33,407 research outputs found
Accurate Galactic 21-cm H I measurements with the NRAO Green Bank Telescope
Aims: We devise a data reduction and calibration system for producing
highly-accurate 21-cm H I spectra from the Green Bank Telescope (GBT) of the
NRAO.
Methods: A theoretical analysis of the all-sky response of the GBT at 21 cm
is made, augmented by extensive maps of the far sidelobes. Observations of
radio sources and the Moon are made to check the resulting aperture and main
beam efficiencies.
Results: The all-sky model made for the response of the GBT at 21 cm is used
to correct for "stray" 21-cm radiation reaching the receiver through the
sidelobes rather than the main beam. This reduces systematic errors in 21-cm
measurements by about an order of magnitude, allowing accurate 21-cm H I
spectra to be made at about 9' angular resolution with the GBT. At this
resolution the procedures discussed here allow for measurement of total
integrated Galactic H I line emission, W, with errors of 3 K km s^-1,
equivalent to errors in optically thin N_HI of 5 x 10^18 cm^-2.Comment: 49 pages, 25 figures; A&A, in pres
Investigation of punctured LDPC codes and time-diversity on free-space optical links
In this paper, we analyze the behavior of DVB-S2 un-punctured/punctured low-density parity-check (LDPC) coded on-off-keying (OOK) under atmospheric turbulence conditions by utilizing time diversity. A performance characterization between these schemes is evaluated, where punctured LDPC code provides a penalty of around 0.1 to 0.2 dB against unpunctured LDPC codes but still provides a coding gain of several dB against uncoded OOK. The combination of channel coding and a bit interleaver results in performance improvements in turbulence conditions. For example, such a system can achieve a coding gain of 16.7 dB in moderate turbulence conditions compared to uncoded OOK
Harmonization of space-borne infra-red sensors measuring sea surface temperature
Sea surface temperature (SST) is observed by a constellation of sensors, and SST retrievals
are commonly combined into gridded SST analyses and climate data records (CDRs). Differential
biases between SSTs from different sensors cause errors in such products, including feature artefacts.
We introduce a new method for reducing differential biases across the SST constellation, by reconciling
the brightness temperature (BT) calibration and SST retrieval parameters between sensors. We use the
Advanced Along-Track Scanning Radiometer (AATSR) and the Sea and Land Surface Temperature
Radiometer (SLSTR) as reference sensors, and the Advanced Very High Resolution Radiometer
(AVHRR) of the MetOp-A mission to bridge the gap between these references. Observations across a
range of AVHRR zenith angles are matched with dual-view three-channel skin SST retrievals from
the AATSR and SLSTR. These skin SSTs act as the harmonization reference for AVHRR retrievals
by optimal estimation (OE). Parameters for the harmonized AVHRR OE are iteratively determined,
including BT bias corrections and observation error covariance matrices as functions of water-vapor
path. The OE SSTs obtained from AVHRR are shown to be closely consistent with the reference sensor
SSTs. Independent validation against drifting buoy SSTs shows that the AVHRR OE retrieval is stable
across the reference-sensor gap. We discuss that this method is suitable to improve consistency across
the whole constellation of SST sensors. The approach will help stabilize and reduce errors in future
SST CDRs, as well as having application to other domains of remote sensing
Titan's atmosphere as observed by Cassini/VIMS solar occultations: CH, CO and evidence for CH absorption
We present an analysis of the VIMS solar occultations dataset, which allows
us to extract vertically resolved information on the characteristics of Titan's
atmosphere between 100-700 km with a characteristic vertical resolution of 10
km. After a series of data treatment procedures, 4 occultations out of 10 are
retained. This sample covers different seasons and latitudes of Titan. The
transmittances show clearly the evolution of the haze and detect the detached
layer at 310 km in Sept. 2011 at mid-northern latitudes. Through the inversion
of the transmission spectra with a line-by-line radiative transfer code we
retrieve the vertical distribution of CH and CO mixing ratio. The two
methane bands at 1.4 and 1.7 {\mu}m are always in good agreement and yield an
average stratospheric abundance of %. This is significantly less
than the value of 1.48% obtained by the GCMS/Huygens instrument. The analysis
of the residual spectra after the inversion shows that there are additional
absorptions which affect a great part of the VIMS wavelength range. We
attribute many of these additional bands to gaseous ethane, whose near-infrared
spectrum is not well modeled yet. Ethane contributes significantly to the
strong absorption between 3.2-3.5 {\mu}m that was previously attributed only to
C-H stretching bands from aerosols. Ethane bands may affect the surface windows
too, especially at 2.7 {\mu}m. Other residual bands are generated by stretching
modes of C-H, C-C and C-N bonds. In addition to the C-H stretch from aliphatic
hydrocarbons at 3.4 {\mu}m, we detect a strong and narrow absorption at 3.28
{\mu}m which we tentatively attribute to the presence of PAHs in the
stratosphere. C-C and C-N stretching bands are possibly present between 4.3-4.5
{\mu}m. Finally, we obtain the CO mixing ratio between 70-170 km. The average
result of ppm is in good agreement with previous studies.Comment: 51 pages, 28 figure
Information content of ozone retrieval algorithms
The algorithms are characterized that were used for production processing by the major suppliers of ozone data to show quantitatively: how the retrieved profile is related to the actual profile (This characterizes the altitude range and vertical resolution of the data); the nature of systematic errors in the retrieved profiles, including their vertical structure and relation to uncertain instrumental parameters; how trends in the real ozone are reflected in trends in the retrieved ozone profile; and how trends in other quantities (both instrumental and atmospheric) might appear as trends in the ozone profile. No serious deficiencies were found in the algorithms used in generating the major available ozone data sets. As the measurements are all indirect in someway, and the retrieved profiles have different characteristics, data from different instruments are not directly comparable
GREAT/SOFIA atmospheric calibration
The GREAT observations need frequency-selective calibration across the
passband for the residual atmospheric opacity at flight altitude. At these
altitudes the atmospheric opacity has both narrow and broad spectral features.
To determine the atmospheric transmission at high spectral resolution, GREAT
compares the observed atmospheric emission with atmospheric model predictions,
and therefore depends on the validity of the atmospheric models. We discusse
the problems identified in this comparison with respect to the observed data
and the models, and describe the strategy used to calibrate the science data
from GREAT/SOFIA during the first observing periods.Comment: 14 pages, 4 figure
High resolution forecast models of water vapour over mountains: comparison of results from the UM and MERIS
Propagation delay due to variable tropospheric water vapor (WV) is one of the most intractable problems for radar interferometry, particularly over mountains. The WV field can be simulated by an atmospheric model, and the difference between the two fields is used to correct the radar interferogram. Here, we report our use of the U.K. Met Office Unified Model in a nested mode to produce high-resolution forecast fields for the 3-km-high Mount Etna volcano. The simulated precipitablewater field is validated against that retrieved from the Medium Resolution Imaging Spectrometer (MERIS) radiometer on the Envisat satellite, which has a resolution of 300 m. Two case studies, one from winter (November 24, 2004) and one from summer (June 25, 2005), show that the mismatch between the model and the MERIS fields (rms = 1.1 and 1.6 mm, respectively) is small. One of the main potential sources of error in the models is the timing of the WV field simulation. We show that long-wavelength upper tropospheric troughs of low WV could be identified in both the model output and Meteosat WV imagery for the November 24, 2004 case and used to choose the best time of model output. © 2007 IEEE
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