4,477 research outputs found
Ellipsoidal corrections for geoid undulation computations
The computation of accurate geoid undulations is usually done combining potential coefficient information and terrestrial gravity data in a cap surrounding the computation point. In doing this a spherical approximation is made that can cause the errors that were investigated. The equations dealing with ellipsoidal corrections developed by Lelgemann and by Moritz were used to develop a computational procedure considering the ellipsoid as a reference surface. Terms in the resulting expression for the geoid undulation are identified as ellipsoidal correction terms. These equations were developed for the case where the Stokes function is used, and for the case where the modified Stokes function is used. For a cap of 20 deg the correction can reach -33 cm
Mean gravity anomalies and sea surface heights derived from GEOS-3 altimeter data
Approximately 2000 GEOS-3 altimeter arcs were analyzed to improve knowledge of the geoid and gravity field. An adjustment procedure was used to fit the sea surface heights (geoid undulations) in an adjustment process that incorporated cross-over constraints. The error model used for the fit was a one or two parameter model which was designed to remove altimeter bias and orbit error. The undulations on the adjusted arcs were used to produce geoid maps in 20 regions. The adjusted data was used to derive 301 5 degree equal area anomalies and 9995 1 x 1 degree anomalies in areas where the altimeter data was most dense, using least squares collocation techniques. Also emphasized was the ability of the altimeter data to imply rapid anomaly changes of up to 240 mgals in adjacent 1 x 1 degree blocks
Gridding of near vertical unrectified space photographs
Gridding of near vertical unrectified space photograph
Thermalization of Heavy Quarks in the Quark-Gluon Plasma
Charm- and bottom-quark rescattering in a Quark-Gluon Plasma (QGP) is
investigated with the objective of assessing the approach towards
thermalization. Employing a Fokker-Planck equation to approximate the collision
integral of the Boltzmann equation we augment earlier studies based on
perturbative parton cross sections by introducing resonant heavy-light quark
interactions. The latter are motivated by recent QCD lattice calculations which
indicate the presence of "hadronic" states in the QGP. We model these states by
colorless (pseudo-) scalar and (axial-) vector D- and B-mesons within a
heavy-quark effective theory framework. We find that the presence of these
states at moderate QGP temperatures substantially accelerates the kinetic
equilibration of c-quarks as compared to using perturbative interactions. We
also comment on consequences for -meson observables in ultra-relativistic
heavy-ion collisions.Comment: 14 pages, 5 figures, v2: Added references, v2: Added further
references, some typos correcte
Thermal Electromagnetic Radiation in Heavy-Ion Collisions
We review the potential of precise measurements of electromagnetic probes in
relativistic heavy-ion collisions for the theoretical understanding of strongly
interacting matter. The penetrating nature of photons and dileptons implies
that they can carry undistorted information about the hot and dense regions of
the fireballs formed in these reactions and thus provide a unique opportunity
to measure the electromagnetic spectral function of QCD matter as a function of
both invariant mass and momentum. In particular we report on recent progress on
how the medium modifications of the (dominant) isovector part of the vector
current correlator ( channel) can shed light on the mechanism of chiral
symmetry restoration in the hot and/or dense environment. In addition, thermal
dilepton radiation enables novel access to (a) the fireball lifetime through
the dilepton yield in the low invariant-mass window , and (b) the early temperatures of the fireball
through the slope of the invariant-mass spectrum in the intermediate-mass
region (). The investigation of
the pertinent excitation function suggests that the beam energies provided by
the NICA and FAIR projects are in a promising range for a potential discovery
of the onset of a first order phase transition, as signaled by a non-monotonous
behavior of both low-mass yields and temperature slopes.Comment: 5 pages, 4 figures; contribution to the NICA White Paper (EPJA
topical issue
Methods for the computation of detailed geoids and their accuracy
Two methods for the computation of geoid undulations using potential coefficients and 1 deg x 1 deg terrestrial anomaly data are examined. It was found that both methods give the same final result but that one method allows a more simplified error analysis. Specific equations were considered for the effect of the mass of the atmosphere and a cap dependent zero-order undulation term was derived. Although a correction to a gravity anomaly for the effect of the atmosphere is only about -0.87 mgal, this correction causes a fairly large undulation correction that was not considered previously. The accuracy of a geoid undulation computed by these techniques was estimated considering anomaly data errors, potential coefficient errors, and truncation (only a finite set of potential coefficients being used) errors. It was found that an optimum cap size of 20 deg should be used. The geoid and its accuracy were computed in the Geos 3 calibration area using the GEM 6 potential coefficients and 1 deg x 1 deg terrestrial anomaly data. The accuracy of the computed geoid is on the order of plus or minus 2 m with respect to an unknown set of best earth parameter constants
Accuracy of the determination of mean anomalies and mean geoid undulations from a satellite gravity field mapping mission
Improved knowledge of the Earth's gravity field was obtained from new and improved satellite measurements such as satellite to satellite tracking and gradiometry. This improvement was examined by estimating the accuracy of the determination of mean anomalies and mean undulations in various size blocks based on an assumed mission. In this report the accuracy is considered through a commission error due to measurement noise propagation and a truncation error due to unobservable higher degree terms in the geopotential. To do this the spectrum of the measurement was related to the spectrum of the disturbing potential of the Earth's gravity field. Equations were derived for a low-low (radial or horizontal separation) mission and a gradiometer mission. For a low-low mission of six month's duration, at an altitude of 160 km, with a data noise of plus or minus 1 micrometers sec for a four second integration time, we would expect to determine 1 deg x 1 deg mean anomalies to an accuracy of plus or minus 2.3 mgals and 1 deg x 1 deg mean geoid undulations to plus or minus 4.3 cm. A very fast Fortran program is available to study various mission configurations and block sizes
The determination of gravity anomalies from geoid heights using the inverse Stokes' formula, Fourier transforms, and least squares collocation
A numerical method for the determination of gravity anomalies from geoid heights is described using the inverse Stokes formula. This discrete form of the inverse Stokes formula applies a numerical integration over the azimuth and an integration over a cubic interpolatory spline function which approximates the step function obtained from the numerical integration. The main disadvantage of the procedure is the lack of a reliable error measure. The method was applied on geoid heights derived from GEOS-3 altimeter measurements in the calibration area of the GEOS-3 satellite
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