410 research outputs found
Orbit determination of space objects based on sparse optical data
While building up a catalog of Earth orbiting objects, if the available
optical observations are sparse, not deliberate follow ups of specific objects,
no orbit determination is possible without previous correlation of observations
obtained at different times. This correlation step is the most computationally
intensive, and becomes more and more difficult as the number of objects to be
discovered increases. In this paper we tested two different algorithms (and the
related prototype software) recently developed to solve the correlation problem
for objects in geostationary orbit (GEO), including the accurate orbit
determination by full least squares solutions with all six orbital elements.
Because of the presence in the GEO region of a significant subpopulation of
high area to mass objects, strongly affected by non-gravitational
perturbations, it was actually necessary to solve also for dynamical parameters
describing these effects, that is to fit between 6 and 8 free parameters for
each orbit. The validation was based upon a set of real data, acquired from the
ESA Space Debris Telescope (ESASDT) at the Teide observatory (Canary Islands).
We proved that it is possible to assemble a set of sparse observations into a
set of objects with orbits, starting from a sparse time distribution of
observations, which would be compatible with a survey capable of covering the
region of interest in the sky just once per night. This could result in a
significant reduction of the requirements for a future telescope network, with
respect to what would have been required with the previously known algorithm
for correlation and orbit determination.Comment: 20 pages, 8 figure
Off-diagonal generalized vector dominance in DIS and QCD
We review the generalized vector dominance (GVD) approach to DIS at small
values of the scaling variable, x. In particular, we concentrate on a recent
formulation of GVD that explicitly incorporates the configuration of the
gamma^* -> q qbar transition and a QCD-inspired ansatz for the (qqbar)p
scattering amplitude. The destructive interference, originally introduced in
off-diagonal GVD is traced back to the generic strcuture of two-gluon exchange.
Asymptotically, the transverse photoabsorption cross section behaves as (ln
Q^2)/Q^2, implying a logarithmic violation of scaling for F_2, while the
longitudinal-to-transverse ratio decreases as 1/\ln Q^2. We also briefly
comment on vector-meson production.Comment: 21 pages latex, 12 postscript files for figures, talk given at the
XXI School of Theoretical Physics, Ustron, Poland, September 199
Vector-Meson Electroproduction from Generalized Vector Dominance
Including destructively interfering off-diagonal transitions of
diffraction-dissociation type, we arrive at a formulation of GVD for exclusive
vector-meson production in terms of a continuous spectral representation of
dipole form. The transverse cross-section sigma_T for gamma* p -> V p behaves
asymptotically as 1/Q^4, while R_V = sigma_L/sigma_T becomes asymptotically
constant. Contributions violating s-channel helicity conservation stay at the
10-15% level established in low-energy photoproduction and diffractive
hadron--hadron interactions. The data for phi- and rho-meson production for 0
\lsim Q^2 \lsim 20 GeV^2 from HERA are found to be in agreement with these
predictions.Comment: 16 pages, latex, 3 figure
The Color Dipole Picture of low-x DIS: Model-Independent and Model-Dependent Results
We present a detailed examination of the color-dipole picture (CDP) of
low- deep inelastic scattering. We discriminate model-independent results,
not depending on a specific parameterization of the dipole cross section, from
model-dependent ones. The model-independent results include the ratio of the
longitudinal to the transverse photoabsorption cross section at large , or
equivalently the ratio of the longitudinal to the unpolarized proton structure
function, , as well as the low- scaling
behavior of the total photoabsorption cross section as for
, and as for .
