SEARCH · Search NASA

Results for “BACKSCATTERING”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6

All sources NASA NTRS DOE OSTI

NASA NTRS1983

Radar backscattering properties of corn and soybeans at frequencies of 1.6, 4.75, and 13.3. GHz

The NASA Johnson Space Center made an observational study of the radar-backscattering properties of corn and soybeans in commercial fields in a test site in Webster County, IA. Aircraft-based radar scatterometers measured the backscattering coefficient of the crops at three frequencies, 1.6 GHz (L-band), 4.75 GHz (C-band), and 13.3 GHz (Ku-band), at 10 sensor look-angles (5 to 50 degrees from the nadir in steps of 5 degrees), and with several polarization combinations. Among other findings, it was determined that: (1) row direction differences among fields affected significantly the radar-backscattering coefficient of the fields when the radar system used like-polarization at look-angles from 5 to 25 degrees; (2) row-direction differences had no effect on radar backscattering when the system used either cross-polarization or look-angles greater than 25 degrees regardless of the polarization; (3) wet surface-soil moisture conditions resulted in significantly poorer spectral separability of the two crops as compared to dry-soil conditions; and (4) on the dry-soil date, the best channel for separating corn from soybeans was the C-band cross-polarized measurement at a look-angle of 50 degrees.

Paris, J. F.↗

NASA NTRS1984

Snowcover influence on backscattering from terrain

The effects of snowcover on the microwave backscattering from terrain in the 8-35 GHz region are examined through the analysis of experimental data and by application of a semiempirical model. The model accounts for surface backscattering contributions by the snow-air and snow-soil interfaces, and for volume backscattering contributions by the snow layer. Through comparisons of backscattering data for different terrain surfaces measured both with and without snowcover, the masking effects of snow are evaluated as a function of snow water equivalent and liquid water content. The results indicate that with dry snowcover it is not possible to discriminate between different types of ground surface (concrete, asphalt, grass, and bare ground) if the snow water equivalent is greater than about 20 cm (or a depth greater than 60 cm for a snow density of 0.3 g/cu cm). For the same density, however, if the snow is wet, a depth of 10 cm is sufficient to mask the underlying surface.

Ulaby, F. T.↗

NASA NTRS1985

Ground-based measurements of atmospheric backscatter and absorption using coherent CO2 lidar

In the present derivation of vertical profiles of atmospheric absorption and backscatter coefficients from coherent pulsed Doppler lidar by means of a slant path method, a strong seasonal variation of absorption and backscatter is evident throughout the lower troposphere. Measurements beyond the lower troposphere are hampered by modest pulse energy and lidar beam absorption, although small backscatter values are occasionally observed at midtropospheric levels during the winter months, when absorption is minimal. While a monomodal lognormal backscatter distribution is found within the lower boundary layer, evidence of a bimodal lognormal distribution is found at higher levels.

Rothermel, J.↗

NASA NTRS1986

Correction of Doppler-broadened Rayleigh backscattering effects in H2O dial measurements

A general method of solutions for treating effects of Doppler-broadened Rayleigh backscattering in H2O Differential Absorption Lidar (DIAL) measurements are described and discussed. Errors in vertical DIAL measuremtns caused by this laser line broadening effect can be very large and, therfore, this effect has to be accounted for accurately. To analyze and correct effects of Doppler-broadened Rayleigh backscattering in DIAL experiments, a generalized DIAL approximation was derived starting from a lidar equation, which includes Doppler broadening. To evaluate the accuracy of H2O DIAL measurements, computer simulations were performed. It was concluded that correction of Doppler broadened Rayleigh backscattering is possible with good accuracy in most cases of tropospheric H2O DIAL measurements, but great care has to be taken when layers with steep gradients of Mie backscattering like clouds or inversion layers are present.

Ansmann, A.↗

NASA NTRS1987

Electromagnetic backscattering by plates and disks

With the recent development of diffraction coefficients for imperfectly conducting half-planes, it has become possible to analyze a wide variety of problems for which the impedance surface boundary condition applies. This impedance boundary condition, while approximate, was utilized to extend the usefulness of the Uniform Geometrical Theory of Diffraction (UTD) beyond the perfectly conducting geometries. These half-plane diffraction coefficients are used to analyze patterns of an antenna in the presence of an imperfectly conducting flat polygonal plate. The Geometrical Theory of Diffraction (GTD) techniques were also used to investigate the backscattering from perfectly conducting plates. To further improve the soft polarization results for wide angles, a model for the creeping wave or circulating current on the edge of the disk was obtained and used to find an additional component of the backscattered field. The backscattering from a square plate was then analyzed using GTD. Backscattering in both the principal and off-principal planes was examined.

