82 research outputs found
The notion of histogram of forces : a new way to represent the relative position of 2D-objects
The assessment of the directional spatial relations (such as "to the right of", "to the south of" . . .) between 2D-objects
relies generally on the computation of a histogram of angles, which is supposed to provide a reasonably good representation o f
the relative position of an object with regard to another . In this paper, we introduce the notion of histogram of forces . It generalize s
and supersedes the one of histogram of angles . The objects are handled as longitudinal sections (1D-entities) . It is thus possibl e
to benefit in full by the power of integral calculus and to ensure a rapid processing of raster data as well as of vector data unde r
explicit consideration of both angular and metric information .L'évaluation des relations spatiales directionnelles (telles que « à droite de », « au sud de » ...) entre objets 2D repose généralement sur la constitution d'un histogramme d'angles. Un tel histogramme est supposé constituer une bonne représentation de la position relative d'un objet par rapport à un autre. Dans cet article, nous introduisons la notion d'histogramme de forces. Elle généralise et supplante celle d'histogramme d'angles. La manipulation des objets (entités de dimension 2) est ramenée à celle de leurs sections longitudinales (entités de dimension 1), non pas à celle de points. Il est ainsi possible de bénéficier de la puissance du calcul intégral et d'assurer un traitement incomparablement plus rapide aussi bien de données rasters que vecteurs, tout en tenant compte explicitement aussi bien de l'information angulaire que de l'information métrique
Describing topological relationships in words: Refinements.
Abstract-In earlier work, we introduced a method for generating linguistic descriptions of the topological relationships between two-dimensional objects. The input to the system is a pair of rasterized objects and the output is a set of propositions about their spatial relationships expressed in natural language. The method relies on finding one or two Allen relations that best describe the relationships along a direction of major object interaction. In this paper, we address some of the issues related to the use of Allen relations for describing two-dimensional object configurations, and we propose two extensions in order to solve problems encountered in the original algorithm. Global subsethood-based information is used to suppress counterintuitive descriptions and an ancillary method for generating alternative descriptions is introduced
Gravitational wave detection using pulsars: status of the Parkes Pulsar Timing Array project
The first direct detection of gravitational waves may be made through
observations of pulsars. The principal aim of pulsar timing array projects
being carried out worldwide is to detect ultra-low frequency gravitational
waves (f ~ 10^-9 to 10^-8 Hz). Such waves are expected to be caused by
coalescing supermassive binary black holes in the cores of merged galaxies. It
is also possible that a detectable signal could have been produced in the
inflationary era or by cosmic strings. In this paper we review the current
status of the Parkes Pulsar Timing Array project (the only such project in the
Southern hemisphere) and compare the pulsar timing technique with other forms
of gravitational-wave detection such as ground- and space-based interferometer
systems.Comment: Accepted for publication in PAS
Timing stability of millisecond pulsars and prospects for gravitational-wave detection
Analysis of high-precision timing observations of an array of approx. 20
millisecond pulsars (a so-called "timing array") may ultimately result in the
detection of a stochastic gravitational-wave background. The feasibility of
such a detection and the required duration of this type of experiment are
determined by the achievable rms of the timing residuals and the timing
stability of the pulsars involved. We present results of the first long-term,
high-precision timing campaign on a large sample of millisecond pulsars used in
gravitational-wave detection projects. We show that the timing residuals of
most pulsars in our sample do not contain significant low-frequency noise that
could limit the use of these pulsars for decade-long gravitational-wave
detection efforts. For our most precisely timed pulsars, intrinsic
instabilities of the pulsars or the observing system are shown to contribute to
timing irregularities on a five-year timescale below the 100 ns level. Based on
those results, realistic sensitivity curves for planned and ongoing timing
array efforts are determined. We conclude that prospects for detection of a
gravitational-wave background through pulsar timing array efforts within five
years to a decade are good.Comment: 21 pages, 5 figures, submitted to MNRA
Shapiro Effect as a Possible Cause of the Low-Frequency Pulsar Timing Noise in Globular Clusters
A prolonged timing of millisecond pulsars has revealed low-frequency
uncorrelated noise, presumably of astrophysical origin, in the pulse arrival
time (PAT) residuals for some of them. In most cases, pulsars in globular
clusters show a low-frequency modulation of their rotational phase and spin
rate. The relativistic time delay of the pulsar signal in the curved space time
of randomly distributed and moving globular cluster stars (the Shapiro effect)
is suggested as a possible cause of this modulation.
Given the smallness of the aberration corrections that arise from the
nonstationarity of the gravitational field of the randomly distributed ensemble
of stars under consideration, a formula is derived for the Shapiro effect for a
pulsar in a globular cluster. The derived formula is used to calculate the
autocorrelation function of the low-frequency pulsar noise, the slope of its
power spectrum, and the behavior of the statistic that characterizes
the spectral properties of this noise in the form of a time function. The
Shapiro effect under discussion is shown to manifest itself for large impact
parameters as a low-frequency noise of the pulsar spin rate with a spectral
index of n=-1.8 that depends weakly on the specific model distribution of stars
in the globular cluster. For small impact parameters, the spectral index of the
noise is n=-1.5.Comment: 23 pages, 6 figure
Pulsar Timing and its Application for Navigation and Gravitational Wave Detection
Pulsars are natural cosmic clocks. On long timescales they rival the
precision of terrestrial atomic clocks. Using a technique called pulsar timing,
the exact measurement of pulse arrival times allows a number of applications,
ranging from testing theories of gravity to detecting gravitational waves. Also
an external reference system suitable for autonomous space navigation can be
defined by pulsars, using them as natural navigation beacons, not unlike the
use of GPS satellites for navigation on Earth. By comparing pulse arrival times
measured on-board a spacecraft with predicted pulse arrivals at a reference
location (e.g. the solar system barycenter), the spacecraft position can be
determined autonomously and with high accuracy everywhere in the solar system
and beyond. We describe the unique properties of pulsars that suggest that such
a navigation system will certainly have its application in future astronautics.
We also describe the on-going experiments to use the clock-like nature of
pulsars to "construct" a galactic-sized gravitational wave detector for
low-frequency (f_GW ~1E-9 - 1E-7 Hz) gravitational waves. We present the
current status and provide an outlook for the future.Comment: 30 pages, 9 figures. To appear in Vol 63: High Performance Clocks,
Springer Space Science Review
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
Precision measurement of the speed of propagation of neutrinos using the MINOS detectors
We report a two-detector measurement of the propagation speed of neutrinos over a baseline of 734 km. The measurement was made with the NuMI beam at Fermilab between the near and far MINOS detectors. The fractional difference between the neutrino speed and the speed of light is determined to be (v/c-1) = (1.0±1.1) × 10^−6, consistent with relativistic neutrinos
Precision measurement of the speed of propagation of neutrinos using the MINOS detectors
We report a two-detector measurement of the propagation speed of neutrinos over a baseline of 734 km. The measurement was made with the NuMI beam at Fermilab between the near and far MINOS detectors. The fractional difference between the neutrino speed and the speed of light is determined to be (v/c−1)=(1.0±1.1)×10−6, consistent with relativistic neutrinos
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