6,456 research outputs found
An operational satellite scatterometer for wind vector measurements over the ocean
Performance requirements and design characteristics of a microwave scatterometer wind sensor for measuring surface winds over the oceans on a global basis are described. Scatterometer specifications are developed from user requirements of wind vector measurement range and accuracy, swath width, resolution cell size and measurement grid spacing. A detailed analysis is performed for a baseline fan-beam scatterometer design, and its performance capabilities for meeting the SeaSat-A user requirements. Various modes of operation are discussed which will allow the resolution of questions concerning the effects of sea state on the scatterometer wind sensing ability and to verify design boundaries of the instrument
A traveling-wave THz photomixer based on angle-tuned phase matching
A traveling-wave THz photomixer based on a free-space optical-THz phase-matching scheme is proposed. A dc-biased coplanar strip line fabricated on low-temperature-grown GaAs serves as the active area of the device, and is illuminated by two noncollinear laser beams which generate interference fringes that are accompanied by THz waves. The device with the laser-power-handling capability over 300 mW and a 3-dB bandwidth of 1.8 THz was experimentally demonstrated. The results show that traveling-wave photomixers have the potential to surpass small-area designs
A 3D radiative transfer framework: I. non-local operator splitting and continuum scattering problems
We describe a highly flexible framework to solve 3D radiation transfer
problems in scattering dominated environments based on a long characteristics
piece-wise parabolic formal solution and an operator splitting method. We find
that the linear systems are efficiently solved with iterative solvers such as
Gauss-Seidel and Jordan techniques. We use a sphere-in-a-box test model to
compare the 3D results to 1D solutions in order to assess the accuracy of the
method. We have implemented the method for static media, however, it can be
used to solve problems in the Eulerian-frame for media with low velocity
fields.Comment: A&A, in press. 14 pages, 19 figures. Full resolution figures
available at ftp://phoenix.hs.uni-hamburg.de/preprints/3DRT_paper1.pdf HTML
version (low res figures) at
http://hobbes.hs.uni-hamburg.de/~yeti/PAPERS/3drt_paper1/index.htm
Frequency and time profiles of metric wave isolated Type I solar noise storm bursts at high spectral and temporal resolution
Type I noise storms constitute a sizeable faction of the active-Sun radio
emission component. Observations of isolated instances of such bursts, in the
swept-frequency-mode at metric wavelengths, have remained sparse, with several
unfilled regions in the frequency coverage. Dynamic spectra of the burst
radiation, in the 30 - 130 MHz band, obtained from the recently commissioned
digital High Resolution Spectrograph (HRS) at the Gauribidanur Radio
Observatory, on account of the superior frequency and time resolution, have
unravelled in explicit detail the temporal and spectral profiles of isolated
bursts. Apart from presenting details on their fundamental emission features,
the time and frequency profile symmetry, with reference to custom-specific
Gaussian distributions, has been chosen as the nodal criterion to statistically
explain the state of the source regions in the vicinity of magnetic
reconnections, the latent excitation agent that contributes to plasma wave
energetics, and the quenching phenomenon that causes damping of the burst
emission.Comment: 9 pages 7 black and white / grey-scale figures (inclusive of 3
composite). MNRAS - accepte
Doppler Aliasing Reduction in Wide-Angle Synthetic Aperture Radar Using Phase Modulated Random Stepped-Frequency Waveforms
This research effort examines the theory, application and results of side-looking airborne radar operation in hot clutter. Hot clutter is an electronic counter-measure used to degrade the performance of airborne radar. Hot clutter occurs by illuminating the ground with an airborne jammer at some velocity, azimuth, elevation, and range from the airborne radar. When the received RCS scattered hot clutter waveform is perfectly coherent with the radar waveform, the radar believes the returns created by the hot clutter jammer resulted from the transmitting radar. Hot clutter degrades radar performance at locations in azimuth and Doppler. The effect of hot clutter is examined for side-looking airborne radar using adaptive and non-adaptive processing. Factored Time Space and Joint Domain Localized adaptive filters are shown to improve radar performance 32 to 36 dB per element per pulse, respectively, over non-adaptive processing in the mainbeam jammer normalized Doppler location when the jammer is not in the radar look direction in azimuth
Disks, Tori, and Cocoons: Emission and Absorption Diagnostics of AGN Environments
One of the most important problems in the study of active galaxies is
understanding the detailed geometry, physics, and evolution of the central
engines and their environments. The leading models involve an accretion disk
and torus structure around a central dense object, thought to be a supermassive
black hole. Gas found in the environment of AGN is associated with different
structures: molecular accretion disks, larger scale atomic tori, ionized and
neutral "cocoons" in which the nuclear regions can be embedded. All of them can
be studied at radio wavelengths by various means. Here, we summarize the work
that has been done to date in the radio band to characterize these structures.
Much has been learned about the central few parsecs of AGN in the last few
decades with contemporary instruments but the picture remains incomplete. In
order to be able to define a more accurate model of this region, significant
advances in sensitivity, spectral and angular resolution, and bandpass
stability are required. The necessary advances will only be provided by the
Square Kilometer Array and we discuss the possibilities that these dramatic
improvements will open for the study of the gas in the central region of AGN.Comment: To appear in "Science with the Square Kilometer Array," eds. C.
Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam); 17
pages, 7 figures (four of them in separate gif/tif files) The full paper with
high resolution images can be downloaded from
http://www.astron.nl/~morganti/Papers/AGNenvironment.ps.g
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