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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 901 records · Page 50
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NASA NTRS1990

The wet tropospheric range correction - Product intercomparisons and the simulated effect for tropical Pacific altimeter retrievals

Four data sets used to correct satellite altimetry for the effects of tropospheric water vapor over the tropical Pacific Ocean are evaluated statistically, with a focus on the period 1979-1983. Satellite altimeter measurements are simulated by degrading model sea-level solutions with selected water-vapor data, and seasonal and interannual variations are investigated. The results are presented in extensive contour maps and characterized in detail. The standard deviation between two satellite-based data sets (Nimbus-7 SMMR and TOVS) was found to be 2-4 cm, while both the rms difference between seasonal cycles of the two sets and the difference between the mean fields were 1-2 cm. Larger differences (4-8 cm) were found between these data sets and those based on operational analyses (ECMWF and Fleet Numerical Oceanography Center). The maximum interannual anomaly in the water-vapor correction was 12 cm, for the eastern Pacific at the peak of the 1982-1983 ENSO event.

Zimbelman, Darrell F.↗
NASA NTRS1990

The variability of atmospheric equivalent temperature for radar altimeter range correction

Two sets of data were used to test the validity of the presently used approximation for radar altimeter range correction due to atmospheric water vapor. The approximation includes an assumption of constant atmospheric equivalent temperature. The first data set includes monthly, three-dimensional, gridded temperature and humidity fields over global oceans for a 10-year period, and the second is comprised of daily or semidaily rawinsonde data at 17 island stations for a 7-year period. It is found that the standard method underestimates the variability of the equivalent temperature, and the approximation could introduce errors of 2 cm for monthly means. The equivalent temperature is found to have a strong meridional gradient, and the highest temporal variabilities are found over western boundary currents. The study affirms that the atmospheric water vapor is a good predictor for both the equivalent temperature and the range correction. A relation is proposed to reduce the error.

Liu, W. Timothy↗
NASA NTRS1990

Calibration correction of an active scattering spectrometer probe to account for refractive index of stratospheric aerosols

The use of the active scattering spectrometer probe (ASAS-X) to measure sulfuric acid aerosols on U-2 and ER-2 research aircraft has yielded results that are at times ambiguous due to the dependence of particles' optical signatures on refractive index as well as physical dimensions. The calibration correction of the ASAS-X optical spectrometer probe for stratospheric aerosol studies is validated through an independent and simultaneous sampling of the particles with impactors; sizing and counting of particles on SEM images yields total particle areas and volumes. Upon correction of calibration in light of these data, spectrometer results averaged over four size distributions are found to agree with similarly averaged impactor results to within a few percent: indicating that the optical properties or chemical composition of the sample aerosol must be known in order to achieve accurate optical aerosol spectrometer size analysis.

Pueschel, R. F.↗
NASA NTRS1991

Improved Radiometric Correction For SAR Images

Correction algorithm reversible and applicable to both detected and complex images. Method described accounts for major effects of transmitter, antenna, receiver, and certain aspects of SAR signal processing upon calibration and provides for appropriate reversible corrections applied in processing. Applicable to both detected (amplitude-only) and complex (amplitude and phase) SAR image data and provides for optional subtraction of estimated noise floor from image.

Freeman, Anthony↗
NASA NTRS1991

Ruling Blazed, Aberration-Corrected Diffraction Gratings

Proposed optoelectromechanical apparatus that functions partly as ruling engine and partly as pantograph rules blazed, aberration-corrected diffraction gratings. Makes possible to combine precision of aberration-corrected, holographically produced grating with diffraction efficiency of blazed grating by using holographically produced grating as model or master pattern that controls ruling engine making blazed grating. Surface of model holographic grating scanned by sensing head mounted on pantographic arm. Ruling stylus mounted on other end of pantographic arm with tip in grating blank.

Leviton, Douglas B.↗
NASA NTRS1990

A new photometric correction technique for the IUE final archive

A substantial fraction of the noise in reduced International Ultraviolet Explorer data is shown to result from limitations of the image processing methods used. An improved method for photometrically correcting spectral images that will be used in the IUE final archive processing software is described. An image processed with this photometric correction software is shown to have not only lower noise overall, but none of the periodic changes in noise amplitude. The improvement that is realized through the approach not only solves a long standing difficulty with IUE data reduction techniques, but substantially enhances the quality of the IUE archive.

