the meridional
flow to reverse the propagation of the dynamo wave is strongly reduced in the
interface dynamo. In particular, in the limit of high diffusivity contrast
relevant to the solar case it is found that a meridional flow of realistic
amplitude cannot reverse the direction of propagation of the dynamo wave. The
implications of this result for the solar dynamo problem are discussed.Comment: 5 pages, 3 figures; MNRAS, in pres
PERSONAL NAVIGATION: EXTENDING MOBILE MAPPING TECHNOLOGIES INTO INDOOR ENVIRONMENTS
This paper discusses some unconventional methods for indoor-outdoor navigation, based on the integration of self-contained sensors, including GPS, IMU, digital barometer, magnetometer compass, and a human locomotion model. The human locomotion model is used as navigation sensor and it is handled by Artificial Intelligence (AI) techniques that form an adaptive knowledge-based system (KBS), which is trained during the GPS signal reception, and is used to support navigation under GPS-denied conditions. A complementary technique used in our solution, which facilitates indoor navigation, is the image-based method (Flash LADAR). In this paper, the system design and an example performance analysis in the mixed indoor-outdoor environment are presented
On the possibility of a bimodal solar dynamo
A simple way to couple an interface dynamo model to a fast tachocline model
is presented, under the assumption that the dynamo saturation is due to a
quadratic process and that the effect of finite shear layer thickness on the
dynamo wave frequency is analoguous to the effect of finite water depth on
surface gravity waves. The model contains one free parameter which is fixed by
the requirement that a solution should reproduce the helioseismically
determined thickness of the tachocline. In this case it is found that, in
addition to this solution, another steady solution exists, characterized by a
four times thicker tachocline and 4-5 times weaker magnetic fields. It is
tempting to relate the existence of this second solution to the occurrence of
grand minima in solar activity.Comment: 4 pages, 1 figure; Astr. Nachr., in pres
Discovery of magnetic fields in hot subdwarfs
We present initial results of a project to measure mean longitudinal magnetic
fields in a group of sdB/OB/O stars. The project was inspired by the discovery
of three super-metal-rich sdOB stars, each having metals (e.g. Ti, V) enhanced
by factors of 10^3 to 10^5. Similar behaviour is observed in chemically
peculiar A stars, where strong magnetic fields are responsible for the
enrichment. With this in mind, we obtained circularly polarised spectra of two
of the super-metal-rich sdOBs, two "normal" sdBs and two sdOs using FORS1 on
the ESO/VLT. By examining circular polarisation in the hydrogen Balmer lines
and in helium lines, we have detected magnetic fields with strengths of 1-2 kG
in most of our targets. This suggests that such fields are relatively common in
hot subdwarfs.Comment: 4 pages, to appear in White Dwarfs, eds. D. Koester, S. Moehler, ASP
Conf. serie
Magnetic deformation of the white dwarf surface structure
The influence of strong, large-scale magnetic fields on the structure and
temperature distribution in white dwarf atmospheres is investigated. Magnetic
fields may provide an additional component of pressure support, thus possibly
inflating the atmosphere compared to the non-magnetic case. Since the magnetic
forces are not isotropic, atmospheric properties may significantly deviate from
spherical symmetry. In this paper the magnetohydrostatic equilibrium is
calculated numerically in the radial direction for either for small deviations
from different assumptions for the poloidal current distribution. We generally
find indication that the scale height of the magnetic white dwarf atmosphere
enlarges with magnetic field strength and/or poloidal current strength. This is
in qualitative agreement with recent spectropolarimetric observations of
Grw+10\degr8247. Quantitatively, we find for e.g. a mean surface poloidal
field strength of 100 MG and a toroidal field strength of 2-10 MG an increase
of scale height by a factor of 10. This is indicating that already a small
deviation from the initial force-free dipolar magnetic field may lead to
observable effects. We further propose the method of finite elements for the
solution of the two-dimensional magnetohydrostatic equilibrium including
radiation transport in the diffusive approximation. We present and discuss
