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At least 649 records · Page 36

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NASA NTRS1991

Spatial and temporal variability of the extent of chemically processed stratospheric air

Simulations of the spatial and temporal variability of the extent of chemically processed air in the Arctic stratosphere have been carried out using a three-dimensional chemistry-transport model for the winters of 1979 and 1989. Chemically processed air is identified in the model as that in which the amounts of hydrogen chloride (HCl) calculated with parameterized loss for conditions appropriate to polar stratospheric cloud (PSC) formation are substantially smaller than those calculated in a model with gas phase chemistry only. It is seen that chemically processed air may be identified over much of the Arctic lower stratosphere from early January to late February, with HCl depletions being larger in 1989 than in 1979. Near the latitude of the Arctic circle, there is important spatial and temporal variability in the extent of chemically processed air. There is some evidence for transport to midlatitudes of processed air during these winters, but the HCl reductions are much smaller and more sporadic than those near the pole. At 62 and 42N, processed air is calculated to occur preferentially over the longitude regions from 60-120E and 270-330E.

Kaye, Jack A.↗

NASA NTRS1991

Interannual variability of the spatial distribution of sea ice in the north polar region

The interannual variability of the monthly average spatial distributions of northern hemisphere sea ice was examined for the 1973-1987 period of the electrically scanning microwave radiometer on the Nimbus 5 satellite and the scanning multichannel microwave radiometer on the Nimbus 7 satellite passive microwave data record. Strong seasonal and spatial contrasts in the amount of interannual variability are mapped and identified. The maps and analyses provide a baseline against which future northern hemisphere sea ice distributions can be compared in the continuing attempt to analyze global climate and to determine whether or not significant climate change is occurring.

Parkinson, Claire L.↗

NASA NTRS1991

Temporal and spatial variability of aerosol optical depth in the Sahel region in relation to vegetation remote sensing

In order to monitor the aerosol characteristics needed for atmospheric correction of remotely sensed data, a network of sun photometers was established in the Sahel region of Senegal, Mali, and Niger. Data analysis suggests that there is a high spatial variability of the aerosol optical thickness tau(a) in the western Sahel region. At a 67 percent confidence level the instantaneous values of tau(a) can be extrapolated approximately 270-400 km with an error tolerance of 50 percent. Spatial variability in the dry season is found to be of a similar magnitude. The ranges of variations in the NDVI in the Sahel region are shown to be approximately 0.02 and 0.01, respectively, due to commonly observed fluctuations in the aerosol optical thickness and aerosol size distribution.

Holben, B. N.↗

NASA NTRS1991

High-power and high-spatial-coherence broad-area power amplifier

A broad-area AlGaAs amplifier operating CW has delivered 425 mW of total power with 342 mW in a single lobe diverging at 1.02 times the diffraction limit (FWHM 0.483 deg, 87.4-micron actual aperture) for a master oscillator input power of 70 mW. The spatial coherence of the amplifier output is 0.97, and the mutual spatial coherence between the oscillator and the amplifier is 0.96.

Andrews, John R.↗

NASA NTRS1989

Near-field structures in three-dimensional spatially-developing wakes

The effects of three-dimensional perturbations on a spatially-developing plane Gaussian wake are studied and quantified using a new numerical code. Results are presented for a elatively simple case since it appears that no 3-D spatially-developing simulations of plane wakes have been performed in the past. The inlet perturbation consists of a 2-D time-periodic forcing plus a spanwise-periodic array of steady streamwise vortices. The distortion of the 2-D vortices caused by the streamwise vortices will be discussed. The vorticity fields are compared to passive scalar fields which simulate the injection of smoke or low heat into the wake.

Buell, Jeffrey C.↗

NASA NTRS1991

A spatial operator algebra for manipulator modeling and control

A recently developed spatial operator algebra for manipulator modeling, control, and trajectory design is discussed. The elements of this algebra are linear operators whose domain and range spaces consist of forces, moments, velocities, and accelerations. The effect of these operators is equivalent to a spatial recursion along the span of a manipulator. Inversion of operators can be efficiently obtained via techniques of recursive filtering and smoothing. The operator algebra provides a high-level framework for describing the dynamic and kinematic behavior of a manipulator and for control and trajectory design algorithms. The interpretation of expressions within the algebraic framework leads to enhanced conceptual and physical understanding of manipulator dynamics and kinematics.