Here, denotes the low- scaling variable, with being
the saturation scale. The model-independent analysis also implies
at any for asymptotically
large energy, . Consistency with pQCD evolution determines the underlying
gluon distribution and the numerical value of in the expression
for the saturation scale, . In the
model-dependent analysis, by restricting the mass of the actively contributing
fluctuations by an energy-dependent upper bound, we extend the
validity of the color-dipole picture to . The
theoretical results agree with the world data on DIS for .Comment: 77 pages, 30 figure
Diffractive production and the total cross section in deep inelastic scattering
We explore the consequences for diffractive production, gamma* p --> X p, in
deep inelastic scattering at low values of x\sim Q^2/W^2 <<1 that follow from
our recent representation of the total photoabsorption cross section,
sigma_{gamma* p}, in the generalized vector dominance/ color dipole
picture(GVD/CDP) that is based on the generic structure of the
two-gluon-exchange from QCD. Sum rules are derived that relate the transverse
and the longitudinal (virtual) photoabsorption cross section to diffractive
forward production of q q-bar states that carry photon quantum numbers
("elastic diffraction"). Agreement with experiment in the W^2 and Q^2
dependence is found for M_X^2/Q^2<<1, where M_X is the mass of the produced
system X. An additional component ("inelastic diffraction"), not actively
contributing to the forward Compton amplitude, is needed for diffractive
production at high values of M_X. Our previous theoretical representation of
the total photoabsorption cross section sigma_{gamma* p}=sigma_{gamma* p}(eta),
in terms of the scaling variable eta=(Q^2+m_0^2)/Lambda^2(W^2) is extended to
include the entire kinematic domain, x==0, where
scaling in eta holds experimentally.Comment: 19 pages with 4 figures,(eps and ps files), Late
Polarization Observables for Two-Pion Production off the Nucleon
We develop polarization observables for the processes
and , using both a helicity and hybrid helicity-transversity
basis. Such observables are crucial if processes that produce final states
consisting of a spin-1/2 baryon and two pseudoscalar mesons are to be fully
exploited for baryon spectroscopy. We derive relationships among the
observables, as well as inequalities that they must satisfy. We also discuss
the observables that must be measured in `complete' experiments, and briefly
examine the prospects for measurement of some of these observables in the near
future.Comment: 20 pages, using revtex
The Influence of Suction on Energy Distribution in Fully Established Turbulent Pipe Flow
The influence of suction on the structure of turbulence in fully established pipe flow has been experimentally investigated. Extensive hot-wire measurements indicate major alterations in the turbulent energy distribution, particularly in the wall layers result from mild suction rates, less than 5 percent of the mean mass flow
Deep inelastic scattering and "elastic" diffraction
We examine the total cross section of virtual photons on protons,
, at low and its
connection with ``elastic'' diffractive production in the two-gluon exchange dynamics for the virtual forward
Compton scattering amplitude. Solely based on the generic structure of
two-gluon exchange, we establish that the cross section is described by the
(imaginary part of the) amplitude for forward scattering of vector
states, . The
generalized vector dominance/color dipole picture (GVD/CDP) is accordingly
established to only rest on the two-gluon-exchange generic structure. This is
explicitly seen by the sum rules that allow one to directly relate the total
cross section to the cross section for elastic diffractive forward production,
, of vector states.Comment: 24 pages, latex file with three eps figures. BI-TP 2002/2
An optical survey for space debris on highly eccentric and inclined MEO orbits
Optical surveys for space debris in high-altitude orbits have been conducted since more than ten years. Originally these efforts concentrated mainly on the geostationary region (GEO). Corresponding observation strategies, processing techniques and cataloguing approaches have been developed and successfully applied. The ESA GEO surveys, e.g., resulted in the detection of a significant population of small-size debris and later in the discovery of high area-to-mass ratio objects in GEO-like orbits. Comparably less experience
(both, in terms of practical observation and strategy definition) is available for eccentric orbits that (at least partly) are in the MEO region, in particular for the Molniya-type orbits. Different survey and follow-up strategies for searching space debris objects in highly-eccentric MEO orbits, and to acquire orbits
which are sufficiently accurate to catalog such objects and to maintain their orbits over longer time spans were developed. Simulations were performed to compare the performance of different survey and cataloguing strategies. Eventually, optical observations were conducted in the framework of an ESA study using ESA’s Space Debris Telescope (ESASDT) the 1-m Zeiss telescope located at the Optical Ground Station (OGS) at the Teide Observatory at Tenerife, Spain. Thirteen nights of surveys of Molniya-type orbits were performed between January and August 2013. Eventually 255 surveys were
performed during these thirteen nights corresponding to about 47 h of observations. In total 30 uncorrelated faint objects were discov-
ered. On average one uncorrelated object was found every 100 min of observations. Some of these objects show a considerable brightness
variation and have a high area-to-mass ratio as determined in the orbit estimation process
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