Balanis, Constantine A.↗

NASA NTRS1986

Correlation of Ultrasonic Polar Backscatter with the Deply Technique for Assessment of Impact Damage in Composite Laminates

Ultrasonic polar backscatter, a quantitative NDE technique introduced by O'Donnell and Miller (1981), was used to characterize impact damages in a 16-ply graphite-epoxy laminate. The results were correlated with the results obtained by the destructive deply technique of Freeman (1984). The size, shape, and orientation of damage correlated well between the polar backscatter technique and the deply technique. There was good quantitative correlation between the areas of damage indicated by the two techniques, suggesting that the polar backscatter technique is sensitive to specific orientations of damage. The polar backscatter technique provides a good qualitative image of the size and shape of the largest zone of damage in each of the principal orientations; a quantitative estimate of the extent of these largest damage zones can then be obtained.

Blodgett, E. D.↗

NASA NTRS1988

Simulation of L-band and HH microwave backscattering from coniferous forest stands - A comparison with SIR-B data

SIR-B images of the Mt. Shasta region of northern California are used to evaluate a composite L-band HH backscattering model of coniferous forest stands. It is found that both SIR-B and simulated backscattering coefficients for eight stands studied have similar trends and relations to average tree height and average number of trees per pixel. Also, the dispersion and distribution of simulated backscattering coefficients from each stand broadly match SIR-B data from the same stand. Although the limited quality and quantity of experimental data makes it difficult to draw any strong conclusions, the comparisons indicate that a stand-based L-band HH composite model seems promising for explaining backscattering features.

Sun, Guo-Qing↗

NASA NTRS1988

Calculation of the effects of ice on the backscatter of a ground plane

Described is a technique for examining the effect of a rough ice layer on the backscatter of a ground plane. The technique is applied to the special case of a rough ice layer that is periodic in space. By assuming that the roughness is periodic, the backscatter of the ground plane can be found from the backscatter of a single period. Backscatter calculations are presented for a single period in which the thickness of the ice layer has a Gaussian shape.

Lambert, K. M.↗

NASA NTRS1989

Altitude and seasonal characteristics of aerosol backscatter at thermal infrared wavelengths using lidar observations from coastal California

A calibrated CO2 lidar has been used to measure boundary layer aerosol backscatter and vertical profiles of tropospheric and lower stratospheric aerosol backscatter over a 1984-1987 time period from a site in Pasadena, California. The lidar data have been taken at two wavelengths, 9.25 and 10.6 microns. Data are presented which show altitude, seasonal, and trend characteristics of backscatter for various air mass histories. Results of trajectory studies indicate the influence of convective activity and other factors on the backscatter profiles.

Menzies, Robert T.↗

NASA NTRS1989

Infrared backscattering

All particles in the atmosphere are not spherical. Moreover, the scattering properties of randomly oriented nonspherical particles are not equivalent to those of spherical particles no matter how the term equivalent is defined. This is especially true for scattering in the backward direction and at the infrared wavelengths at which some atmospheric particles have strong absorption bands. Thus calculations based on Mie theory of infrared backscattering by dry or insoluble atmospheric particles are suspect. To support this assertion, it was noted that peaks in laboratory-measured infrared backscattering spectra show appreciable shifts compared with those calculated using Mie theory. One example is ammonium sulfate. Some success was had in modeling backscattering spectra of ammonium sulfate particles using a simple statistical theory called the continuous distribution of ellipsoids (CDE) theory. In this theory, the scattering properties of an ensemble are calculated. Recently a modified version of this theory was applied to measured spectra of scattering by kaolin particles. The particles were platelike, so the probability distribution of ellipsoidal shapes was chosen to reflect this. As with ammonium sulfate, the wavelength of measured peak backscattering is shifted longward of that predicted by Mie theory.

Bohren, Craig F.↗

NASA NTRS1991

Effects of soil and canopy characteristics on microwave backscattering of vegetation

A frequency modulated continuous wave C-band (4.8 GHz) scatterometer was mounted on an aerial lift truck and backscatter coefficients of corn were acquired as functions of polarizations, view angles, and row directions. As phytomass and green leaf area index increased, the backscatter also increased. Near anthesis when the canopies were fully developed, the major scattering elements were located in the upper 1 m of the 2.8 m tall canopy and little backscatter was measured below that level. C-band backscatter data could provide information to monitor vegetation at large view zenith angles.

Daughtry, C. S. T.↗

NASA NTRS1991

Lidar aerosol and cloud backscatter at 0.53, 1.06 and 1.54 microns

The performance space borne or airborne lidar remote sensor is critically dependent on the magnitude the atmospheric backscatter cross section for aerosol and clouds. Measurements are required to establish a representative global distribution. There has been in particular a lack of observations in the Southern Hemisphere and for remote ocean areas. The Global Backscatter Experiment (GLOBE) was flown on the NASA DC-8 aircraft in November, 1989 and May, 1990 throughout northern and southern Pacific regions. A lidar system to measure aerosol backscatter cross section at the visible and near IR wavelengths of 1.06, 1.54 and 0.53 microns was included in the missions. From the global flight surveys, extensive data have been obtained on aerosol and cloud backscatter cross section and structure throughout the troposphere. These data define the feasibility of proposed lidar applications.

