645 research outputs found
The BaR-SPOrt Experiment
BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation Observations) is an
experiment to measure the linearly polarized emission of sky patches at 32 and
90 GHz with sub-degree angular resolution. It is equipped with high sensitivity
correlation polarimeters for simultaneous detection of both the U and Q stokes
parameters of the incident radiation. On-axis telescope is used to observe
angular scales where the expected polarization of the Cosmic Microwave
Background (CMBP) peaks. This project shares most of the know-how and
sophisticated technology developed for the SPOrt experiment onboard the
International Space Station. The payload is designed to flight onboard long
duration stratospheric balloons both in the Northern and Southern hemispheres
where low foreground emission sky patches are accessible. Due to the weakness
of the expected CMBP signal (in the range of microK), much care has been spent
to optimize the instrument design with respect to the systematics generation,
observing time efficiency and long term stability. In this contribution we
present the instrument design, and first tests on some components of the 32 GHz
radiometer.Comment: 12 pages, 10 figures, Astronomical Telescopes and Instrumentation
(Polaimetry in Astronomy) Hawaii August 2002 SPIE Meetin
Current Status and Perspectives of Cosmic Microwave Background Observations
Measurements of the cosmic microwave background (CMB) radiation provide a
unique opportunity for a direct study of the primordial cosmic plasma at
redshift z ~1000. The angular power spectra of temperature and polarisation
fluctuations are powerful observational objectives as they encode information
on fundamental cosmological parameters and on the physics of the early
universe. A large number of increasingly ambitious balloon-borne and
ground-based experiments have been carried out following the first detection of
CMB anisotropies by COBE-DMR, probing the angular power spectrum up to high
multipoles. The recent data from WMAP provide a new major step forward in
measurements percision. The ESA mission Planck Surveyor, to be launched in
2007, is the third-generation satellite devoted to CMB imaging. Planck is
expected to extract the full cosmological information from temperature
anisotropies and to open up new fronteers in the CMB field.Comment: 6 pages, 1 figure, to appear in "Proc of International Symposium on
Plasmas in the Laboratory and in the Universe: new insights and new
challenges", September 16-19, 2003, Como, Ital
Potential of Radiotelescopes for Atmospheric Line Observations: I. Observation Principles and Transmission Curves for Selected Sites
Existing and planned radiotelescopes working in the millimetre (mm) and
sub-millimetre wavelengths range provide the possibility to be used for
atmospheric line observations. To scrutinize this potential, we outline the
differences and similarities in technical equipment and observing techniques
between ground-based aeronomy mm-wave radiometers and radiotelescopes.
Comprehensive tables summarizing the technical characteristics of existing and
future (sub)-mm radiotelescopes are given. The advantages and disadvantages
using radiotelescopes for atmospheric line observations are discussed. In view
of the importance of exploring the sub-mm and far-infrared wavelengths range
for astronomical observations and atmospheric sciences, we present model
calculations of the atmospheric transmission for selected telescope sites
(DOME-C/Antarctica, ALMA/Chajnantor, JCMT and CSO on Mauna Kea/Hawaii,
KOSMA/Swiss Alpes) for frequencies between 0 and 2000 GHz (150 micron) and
typical atmospheric conditions using the forward model MOLIERE (version~5). For
the DOME-C site, the transmission over a larger range of up to 10 THz (30
micron) is calculated in order to demonstrate the quality of an earth-bound
site for mid-IR observations. All results are available on a dedicated webpage
(http://transmissioncurves.free.fr)Comment: Planetary and Space Science accepted (in press), see also website
http://transmissioncurves.free.f
Imaging the first light: experimental challenges and future perspectives in the observation of the Cosmic Microwave Background Anisotropy
Measurements of the cosmic microwave background (CMB) allow high precision
observation of the Last Scattering Surface at redshift 1100. After the
success of the NASA satellite COBE, that in 1992 provided the first detection
of the CMB anisotropy, results from many ground-based and balloon-borne
experiments have showed a remarkable consistency between different results and
provided quantitative estimates of fundamental cosmological properties. During
2003 the team of the NASA WMAP satellite has released the first improved
full-sky maps of the CMB since COBE, leading to a deeper insight into the
origin and evolution of the Universe. The ESA satellite Planck, scheduled for
launch in 2007, is designed to provide the ultimate measurement of the CMB
temperature anisotropy over the full sky, with an accuracy that will be limited
only by astrophysical foregrounds, and robust detection of polarisation
anisotropy. In this paper we review the experimental challenges in high
precision CMB experiments and discuss the future perspectives opened by second
and third generation space missions like WMAP and Planck.Comment: To be published in "Recent Research Developments in Astronomy &
Astrophysics Astrophysiscs" - Vol I
Measuring GNSS ionospheric total electron content at Concordia, and application to L-band radiometers
In the framework of the project BIS - Bipolar Ionospheric Scintillation and Total Electron Content Monitoring, the ISACCO-DMC0 and ISACCO-DMC1 permanent monitoring stations were installed in 2008.