Rodriguez, G.↗

NASA NTRS1990

Spatial heterodyne spectroscopy - Interferometric performance at any wavelength without scanning

Spatial heterodyne spectroscopy (SHS) employing a two-beam dispersive interferometer producing a Fizeau fringe pattern having wavelength-dependent spatial frequencies is presented. The pattern is recorded on an imaging detector and Fourier transformed to recover the input stream. It is pointed out that spectrometers operating on the SHS principle can achieve the theoretical resolution limit of the gratings without scanning, retaining at the same time the large angular input tolerance and multiplexing properties of conventional scanning Fourier-transform spectrometers. Additionally, broad spectral coverages can be achieved, and field widening can be accomplished without moving parts.

Roesler, F. L.↗

NASA NTRS1990

Recursive dynamics for flexible multibody systems using spatial operators

Due to their structural flexibility, spacecraft and space manipulators are multibody systems with complex dynamics and possess a large number of degrees of freedom. Here the spatial operator algebra methodology is used to develop a new dynamics formulation and spatially recursive algorithms for such flexible multibody systems. A key feature of the formulation is that the operator description of the flexible system dynamics is identical in form to the corresponding operator description of the dynamics of rigid multibody systems. A significant advantage of this unifying approach is that it allows ideas and techniques for rigid multibody systems to be easily applied to flexible multibody systems. The algorithms use standard finite-element and assumed modes models for the individual body deformation. A Newton-Euler Operator Factorization of the mass matrix of the multibody system is first developed. It forms the basis for recursive algorithms such as for the inverse dynamics, the computation of the mass matrix, and the composite body forward dynamics for the system. Subsequently, an alternative Innovations Operator Factorization of the mass matrix, each of whose factors is invertible, is developed. It leads to an operator expression for the inverse of the mass matrix, and forms the basis for the recursive articulated body forward dynamics algorithm for the flexible multibody system. For simplicity, most of the development here focuses on serial chain multibody systems. However, extensions of the algorithms to general topology flexible multibody systems are described. While the computational cost of the algorithms depends on factors such as the topology and the amount of flexibility in the multibody system, in general, it appears that in contrast to the rigid multibody case, the articulated body forward dynamics algorithm is the more efficient algorithm for flexible multibody systems containing even a small number of flexible bodies. The variety of algorithms described here permits a user to choose the algorithm which is optimal for the multibody system at hand. The availability of a number of algorithms is even more important for real-time applications, where implementation on parallel processors or custom computing hardware is often necessary to maximize speed.

Jain, A.↗

NASA NTRS1991

All-optical photochromic spatial light modulators based on photoinduced electron transfer in rigid matrices

A single material (not a multi-element structure) spatial light modulator may be written to, as well as read out from, using light. The device has tailorable rise and hold times dependent on the composition and concentration of the molecular species used as the active components. The spatial resolution of this device is limited only by light diffraction as in volume holograms. The device may function as a two-dimensional mask (transmission or reflection) or as a three-dimensional volume holographic medium. This device, based on optically-induced electron transfer, is able to perform incoherent to coherent image conversion or wavelength conversion over a wide spectral range (ultraviolet, visible, or near-infrared regions).

Beratan, David N.↗

NASA NTRS1992

Quantum spatial propagation of squeezed light in a degenerate parametric amplifier

Differential equations which describe the steady state spatial evolution of nonclassical light are established using standard quantum field theoretic techniques. A Schroedinger equation for the state vector of the optical field is derived using the quantum analog of the slowly varying envelope approximation (SVEA). The steady state solutions are those that satisfy the time independent Schroedinger equation. The resulting eigenvalue problem then leads to the spatial propagation equations. For the degenerate parametric amplifier this method shows that the squeezing parameter obey nonlinear differential equations coupled by the amplifier gain and phase mismatch. The solution to these differential equations is equivalent to one obtained from the classical three wave mixing steady state solution to the parametric amplifier with a nondepleted pump.

Deutsch, Ivan H.↗

NASA NTRS1992

Spatial distribution of gamma-ray bursts observed by BATSE

An analysis of 153 gamma-ray bursts detected by the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory is reported. The number versus intensity distribution does not follow the -3/2 power law expected for a spatially extended homogeneous distribution of sources, but at the same time the angular distribution is isotropic within statistical limits. Taken together these results are consistent with the spatial distribution of any known population of galactic objects, but may be consistent with the bursts being at cosmological distances.

Meegan, C. A.↗

NASA NTRS1991

Spatial variability of outgoing longwave radiation

Spatial distributions of day-to-day variations in the outgoing longwave radiation (OLR) are quantified by examining spatial variations in daily OLR data obtained from the Earth Radiation Budget Experiment scanning radiometer aboard the NOAA-9 meteorological satellite, in the form of daily maps and monthly radiation maps at 2.5 deg latitude-longitude scale. The results show that the variations of OLR during a month are markedly greater in the subtropics than they are in the mid and high altitudes. Results for October 1985 are presented.