Shaw, Richard A.↗
NASA NTRS1991

Verification of the FtCayuga fault-tolerant microprocessor system. Volume 2: Formal specification and correctness theorems

Presented here is a formal specification and verification of a property of a quadruplicately redundant fault tolerant microprocessor system design. A complete listing of the formal specification of the system and the correctness theorems that are proved are given. The system performs the task of obtaining interactive consistency among the processors using a special instruction on the processors. The design is based on an algorithm proposed by Pease, Shostak, and Lamport. The property verified insures that an execution of the special instruction by the processors correctly accomplishes interactive consistency, providing certain preconditions hold, using a computer aided design verification tool, Spectool, and the theorem prover, Clio. A major contribution of the work is the demonstration of a significant fault tolerant hardware design that is mechanically verified by a theorem prover.

Bickford, Mark↗
NASA NTRS1989

A posteriori estimates for shear correction factors in multilayered composite cylinders

A predictor-corrector approach is presented for calculating the composite shear correction factors and analyzing multilayered composite cylinders. In the predictor phase a two-dimensional first-order shear deformation theory is used to predict the gross response characteristics of the cylinder (vibration frequencies, average through-the-thickness displacements, rotations, and transverse shear strain energy per unit area) as well as the in-plane strains and stresses in the thickness direction. The three-dimensional equilibrium equations and constitutive relations are then used to compute the transverse stresses and strains as well as the transverse shear factors, and they are also used to correct the predicted response quantities of the cylinder. For simply supported multilayered cylinders the response quantities obtained by using the proposed approach are shown to be in close agreement with three-dimensional elasticity solutions for a wide range of lamination and geometric parameters. Also, the potential of the proposed approach for use in conjunction with large-scale finite element models of composite cylinders is outlined.

Noor, Ahmed K.↗
NASA NTRS1989

Atmospheric correction of high resolution land surface images

Algorithms to correct for atmospheric-scattering effects in high-spatial resolution land-surface images require the ability to perform rapid and accurate computations of the top-of-atmosphere diffuse radiance field for arbitrarily general surface reflectance distributions (which may be both heterogeneous and non-Lambertian) and atmospheric models. Using three-dimensional radiative transfer (3DRT) theory algorithms are being developed. The methodology used to perform the 3DRT calculations is described. It is shown how these calculations are used to perform atmospheric corrections, and the sensitivity of the retrieved surface reflectances to atmospheric structural parameters is illustrated.

Diner, D. J.↗
NASA NTRS1990

Codes with multi-level error-correcting capabilities

In conventional channel coding, all the information symbols of a message are regarded equally significant, and hence codes are devised to provide equal protection for each information symbol against channel errors. However, in some circumstances, some information symbols in a message are more significant than the other symbols. As a result, it is desirable to devise codes with multilevel error-correcting capabilities. In this paper, block codes with multilevel error correcting capabilities, which are also known as unequal error protection (UEP) codes, are investigated. Several classes of UEP codes are constructed. One class of codes satisfies the Hamming bound on the number of parity-check symbols for systematic linear UEP codes and hence is optimal.

Lin, Mao-Chao↗
NASA NTRS1991

A comparative study of several compressibility corrections to turbulence models applied to high-speed shear layers

Several recently published compressibility corrections to the standard k-epsilon turbulence model are used with the Navier-Stokes equations to compute the mixing region of a large variety of high speed flows. These corrections, specifically developed to address the weakness of higher order turbulence models to accurately predict the spread rate of compressible free shear flows, are applied to two stream flows of the same gas mixing under a large variety of free stream conditions. Results are presented for two types of flows: unconfined streams with either (1) matched total temperatures and static pressures, or (2) matched static temperatures and pressures, and a confined stream.

Viegas, John R.↗
NASA NTRS1991

Active vibration control with model correction on a flexible laboratory grid structure

This paper presents experimental and computational comparisons of three active damping control laws applied to a complex laboratory structure. Two reduced structural models were used with one model being corrected on the basis of measured mode shapes and frequencies. Three control laws were investigated, a time-invariant linear quadratic regulator with state estimation and two direct rate feedback control laws. Experimental results for all designs were obtained with digital implementation. It was found that model correction improved the agreement between analytical and experimental results. The best agreement was obtained with the simplest direct rate feedback control.

