21,620 research outputs found
Application of remote sensing in estimating evapotranspiration in the Platte river basin
A 'resistance model' and a mass transport model for estimating evapotranspiration (ET) were tested on large fields of naturally subirrigated alfalfa. Both models make use of crop canopy temperature data. Temperature data were obtained with an IR thermometer and with leaf thermocouples. A Bowen ratio-energy balance (BREB) model, adjusted to account for underestimation of ET during periods of strong sensible heat advection, was used as the standard against which the resistance and mass transport models were compared. Daily estimates by the resistance model were within 10% of estimates made by the BREB model. Daily estimates by the mass transport model did not agree quite as well. Performance was good on clear and cloudy days and also during periods of non-advection and strong advection of sensible heat. The performance of the mass transport and resistance models was less satisfactory for estimation of fluxes of latent heat for short term periods. Both models tended to overestimate at low LE fluxes
Properties of periodic arrays of symmetric complementary structuresand their application to grid amplifiers
Deschamps' theorem for n-terminal complementary structures is reviewed. An extension to Deschamps' theorem for a class of three-terminal bounded structures with one axis of symmetry is presented. It is shown that, for these structures, a simple relationship between the impedances of the odd mode of the original structure and the admittances of the even mode of the complementary structure exists, and that these modes are orthogonal. Using this, a self-complementary grid amplifier is designed and the measured results are presented
Self-Complementary Structures and Their Application in Grid Amplifiers
An extension to Deschamps’s theorem for a class of 3-terminal bounded structures with one axis of symmetry is presented. For these structures, a simple relationship between the impedance matrix of the odd mode excitation of the original structure and the admittance matrix of the even mode excitation of its complement exists. Using this, a self-complementary grid amplifier is designed and the measured results are presented
Resolution-enhanced Mapping Spectrometer
A familiar mapping spectrometer implementation utilizes two dimensional detector arrays with spectral dispersion along one direction and spatial along the other. Spectral images are formed by spatially scanning across the scene (i.e., push-broom scanning). For imaging grating and prism spectrometers, the slit is perpendicular to the spatial scan direction. For spectrometers utilizing linearly variable focal-plane-mounted filters the spatial scan direction is perpendicular to the direction of spectral variation. These spectrometers share the common limitation that the number of spectral resolution elements is given by the number of pixels along the spectral (or dispersive) direction. Resolution enhancement by first passing the light input to the spectrometer through a scanned etalon or Michelson is discussed. Thus, while a detector element is scanned through a spatial resolution element of the scene, it is also temporally sampled. The analysis for all the pixels in the dispersive direction is addressed. Several specific examples are discussed. The alternate use of a Michelson for the same enhancement purpose is also discussed. Suitable for weight constrained deep space missions, hardware systems were developed including actuators, sensor, and electronics such that low-resolution etalons with performance required for implementation would weigh less than one pound
Morita Equivalence of Noncommutative Supertori
In this paper we study the extension of Morita equivalence of noncommutative
tori to the supersymmetric case. The structure of the symmetry group yielding
Morita equivalence appears to be intact but its parameter field becomes
supersymmetrized having both body and soul parts. Our result is mainly in the
two dimensional case in which noncommutative supertori have been constructed
recently: The group , where denotes Grassmann even
number whose body part belongs to , yields Morita equivalent
noncommutative supertori in two dimensions.Comment: LaTeX 18 pages, the version appeared in JM
A 10 GHz Quasi-Optical Grid Amplifier Using Integrated HBT Differential Pairs
We report the fabrication and testing of a 10 GHz grid amplifier utilizing sixteen GaAs chips each
containing an HBT differential pair plus integral bias/feedback resistors. The overall amplifier consists of
a 4x4 array of unit cells on an RT Duroid™ board having a relative permittivity of 2.2. Each unit cell
consists of an emitter-coupled differential pair at the center, an input antenna which extends horizontally
in both directions from the two base leads, an output antenna which extends vertically in both directions
from the two collector leads, and high inductance bias lines. In operation, the active grid array is placed
between a pair of crossed polarizers. The horizontally polarized input wave passes through the input
polarizer and couples to the input leads. An amplified current then flows on the vertical leads, which
radiate a vertically polarized amplified signal through the output polarizer. The polarizers serve dual
functions, providing both input-output isolation as well as independent impedance matching for the input
and output ports. The grid thus functions essentially as a free-space beam amplifier. Calculations indicate
that output powers of several watts per square centimeter of grid area should be attainable with optimized
structures
Mechanical oscillation and cooling actuated by the optical gradient force
In this work we combine the large per-photon optical gradient force with the
sensitive feedback of a high quality factor whispering-gallery microcavity. The
cavity geometry, consisting of a pair of silica disks separated by a nanoscale
gap, shows extremely strong dynamical backaction, powerful enough to excite
giant coherent oscillations even under heavily damped conditions (mechanical
Q=4). In vacuum, the threshold for regenerative mechanical oscillation is
lowered to an optical input power of only 270-nanoWatts, or roughly 1000 stored
cavity photons, and efficient cooling of the mechanical motion is obtained with
a temperature compression factor of 13-dB for 4-microWatts of dropped optical
input power.Comment: 29 pages, 8 figure
A 100-Element MODFET Grid Amplifier
A 100-element quasi-optical amplifier is presented. The active devices are custom-fabricated modulation-doped field-effect transistors (MODFETs). Common-mode oscillations were suppressed using resistors in the input gate leads. The grid has 9 dB of gain at 10.1 GHz. The 3-dB bandwidth is 1.2 GHz. We present a model for the gain of the grid versus frequency and compare measurement with theory
Unravelling the Mysteries of the Leo Ring: An Absorption Line Study of an Unusual Gas Cloud
Since the 1980's discovery of the large (2x10^9 Msun) intergalactic cloud
known as the Leo Ring, this object has been the center of a lively debate about
its origin. Determining the origin of this object is still important as we
develop a deeper understanding of the accretion and feedback processes that
shape galaxy evolution. We present HST/COS observations of three sightlines
near the Ring, two of which penetrate the high column density neutral hydrogen
gas visible in 21 cm observations of the object. These observations provide the
first direct measurement of the metallicity of the gas in the Ring, an
important clue to its origins. Our best estimate of the metallicity of the ring
is ~10% Zsun, higher than expected for primordial gas but lower than expected
from an interaction. We discuss possible modifications to the interaction and
primordial gas scenarios that would be consistent with this metallicity
measurement.Comment: 11 pages, 7 figures, accepted Ap
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