90 research outputs found
Planetary Radio Interferometry and Doppler Experiment (PRIDE) Technique: a Test Case of the Mars Express Phobos Fly-by. 2. Doppler tracking: Formulation of observed and computed values, and noise budget
Context. Closed-loop Doppler data obtained by deep space tracking networks
(e.g., NASA's DSN and ESA's Estrack) are routinely used for navigation and
science applications. By "shadow tracking" the spacecraft signal, Earth-based
radio telescopes involved in Planetary Radio Interferometry and Doppler
Experiment (PRIDE) can provide open-loop Doppler tracking data when the
dedicated deep space tracking facilities are operating in closed-loop mode
only. Aims. We explain in detail the data processing pipeline, discuss the
capabilities of the technique and its potential applications in planetary
science. Methods. We provide the formulation of the observed and computed
values of the Doppler data in PRIDE tracking of spacecraft, and demonstrate the
quality of the results using as a test case an experiment with ESA's Mars
Express spacecraft. Results. We find that the Doppler residuals and the
corresponding noise budget of the open-loop Doppler detections obtained with
the PRIDE stations are comparable to the closed-loop Doppler detections
obtained with the dedicated deep space tracking facilities
Can the crystallization rate be independent from the crystallization enthalpy? The case of amorphous silicon
The crystallization enthalpy measured in a large series of amorphous silicon (a-Si) materials
varies within a factor of 2 from sample to sample (Kail et al 2011 Phys. Status Solidi RRL 5
361). According to the classical theory of nucleation, this variation should produce large
differences in the crystallization kinetics leading to crystallization temperatures and activation
energies exceeding 550 C and 1.7 eV, respectively, the âstandardâ values measured for a-Si
obtained by self-implantation. In contrast, the observed crystallization kinetics is very similar
for all the samples studied and has no correlation with the crystallization enthalpy. This
discrepancy has led us to propose that crystallization in a-Si begins in microscopic domains
that are almost identical in all samples, independently of their crystallization enthalpy.
Probably the existence of microscopic inhomogeneities also plays a crucial role in the
crystallization kinetics of other amorphous materials and glasses
Venus Express radio occultation observed by PRIDE
Context. Radio occultation is a technique used to study planetary atmospheres
by means of the refraction and absorption of a spacecraft carrier signal
through the atmosphere of the celestial body of interest, as detected from a
ground station on Earth. This technique is usually employed by the deep space
tracking and communication facilities (e.g., NASA's Deep Space Network (DSN),
ESA's Estrack). Aims. We want to characterize the capabilities of the Planetary
Radio Interferometry and Doppler Experiment (PRIDE) technique for radio
occultation experiments, using radio telescopes equipped with Very Long
Baseline Interferometry (VLBI) instrumentation. Methods. We conducted a test
with ESA's Venus Express (VEX), to evaluate the performance of the PRIDE
technique for this particular application. We explain in detail the data
processing pipeline of radio occultation experiments with PRIDE, based on the
collection of so-called open-loop Doppler data with VLBI stations, and perform
an error propagation analysis of the technique. Results. With the VEX test case
and the corresponding error analysis, we have demonstrated that the PRIDE setup
and processing pipeline is suited for radio occultation experiments of
planetary bodies. The noise budget of the open-loop Doppler data collected with
PRIDE indicated that the uncertainties in the derived density and temperature
profiles remain within the range of uncertainties reported in previous Venus'
studies. Open-loop Doppler data can probe deeper layers of thick atmospheres,
such as that of Venus, when compared to closed-loop Doppler data. Furthermore,
PRIDE through the VLBI networks around the world, provides a wide coverage and
range of large antenna dishes, that can be used for this type of experiments
The use of micro-XRD for the study of glaze color decorations
The compounds responsible for the colours and decorations in glass and glazed ceramics include:
colouring agents (transition metal ions), pigments (micro-and nano-precipitates of compounds
that either do not dissolve or recrystallize in the glassy matrix) and opacifiers (microcrystalline
compounds with high light scattering capability). Their composition, structure and range of
stability are highly dependent not only on the composition but also on the procedures followed to
obtain them. Chemical composition of the colorants and crystallites may be obtained by means of
SEM-EDX and WDX. Synchrotron Radiation micro-X-ray Diffraction has a small beam size
adequate (10 to 50 microns footprint size) to obtain the structural information of crystalline
compounds and high brilliance, optimal for determining the crystallites even when present in low
amounts. In addition, in glass decorations the crystallites often appear forming thin layers (from
10 to 100 micrometers thick) and they show a depth dependent composition and crystal structure.