preliminary solutions, again indicating on an expansion of the magnetized
atmosphere.Comment: 14 pages with 14 figure
Break-up stage restoration in multifragmentation reactions
In the case of Xe+Sn at 32 MeV/nucleon multifragmentation reaction break-up
fragments are built-up from the experimentally detected ones using evaluations
of light particle evaporation multiplicities which thus settle fragment
internal excitation. Freeze-out characteristics are extracted from experimental
kinetic energy spectra under the assumption of full decoupling between fragment
formation and energy dissipated in different degrees of freedom. Thermal
kinetic energy is determined uniquely while for freeze-out volume - collective
energy a multiple solution is obtained. Coherence between the solutions of the
break-up restoration algorithm and the predictions of a multifragmentation
model with identical definition of primary fragments is regarded as a way to
select the true value. The broad kinetic energy spectrum of He is
consistent with break-up genesis of this isotope.Comment: 17 pages, 5 figure
Magnetic helicity fluxes in interface and flux transport dynamos
Dynamos in the Sun and other bodies tend to produce magnetic fields that
possess magnetic helicity of opposite sign at large and small scales,
respectively. The build-up of magnetic helicity at small scales provides an
important saturation mechanism. In order to understand the nature of the solar
dynamo we need to understand the details of the saturation mechanism in
spherical geometry. In particular, we want to understand the effects of
magnetic helicity fluxes from turbulence and meridional circulation. We
consider a model with just radial shear confined to a thin layer (tachocline)
at the bottom of the convection zone. The kinetic alpha owing to helical
turbulence is assumed to be localized in a region above the convection zone.
The dynamical quenching formalism is used to describe the build-up of mean
magnetic helicity in the model, which results in a magnetic alpha effect that
feeds back on the kinetic alpha effect. In some cases we compare with results
obtained using a simple algebraic alpha quenching formula. In agreement with
earlier findings, the magnetic alpha effect in the dynamical alpha quenching
formalism has the opposite sign compared with the kinetic alpha effect and
leads to a catastrophic decrease of the saturation field strength with
increasing magnetic Reynolds numbers. However, at high latitudes this quenching
effect can lead to secondary dynamo waves that propagate poleward due to the
opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and
from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &
Extracting Rotational Energy in Supernova Progenitors: Transient Poynting Flux Growth vs. Turbulent Dissipation
Observational evidence for anisotropy in supernovae (SN) may signal the
importance of angular momentum and differential rotation in the progenitors.
Free energy in differential rotation and rotation can be extracted magnetically
or via turbulent dissipation. The importance that magnetohydrodyamic jets and
coronae may play in driving SN motivates understanding large scale dynamos in
SN progenitors. We develop a dynamical large scale interface dynamo model in
which the differential rotation and rotation deplete both through Poynting flux
and turbulent diffusion. We apply the model to a differentially rotating core
surrounded by a convection zone of a SN progenitor from a initial 15
star. Unlike the Sun, the dynamo is transient because the differential rotation
is primarily due to the initial collapse. Up to erg can be
drained into time-integrated Poynting flux and heat, the relative fraction of
which depends on the relative amount of turbulence in the shear layer vs.
convection zone and the fraction of the shear layer into which the magnetic
field penetrates. Both sinks can help facilitate explosions and could lead to
different levels of anisotropy and pulsar kicks. In all cases, the poloidal
magnetic field is much weaker than the toroidal field, and the Poynting flux is
lower than previous estimates which invoke the magnitude of the total magnetic
energy. A signature of a large scale dynamo is that the oscillation of the
associated
Poynting flux on sec time scales, implying the same for the energy
delivery to a SN.Comment: 26 pages, submitted to New Astronomy (revised to incorporate the
depletion of shear energy via both turbulent diffusion and magnetic stress
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