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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Improvement and extension of a radar forest backscattering model

Radar modeling of mangal forest stands, in the Sundarbans area of Southern Bangladesh, was developed. The modeling employs radar system parameters such as wavelength, polarization, and incidence angle, with forest data on tree height, spacing, biomass, species combinations, and water content (including slightly conductive water) both in leaves and trunks of the mangal. For Sundri and Gewa tropical mangal forests, five model components are proposed, which are required to explain the contributions of various forest species combinations in the attenuation and scattering of mangal vegetated nonflooded or flooded surfaces. Statistical data of simulated images (HH components only) were compared with those of SIR-B images both to refine the modeling procedures and to appropriately characterize the model output. The possibility of delineation of flooded or non-flooded boundaries is discussed.

Simonett, David S.↗
NASA NTRS1990

On the angle and wavelength dependencies of the radar backscatter from the icy Galilean moons of Jupiter

This paper reports new developments in the buried crater model that has proved successful in explaining the anomalous strengths and polarizations of the radar echoes from the icy Galilean moons of Jupiter (Europa, Ganymede, and Callisto). The theory is extended to make predictions of the radar cross sections at all points on the surface of the moon, to compute the shape and strength of the power spectra, and to model a wavelength dependence that has been observed.

Gurrola, Eric M.↗
NASA NTRS1990

Measurements of sea spikes in microwave backscatter at moderate incidence

Quantitative measurements of the relationship between the friction velocity and sea spikes in the mean normalized radar cross section (MNRCS) are presented. The intensity threshold for the definition of sea spike contribution to the MNRCS is used to count large sea spikes associated with a positive mean Doppler frequency. The proposed method is applied to Ku-band measurements made with a dual-polarized Doppler scatterometer at a moderate incidence angle from a fixed platform in the North Sea. The variation of the sea spike contribution to the MNRCS with friction velocity was found to be consistent with Phillips' (1988) prediction of a cubic dependence.

Jessup, A. T.↗
NASA NTRS1989

Aerosol speckle effects on atmospheric pulsed lidar backscattered signals

Lidar systems using atmospheric aerosols as targets exhibit return signal amplitude and power fluctuations which indicate speckle effects. The effects of refractive turbulence along the path on the aerosol speckle field propagation and on the decorrelation time are studied for coherent pulsed lidar systems.

Murty, S. R.↗
NASA NTRS1989

The discrimination of sea ice types using SAR backscatter statistics

X-band (HH) synthetic aperture radar (SAR) data of sea ice collected during the Marginal Ice Zone Experiment in March and April of 1987 was statistically analyzed with respect to discriminating open water, first-year ice, multiyear ice, and Odden. Odden are large expanses of nilas ice that rapidly form in the Greenland Sea and transform into pancake ice. A first-order statistical analysis indicated that mean versus variance can segment out open water and first-year ice, and skewness versus modified skewness can segment the Odden and multilayer categories. In additions to first-order statistics, a model has been generated for the distribution function of the SAR ice data. Segmentation of ice types was also attempted using textural measurements. In this case, the general co-occurency matrix was evaluated. The textural method did not generate better results than the first-order statistical approach.

Shuchman, Robert A.↗
NASA NTRS1989

Calibration of quadpolarization SAR data using backscatter statistics

A new technique is described for calibration of complex multipolarization SAR imagery. Scatterer reciprocity and lack of correlation between like- and cross-polarized radar echoes for natural targets are used to remove cross-polarized contamination in the radar data channels without the use of known ground targets. If known targets are available, all data channels can be calibrated relative to one another and absolutely as well. The method is verified with airborne SAR data.

Klein, Jeffrey D.↗
NASA NTRS1991

Satellite observations of polar mesospheric clouds by the solar backscattered ultraviolet spectral radiometer - Evidence of a solar cycle dependence

Results are presented on eight years of satellite observations of the polar mesospheric clouds (PMCs) by the SBUV spectral radiometer, showing that PMCs occur in the summertime polar cap regions of both hemispheres and that they exhibit year-to-year variability. It was also found that the increase in the PMC occurrence frequency was inversely correlated with solar activity. Two kinds of hemispherical asymmetries could be identified: (1) PMCs in the Northern Hemisphere were significantly brighter than in the Southern Hemisphere, in accordance with previous results derived from SME data; and (2) the solar cycle response in the south is more pronounced than in the north. The paper also describes the cloud detection algorithm.

Thomas, Gary E.↗
NASA NTRS1991

The modeling of CO2 lidar backscatter from stratospheric aerosols

Factors that affect modeling for the scattering properties of stratospheric aerosols at CO2 laser wavelengths are discussed. Particular attention is given to the relevant stratospheric aerosol measurements, both in situ and remote, and of the techniques for modeling optical properties at wavelengths close to 10.6 microns. The results obtained by such modeling are compared with published CO2 lidar data.

Kent, G. S.↗
NASA NTRS1991

Thermal protection for hypervelocity flight in earth's atmosphere by use of radiation backscattering ablating materials

A heat-shield-material response code predicting the transient performance of a material subject to the combined convective and radiative heating associated with the hypervelocity flight is developed. The code is dynamically interactive to the heating from a transient flow field, including the effects of material ablation on flow field behavior. It accomodates finite time variable material thickness, internal material phase change, wavelength-dependent radiative properties, and temperature-dependent thermal, physical, and radiative properties. The equations of radiative transfer are solved with the material and are coupled to the transfer energy equation containing the radiative flux divergence in addition to the usual energy terms.

Howe, John T.↗
NASA NTRS1991

An analytic solution of the transient behavior of backscattering thermal protective coatings exposed to combined radiative and convective heating

An analytic solution of the material response to combined radiative and convective heating is presented. The solution includes the equations of radiative transfer (within the material), coupled to a transient energy equation which contains both radiative and convective terms. The analysis allows for unlimited spectral detail, but assumes that within the range of applicability, the various material properties do not vary significantly with temperature. Also, to facilitate development of the analytic solution, it is assumed that scattering within the material dominates absorption, and the material exposed surface does not ablate. The exposed surface boundary condition includes convective heating and spectral radiation, some of which is absorbed by the surface and some which penetrates the surface.

Cornelison, Charles J.↗
NASA NTRS1991

Models of the diffuse radar backscatter from Mars

The topographies of several debris flow units near the Mount St. Helens Volcano were measured at lateral scales of millimeters to meters in September 1990. The objective was to measure the surface roughness of the debris flows at scales smaller than, on the order of, and larger that the radar wavelength of common remote sensing radars. A laser profiling system and surveying instruments were used to obtain elevation data for square areas that varied in size from 10 to 32 cm. The elevation data were converted to estimates of the power spectrum of surface roughness. The conversions were based upon standard periodogram techniques, and upon a modified spectral estimation technique that was developed.

England, A. W.↗
NASA NTRS1991

Comparison of 2 micron Ho and 10 micron CO2 lidar for atmospheric backscatter and Doppler windshear detection

The development of eye-safe, solid-state Lidar systems is discussed, with an emphasis on Coherent Doppler Lidar for Atmospheric Wind Measurements. The following subject areas are covered: tunable Ho DIAL (Differential Absorption Lidar)/lidar atmospheric measurements; atmospheric turbulence measurements and detector arrays; diurnal measurements of C(sub n)(sup 2) for KSC lidar measurements; and development of single-frequency Ho laser/lidar.

Killinger, Dennis↗