Their nature and distribution across the glass/glazes decorations gives direct information on the
technology of production and stability and may be related to the color and appearance.
A selection of glass and glaze coloring agents and decorations are studied by means of SR-micro-
XRD and SEM-EDX including: manganese brown, antimony yellow, red copper lusters and cobalt
blue. The selection includes Medieval (Islamic, and Hispano Moresque) and renaissance tin
glazed ceramics from the 10th to the 17th century AD
Theoretical approach to two-dimensional traffic flow models
In this paper we present a theoretical analysis of a recently proposed
two-dimensional Cellular Automata model for traffic flow in cities with the
novel ingredient of turning capability. Numerical simulations of this model
show that there is a transition between a freely moving phase with high
velocity to a jammed state with low velocity. We study the dynamics of such a
model starting with the microscopic evolution equation, which will serve as a
basis for further analysis. It is shown that a kinetic approach, based on the
Boltzmann assumption, is able to provide a reasonably good description of the
jamming transition. We further introduce a space-time continuous
phenomenological model leading to a couple of partial differential equations
whose preliminary results agree rather well with the numerical simulations.Comment: 15 pages, REVTeX 3.0, 7 uuencoded figures upon request to
[email protected]
Two-Vierbein Formalism for String-Inspired Axionic Gravity
Using independent left and right vierbeins to describe graviton plus axion as
suggested by string mechanics, O(d,d) duality can be realized linearly.Comment: 14 pg., (uuencoded dvi file; fixed uuencoding so file is unprotected)
ITP-SB-93-
Search for Duality Symmetries in p-Brane Theories
The requirement of an duality symmetry, mixing the worldvolume field
equations with Bianchi identities, leads to a highly nonlinear equation
involving the transformation parameters and certain worldvolume currents. In
general, this equation seems to admit a solution only for a two parameter
subgroup of the seeked . These transformations also leave invariant the
first class constraints generating the worldvolume reparametrizations. In the
special case of --branes in dimensions, the full is realized.Comment: 7 pages, plain tex, contribution to the Proc. of the Gursey Memorial
Conf. I on Strings and Symmetries, Istanbul, Turkey, June 6-10, 199
A monitoring campaign (2013-2020) of ESA's Mars Express to study interplanetary plasma scintillation
The radio signal transmitted by the Mars Express (MEX) spacecraft was
observed regularly between the years 2013-2020 at X-band (8.42 GHz) using the
European Very Long Baseline Interferometry (EVN) network and University of
Tasmania's telescopes. We present a method to describe the solar wind
parameters by quantifying the effects of plasma on our radio signal. In doing
so, we identify all the uncompensated effects on the radio signal and see which
coronal processes drive them. From a technical standpoint, quantifying the
effect of the plasma on the radio signal helps phase referencing for precision
spacecraft tracking. The phase fluctuation of the signal was determined for
Mars' orbit for solar elongation angles from 0 - 180 deg. The calculated phase
residuals allow determination of the phase power spectrum. The total electron
content (TEC) of the solar plasma along the line of sight is calculated by
removing effects from mechanical and ionospheric noises. The spectral index was
determined as which is in agreement with Kolomogorov's
turbulence. The theoretical models are consistent with observations at lower
solar elongations however at higher solar elongation (160 deg) we see the
observed values to be higher. This can be caused when the uplink and downlink
signals are positively correlated as a result of passing through identical
plasma sheets.Comment: The paper has 13 figures and one table. It has been accepted for
publication in PASA and the article will receive its DOI in a week's tim
Superspace Duality in Low-Energy Superstrings
We extend spacetime duality to superspace, including fermions in the
low-energy limits of superstrings. The tangent space is a curved, extended
superspace. The geometry is based on an enlarged coordinate space where the
vanishing of the d'Alembertian is as fundamental as the vanishing of the curl
of a gradient.Comment: 27 pg., (uuencoded compressed postscript file; NO CHANGES to output)
ITP-SB-93-2
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