469 research outputs found
An airport wind shear detection and warning system using Doppler radar: A feasibility study
A feasibility study was conducted to determine whether ground based Doppler radar could measure the wind along the path of an approaching aircraft with sufficient accuracy to predict aircraft performance. Forty-three PAR approaches were conducted, with 16 examined in detail. In each, Doppler derived longitudinal winds were compared to aircraft measured winds; in approximately 75 percent of the cases, the Doppler and aircraft winds were in acceptable agreement. In the remaining cases, errors may have been due to a lack of Doppler resolution, a lack of co-location of the two sampling volumes, the presence of eddy or vortex like disturbances within the pulse volume, or the presence of point targets in antenna side lobes. It was further concluded that shrouding techniques would have reduced the side lobe problem. A ground based Doppler radar operating in the optically clear air, provides the appropriate longitudinal winds along an aircraft's intended flight path
Jet transport performance in thunderstorm wind shear conditions
Several hours of three dimensional wind data were collected in the thunderstorm approach-to-landing environment, using an instrumented Queen Air airplane. These data were used as input to a numerical simulation of aircraft response, concentrating on fixed-stick assumptions, while the aircraft simulated an instrument landing systems approach. Output included airspeed, vertical displacement, pitch angle, and a special approach deterioration parameter. Theory and the results of approximately 1000 simulations indicated that about 20 percent of the cases contained serious wind shear conditions capable of causing a critical deterioration of the approach. In particular, the presence of high energy at the airplane's phugoid frequency was found to have a deleterious effect on approach quality. Oscillations of the horizontal wind at the phugoid frequency were found to have a more serious effect than vertical wind. A simulation of Eastern flight 66, which crashed at JFK in 1975, served to illustrate the points of the research. A concept of a real-time wind shear detector was outlined utilizing these results
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Resolving Apparent Conflicts between Oceanographic and Antarctic Climate Records and Evidence for a Decrease in pCO2 during the Oligocene through Early Miocene (34–16 Ma)
An apparent mismatch between published oxygen isotopic data and other paleoclimate proxies for the span from 26–16 Ma is resolved by calibration against global sea-level estimates obtained from backstripping continental margin stratigraphy. Ice-volume estimates from calibrated oxygen isotope data compare favorably with stratigraphic and palynological data from Antarctica, and with estimates of atmospheric pCO2 throughout the Oligocene to early Miocene (34–16 Ma). Isotopic evidence for an East Antarctic Ice Sheet (EAIS) as much as 30% larger than its present-day volume at glacial maxima during that span is consistent with seismic reflection and stratigraphic evidence for an ice sheet covering much of the Antarctic continental shelf at the same glacial maxima. Palynological data suggest long-term cooling during the Oligocene, with cold near-tundra environments developing along the coast at glacial minima no later than the late Oligocene. A possible mechanism for this long-term cooling is a decrease in atmospheric pCO2 from the middle Eocene to Oligocene, reaching near pre-industrial levels by the latest Oligocene, and remaining at those depressed levels throughout the Miocene
Nanopillar Arrays on Semiconductor Membranes as Electron Emission Amplifiers
A new transmission-type electron multiplier was fabricated from
silicon-on-insulator (SOI) material by integrating an array of one dimensional
(1D) silicon nanopillars onto a two dimensional (2D) silicon membrane. Primary
electrons are injected into the nanopillar-membrane system from the flat
surface of the membrane, while electron emission from the other side is probed
by an anode. The secondary electron yield (SEY) from nanopillars is found to be
about 1.8 times that of plane silicon membrane. This gain in electron number is
slightly enhanced by the electric field applied from the anode. Further
optimization of the dimensions of nanopillars and membrane and application of
field emission promise an even higher gain for detector applications and allow
for probing of electronic/mechanical excitations in nanopillar-membrane system
excited by incident particles or radiation.Comment: 4 figure
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Is There a Role for Sequence Stratigraphy in Chronostratigraphy?
Sequence stratigraphy revolutionized the field of stratigraphy in the late 1970s and 1980s by providing an interpretive depositional framework for integrating diverse stratigraphic data at the scale of sedimentary basins. However, a lack of consensus on criteria for recognizing, mapping and hence dating sequence boundaries, interpretations of uneven quality, and doubts about the universal eustatic origin and global synchrony of unconformity-related sequences limit the usefulness of sequence stratigraphy in chronostratigraphy
Josephson Junctions defined by a Nano-Plough
We define superconducting constrictions by ploughing a deposited Aluminum
film with a scanning probe microscope. The microscope tip is modified by
electron beam deposition to form a nano-plough of diamond-like hardness, what
allows the definition of highly transparent Josephson junctions. Additionally a
dc-SQUID is fabricated to verify appropriate functioning of the junctions. The
devices are easily integrated in mesoscopic devices as local radiation sources
and can be used as tunable on-chip millimeter wave sources
Red Blood Cells and Turbulence
Measurements were made of the turbulence intensity of blood of various hematocrits (volume percentage of red cells in blood) flowing through an orifice. The maximum relative turbulence intensity was found to occur in the hematocrit range of 20% - 30%
Adiabatic steering and determination of dephasing rates in double dot qubits
We propose a scheme to prepare arbitrary superpositions of quantum states in
double quantum--dots irradiated by coherent microwave pulses. Solving the
equations of motion for the dot density matrix, we find that dephasing rates
for such superpositions can be quantitatively infered from additional electron
current pulses that appear due to a controllable breakdown of coherent
population trapping in the dots.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Determination of the complex microwave photoconductance of a single quantum dot
A small quantum dot containing approximately 20 electrons is realized in a
two-dimensional electron system of an AlGaAs/GaAs heterostructure. Conventional
transport and microwave spectroscopy reveal the dot's electronic structure. By
applying a coherently coupled two-source technique, we are able to determine
the complex microwave induced tunnel current. The amplitude of this
photoconductance resolves photon-assisted tunneling (PAT) in the non-linear
regime through the ground state and an excited state as well. The out-of-phase
component (susceptance) allows to study charge relaxation within the quantum
dot on a time scale comparable to the microwave beat period.Comment: 5.5 pages, 6 figures, accepted by Phys. Rev. B (Jan. B15 2001
Shock Waves in Nanomechanical Resonators
The dream of every surfer is an extremely steep wave propagating at the
highest speed possible. The best waves for this would be shock waves, but are
very hard to surf. In the nanoscopic world the same is true: the surfers in
this case are electrons riding through nanomechanical devices on acoustic waves
[1]. Naturally, this has a broad range of applications in sensor technology and
for communication electronics for which the combination of an electronic and a
mechanical degree of freedom is essential. But this is also of interest for
fundamental aspects of nano-electromechanical systems (NEMS), when it comes to
quantum limited displacement detection [2] and the control of phonon number
states [3]. Here, we study the formation of shock waves in a NEMS resonator
with an embedded two-dimensional electron gas using surface acoustic waves. The
mechanical displacement of the nano-resonator is read out via the induced
acoustoelectric current. Applying acoustical standing waves we are able to
determine the anomalous acoustocurrent. This current is only found in the
regime of shock wave formation. We ontain very good agreement with model
calculations.Comment: 14 Pages including 4 figure
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