The principal scope of the stations is to measure the ionospheric total electron content (TEC) and to monitor the ionospheric scintillations, using high-sampling-frequency global positioning system (GPS) ionospheric scintillation and TEC monitor (GISTM) receivers. The disturbances that
the ionosphere can induce on the electromagnetic signals emitted by the
Global Navigation Satellite System constellations are due to the presence
of electron density anomalies in the ionosphere, which are particularly frequent
at high latitudes, where the upper atmosphere is highly sensitive to
perturbations coming from outer space. With the development of present and future low-frequency space-borne microwave missions (e.g., Soil Moisture and Ocean Salinity [SMOS], Aquarius, and Soil Moisture Active
Passive missions), there is an increasing need to estimate the effects of the ionosphere on the propagation of electromagnetic waves that affects
satellite measurements. As an example, how the TEC data collected at Concordia station are useful for the calibration of the European Space Agency SMOS data within the framework of an experiment promoted by the European Space Agency (known as DOMEX) will be discussed.
The present report shows the ability of the GISTM station to monitor ionospheric scintillation and TEC, which indicates that only the use of continuous GPS measurements can provide accurate information on TEC
variability, which is necessary for continuous calibration of satellite data
Weekly Gridded Aquarius L-band Radiometer-Scatterometer Observations and Salinity Retrievals over the Polar Regions - Part 2: Initial Product Analysis
Following the development and availability of Aquarius weekly polar-gridded products, this study presents the spatial and temporal radiometer and scatterometer observations at L band (frequency1.4 GHz) over the cryosphere including the Greenland and Antarctic ice sheets, sea ice in both hemispheres, and over sub-Arctic land for monitoring the soil freeze-thaw state. We provide multiple examples of scientific applications for the L-band data over the cryosphere. For example, we show that over the Greenland Ice Sheet, the unusual 2012 melt event lead to an L-band brightness temperature (TB) sustained decrease of 5 K at horizontal polarization. Over the Antarctic ice sheet, normalized radar cross section (NRCS) observations recorded during ascending and descending orbits are significantly different, highlighting the anisotropy of the ice cover. Over sub-Arctic land, both passive and active observations show distinct values depending on the soil physical state (freeze-thaw). Aquarius sea surface salinity (SSS) retrievals in the polar waters are also presented. SSS variations could serve as an indicator of fresh water input to the ocean from the cryosphere, however the presence of sea ice often contaminates the SSS retrievals, hindering the analysis. The weekly grided Aquarius L-band products used a redistributed by the US Snow and Ice Data Center at http:nsidc.orgdataaquariusindex.html, and show potential for cryospheric studies
The Large-Scale Polarization Explorer (LSPE)
The LSPE is a balloon-borne mission aimed at measuring the polarization of
the Cosmic Microwave Background (CMB) at large angular scales, and in
particular to constrain the curl component of CMB polarization (B-modes)
produced by tensor perturbations generated during cosmic inflation, in the very
early universe. Its primary target is to improve the limit on the ratio of
tensor to scalar perturbations amplitudes down to r = 0.03, at 99.7%
confidence. A second target is to produce wide maps of foreground polarization
generated in our Galaxy by synchrotron emission and interstellar dust emission.
These will be important to map Galactic magnetic fields and to study the
properties of ionized gas and of diffuse interstellar dust in our Galaxy. The
mission is optimized for large angular scales, with coarse angular resolution
(around 1.5 degrees FWHM), and wide sky coverage (25% of the sky). The payload
will fly in a circumpolar long duration balloon mission during the polar night.
Using the Earth as a giant solar shield, the instrument will spin in azimuth,
observing a large fraction of the northern sky. The payload will host two
instruments. An array of coherent polarimeters using cryogenic HEMT amplifiers
will survey the sky at 43 and 90 GHz. An array of bolometric polarimeters,
using large throughput multi-mode bolometers and rotating Half Wave Plates
(HWP), will survey the same sky region in three bands at 95, 145 and 245 GHz.
The wide frequency coverage will allow optimal control of the polarized
foregrounds, with comparable angular resolution at all frequencies.Comment: In press. Copyright 2012 Society of Photo-Optical Instrumentation
Engineers. One print or electronic copy may be made for personal use only.
Systematic reproduction and distribution, duplication of any material in this
paper for a fee or for commercial purposes, or modification of the content of
the paper are prohibite
Toward vicarious calibration of microwave remote-sensing satellites in arid environments
The Soil Moisture and Ocean Salinity (SMOS)
satellite marks the commencement of dedicated global surface
soil moisture missions, and the first mission to make passive microwave observations at L-band. On-orbit calibration is an essential part of the instrument calibration strategy, but on-board beam-filling targets are not practical for such large apertures. Therefore, areas to serve as vicarious calibration targets need to be identified. Such sites can only be identified through field
experiments including both in situ and airborne measurements. For this purpose, two field experiments were performed in central Australia. Three areas are studied as follows: 1) Lake Eyre, a typically dry salt lake; 2) Wirrangula Hill, with sparse vegetation and a dense cover of surface rock; and 3) Simpson Desert, characterized by dry sand dunes. Of those sites, only Wirrangula
Hill and the Simpson Desert are found to be potentially
suitable targets, as they have a spatial variation in brightness temperatures of <4 K under normal conditions. However, some limitations are observed for the Simpson Desert, where a bias of 15 K in vertical and 20 K in horizontal polarization exists between model predictions and observations, suggesting a lack of understanding of the underlying physics in this environment.
Subsequent comparison with model predictions indicates a SMOS bias of 5 K in vertical and 11 K in horizontal polarization, and an unbiased root mean square difference of 10 K in both polarizations for Wirrangula Hill. Most importantly, the SMOS observations show that the brightness temperature evolution is dominated by regular seasonal patterns and that precipitation events have only little impact
- …