Smith, G. L.↗

NASA NTRS1992

Spatial Operator Algebra for multibody system dynamics

The Spatial Operator Algebra framework for the dynamics of general multibody systems is described. The use of a spatial operator-based methodology permits the formulation of the dynamical equations of motion of multibody systems in a concise and systematic way. The dynamical equations of progressively more complex grid multibody systems are developed in an evolutionary manner beginning with a serial chain system, followed by a tree topology system and finally, systems with arbitrary closed loops. Operator factorizations and identities are used to develop novel recursive algorithms for the forward dynamics of systems with closed loops. Extensions required to deal with flexible elements are also discussed.

Rodriguez, G.↗

NASA NTRS1991

The application of high spectral and spatial resolution imaging spectrometers for locating downed aircraft

The utility of high-resolution imaging spectrometer data is examined as an aid in locating downed aircraft by using a unique spectral signature while not requiring the extremely high spatial resolution needed to identify an aircraft by shape. Ground spectral measurements of several airplane wings, overflight spectral measurements of aircraft scenes, and the rationale for the chosen spectral signature are presented. It is concluded that imaging spectrometers which can detect and spatially locate a narrow-band spectral signature filling only a few pixels appear to have a utility for search and rescue aircraft or satellite systems as a aid in locating small downed aircraft. This spectral feature would have to be added to the surface coatings applied to aircraft. Proposed for use as such a spectral signature is a significant negative reflectance slope, in the 520 to 580 nm interval.

Gatlin, James A.↗

NASA NTRS1992

Effect of spatial resolution on apparent sensitivity to initial conditions of a decaying flow as it becomes turbulent

In order to check for spurious chaos and obtain superior solutions for decaying Navier-Stokes flows, an investigation is conducted of the effect of spatial resolution on numerical results. The fourth-order finite difference method results obtained with grids of 32-cubed and 64-cubed points, and those of a pseudospectral method for 128-cubed points, indicate that the sensitivity of initially neighboring solutions to small changes in initial conditions increases with improving spatial resolution.

Deissler, Robert G.↗

NASA NTRS1992

Spatial and temporal temperature distribution optimization for a geostationary antenna

The Geostationary Microwave Precipitation Radiometer antenna is considered and a thermal design analysis is performed to determine a design that would minimize on-orbit antenna temporal and spatial temperature gradients. The final design is based on an optically opaque radome which covered the antenna. The average orbital antenna temperature is found to be 9 C with maximum temporal and spatial variations of 34 C and 1 C, respectively. An independent thermal distortion analysis showed that this temporal variation would give an antenna figure error of 14 microns.

Tsuyuki, G.↗

NASA NTRS1992

Coherent summation of spatially distorted laser Doppler signals by using a two-dimensional heterodyne detector array

Phase-sensitive coherent summation of individual heterodyne detector array signals was demonstrated for the enhanced detection of spatially distorted laser Doppler returns. With the use of a 2 x 2 heterodyne detector array, the phase and amplitude of a time-varying speckle pattern was detected, and the signal-to-noise ratio of the Doppler shift estimate was shown to be improved by a factor of 2, depending on the extent of spatial coherence loss. These results are shown to agree with a first-order analysis and indicate the advantage of coherent summation for both short-range laser Doppler velocimetry and long-range atmospheric coherent lidar.

Chan, Kin P.↗

NASA NTRS1992

Spatial patterns of increases and decreases in the length of the sea ice season in the north polar region, 1979-1986

Recently it was reported that sea ice extents in the Northern Hemisphere showed a very slight but statistically significant decrease over the 8.8-year period of the Nimbus 7 scanning multichannel microwave radiometer (SMMR) data set. In this paper the same SMMR data are used to reveal spatial patterns in increasing and decreasing sea ice coverage. Specifically, the length of the ice season is mapped for each full year of the SMMR data set (1979-1986), and the trends over the 8 years in these ice season lengths are also mapped. These trends show considerable spatial coherence, with a shortening in the sea ice season apparent in much of the eastern hemisphere of the north polar ice cover, particularly in the Sea of Okhotsk, the Barents Sea, and the Kara Sea, and a lengthening of the sea ice season apparent in much of the western hemisphere of the north polar ice cover, particularly in Davis Strait, the Labrador Sea, and the Beaufort Sea.

Parkinson, Claire L.↗