Schamel, George C., II↗
NASA NTRS1992

User guide for WIACX: A transonic wind-tunnel wall interference assessment and correction procedure for the NTF

A three dimensional transonic Wind-tunnel Interference Assessment and Correction (WIAC) procedure developed specifically for use in the National Transonic Facility (NTF) at NASA Langley Research Center is discussed. This report is a user manual for the codes comprising the correction procedure. It also includes listings of sample procedures and input files for running a sample case and plotting the results.

Garriz, Javier A.↗
NASA NTRS1992

Effects and correction of magneto-optic spatial light modulator phase errors in an optical correlator

The optical phase errors introduced into an optical correlator by the input and filter plane magnetooptic spatial light modulators have been studied. The magnitude of these phase errors is measured and characterized, their effects on the correlation results are evaluated, and a means of correction by a design modification of the binary phase-only optical-filter function is presented. The efficacy of the phase-correction technique is quantified and is found to restore the correlation characteristics to those obtained in the absence of errors, to a high degree. The phase errors of other correlator system elements are also discussed and treated in a similar fashion.

Downie, John D.↗
NASA NTRS1992

Atmospheric correction algorithm for NOAA-AVHRR products - Theory and application

The present evaluation of the effects of atmospheric constituents on NOAA AVHRR visible and near-IR data first gives attention to the general remote sensing equation and then addresses the magnitude of the atmospheric effects for AVHRR solar bands with respect to their impact on the normalized difference vegetation index (NDVI) and the surface bidirectional reflectance. Possible approaches to the acquisition of atmospheric data are then considered, and examples of atmospheric correction of surface reflectance and NDVI are discussed. The correction of the aerosol effect is essential in the case of NDVI study of densely vegetated areas.

Tanre, Didier↗
NASA NTRS1992

The effect of temperature on attenuation-correction schemes in rain using polarization propagation differential phase shift

The study elucidates and quantifies differences in the response of the rate of change of polarization propagation differential phase shift Phi, the rate of attenuation for a horizontally/vertically polarized wave A(H,V), and the rate of polarization differential attenuation A(H-V) to temperature. It is shown that if the effects of temperature when estimating A(H) and A(H-V) from Phi are neglected, the average fractional standard error increases only slightly at 9 GHz but significantly at 5 and 3 GHz. Errors at 5 and 3 GHz are about two to three times those at 9 GHz. The performance of Phi-based schemes of attenuation correction at these lower frequencies is much more significantly degraded by temperature uncertainty than at 9 GHz. It is concluded that it is best to use Phi to correct for attenuation at the least-attenuating frequencies.

Jameson, A. R.↗
NASA NTRS1991

An investigation of error correcting techniques for OMV and AXAF

The original objectives of this project were to build a test system for the NASA 255/223 Reed/Solomon encoding/decoding chip set and circuit board. This test system was then to be interfaced with a convolutional system at MSFC to examine the performance of the concantinated codes. After considerable work, it was discovered that the convolutional system could not function as needed. This report documents the design, construction, and testing of the test apparatus for the R/S chip set. The approach taken was to verify the error correcting behavior of the chip set by injecting known error patterns onto data and observing the results. Error sequences were generated using pseudo-random number generator programs, with Poisson time distribution between errors and Gaussian burst lengths. Sample means, variances, and number of un-correctable errors were calculated for each data set before testing.

Ingels, Frank↗
NASA NTRS1992

Overview of atmospheric correction and radiometric calibration efforts during FIFE

The primary responsibility within the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) for understanding the radiometric properties of the remote sensing instrumentation and the atmospheric optical properties affecting measurements with these instruments rested within the atmospheric corrections and radiometric calibration subgroup of FIFE, called the correction/calibration group for short. Specific activities included (1) calibration of remote sensing instrumentation used on aircraft and surface platforms, (2) formulation of recommendations for calibrating data from satellite sensors, (3) measurement and analysis of atmospheric properties, (4) development and evaluation of radiative transfer algorithms which perform the atmospheric correction of remotely sensed data, and (5) derivation and evaluation of surface properties such as reflectances and temperatures. A review of the activities performed by this group and a discussion of the importance of these activities in retrieving surface parameters is presented here.

Halthore, Rangasayi N.↗