Spinhirne, James D.↗

NASA NTRS1991

Southern Hemisphere tropospheric aerosol backscatter measurements - Implications for a laser wind system

To identify the aerosol size fraction that is responsible for the 10.6-micron backscatter and the morphological composition of these particles, values of backscatter coefficient at 0.532 micron, 0.694 micron, and 10.6 microns were determined both by direct measurement and by Mie computation from measured microphysical properties of aerosols over eastern Australia between 4 deg and 41 deg S. The values of backscatter coefficient determined from direct measurements at 10.6 microns were found to agree well with values obtained indirectly from calculations based on measured aerosol particle size distributions and properties. It was also found that backscatter at 10.6 microns and at visible wavelengths were sensitive to different regions of the aerosol size spectrum, with considerable day-to-day variability observed at all wavelengths.

Gras, J. L.↗

NASA NTRS1991

C-band backscattering from corn canopies

A frequency-modulatad continuous-wave C-band (4.8 GHz) scatterometer was mounted on an aerial lift truck, and backscatter coefficients of corn (Zea mays L.) were acquired as functions of polarizations, view angles, and row directions. As phytomass and green-leaf area index increased, the backscatter also increased. Near anthesis, when the canopies were fully developed, the major scattering elements were located in the upper 1 m of the 2.8 m tall canopy and little backscatter was measured below that level for view angles of 30 deg or greater. C-band backscatter data could provide information to monitor tillage operations at small view zenith angles and vegetation at large view zenith angles.

Daughtry, C. S. T.↗

NASA NTRS1991

Multi-frequency and polarimetric radar backscatter signatures for discrimination between agricultural crops at the Flevoland experimental test site

We describe the calibration and analysis of multi-frequency, multi-polarization radar backscatter signatures over an agriculture test site in the Netherlands. The calibration procedure involved two stages: in the first stage, polarimetric and radiometric calibrations (ignoring noise) were carried out using square-base trihedral corner reflector signatures and some properties of the clutter background. In the second stage, a novel algorithm was used to estimate the noise level in the polarimetric data channels by using the measured signature of an idealized rough surface with Bragg scattering (the ocean in this case). This estimated noise level was then used to correct the measured backscatter signatures from the agriculture fields. We examine the significance of several key parameters extracted from the calibrated and noise-corrected backscatter signatures. The significance is assessed in terms of the ability to uniquely separate among classes from 13 different backscatter types selected from the test site data, including eleven different crops, one forest and one ocean area. Using the parameters with the highest separation for a given class, we use a hierarchical algorithm to classify the entire image. We find that many classes, including ocean, forest, potato, and beet, can be identified with high reliability, while the classes for which no single parameter exhibits sufficient separation have higher rates of misclassification. We expect that modified decision criteria involving simultaneous consideration of several parameters increase performance for these classes.

Freeman, A.↗

NASA NTRS1992

Multi-wavelength Airborne Lidar Intercomparisons of Aerosol and Cirrus Backscatter over the Pacific Ocean

Airborne CO2 and Nd:YAG lidar instruments were flown on the NASA DC-8 research aircraft during two extended missions over the Pacific. The objectives during these flights were to characterize aerosol backscatter levels at infrared wavelengths in relatively pristine regions of the troposphere, and to observe cloud effects on lidar signals. In addition, the three wavelength Nd:YAG data were processed using independent temperature profile information, in order to deduce the Rayleigh scattering component of the total backscatter at each wavelength. The data can be used to better understand the capabilities of various airborne or Earth-orbiting lidar designs to provide backscatter information from aerosols and clouds, or to provide Doppler or DIAL information from aerosol and cloud backscatter.

Menzies, Robert T.↗

NASA NTRS1992

Comparison of radar backscatter from Antarctic and Arctic sea ice

Two ship-based step-frequency radars, one at C-band (5.3 GHz) and one at Ku-band (13.9 GHz), measured backscatter from ice in the Weddell Sea. Most of the backscatter data were from first-year (FY) and second-year (SY) ice at the ice stations where the ship was stationary and detailed snow and ice characterizations were performed. The presence of a slush layer at the snow-ice interface masks the distinction between FY and SY ice in the Weddell Sea, whereas in the Arctic the separation is quite distinct. The effect of snow-covered ice on backscattering coefficients (sigma0) from the Weddell Sea region indicates that surface scattering is the dominant factor. Measured sigma0 values were compared with Kirchhoff and regression-analysis models. The Weibull power-density function was used to fit the measured backscattering coefficients at 45 deg.

Hosseinmostafa, R.↗

NASA NTRS1993

Santa Barbara microwave backscattering model for woodlands

The Santa Barbara microwave backscattering model for woodland vegetation with discontinuous tree canopies is described, with an emphasis on the construction of the model from probability-weighted sub-components. The modelling approach is to treat individual tree crowns as scatterers and attenuators, using the probabilities of scattering and attenuation to compute total backscatter. Four major model components are defined: surface backscattering, crown volume scattering, multi-path interactions between crown and ground, and double-bounce trunk-ground interactions. Each component is divided into subcomponents having distinct scattering and attenuation paths. The scattering of each subcomponent is computed and weighted by the probability of its occurrence. Total backscatter from a simulated woodland stand is computed by incoherent summation of the components. Recent revisions to the model have modified the subcomponent definitions and improved the probability formulation.

Wang, Y.↗