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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The effect of vegetation type, microrelief, and incidence angle on radar backscatter

The NASA/JPL Synthetic Aperture Radar (SAR) was flown over a 20 x 110 km test site in the Texas High Plains regions north of Lubbock during February/March 1984. The effect of incidence angle was investigated by comparing the pixel values of the calibrated and uncalibrated images. Ten-pixel-wide transects along the entire azimuth were averaged in each of the two scenes, and plotted against the calculated incidence angle of the center of each range increment. It is evident from the graphs that both the magnitudes and patterns exhibited by the corresponding transect means of the two images are highly dissimilar. For each of the cross-poles, the uncalibrated image displayed very distinct and systematic positive trends through the entire range of incidence angles. The two like-poles, however, exhibited relatively constant returns. In the calibrated image, the cross-poles exhibited a constant return, while the like-poles demonstrated a strong negative trend across the range of look-angles, as might be expected.

Owe, M.↗
NASA NTRS2022

Progress in Time-Series Soil Moisture Retrieval Using L- and S-Band Radar Backscatter

L- and S-band observations from NASA's Passive/Active L/S band (PALS) sensor from the SMEX02 campaign were used to estimate soil moisture. The retrieval process is based on the “alpha approximation” method. This method utilizes a time-series of normalized radar backscatter measurements as well as ancillary information to estimate soil moisture over the Walnut Creek watershed. The resulting retrieved soil moistures are compared to in-situ soil moisture measurements at multiple test sites within the watershed. The calculations show reasonable results for both L- and S-band and provide further insight into the use of L- and S-bands for the upcoming NASA/ISRO mission.

Soil moisture↗
NASA NTRS1990

Near-nadir radar backscatter from a well-developed sea

The Kirchhoff approximation for the case of well-developed seas is analyzed. In this peculiar case, the equilibrium range in the wave number spectrum corresponds to a cascade pattern in the surface geometry. Its high wave number cutoff is shown to be a major factor of the radar backscatter. This intrinsic scale is evaluated, and both the geometrical and the physical optics terms are related to major parameters of wind-wave dynamics. The range of validity of the Kirchhoff approximation and the relative importance of the diffraction correction are analyzed. Finally, the radar cross section sigma exp 0 of well-developed seas is compared with that of poorly developed seas. The great qualitative difference shown in the wind speed dependence of sigma exp 0 for these two regimes is pointed out as a source of a considerable error trend recently discovered in satellite altimeter wind measurements.

Glazman, Roman E.↗
NASA NTRS1984

Radar backscatter modelling

The terrain analysis software package was restructured and documentation was added. A program was written to test Johnson Space Center's four band scatterometer data for spurious signals data. A catalog of terrain roughness statistics and calibrated four frequency multipolarization scatterometer data is being published to support the maintenance of Death Valley as a radar backscatter calibration test site for all future airborne and spacecraft missions. Test pits were dug through sand covered terrains in the Eastern Sahara to define the depth and character of subsurface interfaces responsible for either backscatter or specular response in SIR-A imagery. Blocky sandstone bedrock surfaces at about 1 m depth were responsible for the brightest SIR-A returns. Irregular very dense CaCO3 cemented sand interfaces were responsible for intermediate grey tones. Ancient river valleys had the weakest response. Reexamination of SEASAT l-band imagery of U.S. deserts continues.

Schaber, G. G.↗
NASA NTRS1993

Radar backscattering from snow facies of the Greenland ice sheet: Results from the AIRSAR 1991 campaign

In June 1991, the NASA/JPL airborne SAR (AIRSAR) acquired C- (lambda = 5.6cm), L- (lambda = 24cm), and P- (lambda = 68m) band polarimetric SAR data over the Greenland ice sheet. These data are processed using version 3.55 of the AIRSAR processor which provides radiometrically and polarimetrically calibrated images. The internal calibration of the AIRSAR data is cross-checked using the radar response from corner reflectors deployed prior to flight in one of the scenes. In addition, a quantitative assessment of the noise power level at various frequencies and polarizations is made in all the scenes. Synoptic SAR data corresponding to a swath width of about 12 by 50 km in length (compared to the standard 12 x 12 km size of high-resolution scenes) are also processed and calibrated to study transitions in radar backscatter as a function of snow facies at selected frequencies and polarizations. The snow facies on the Greenland ice sheet are traditionally categorized based on differences in melting regime during the summer months. The interior of Greenland corresponds to the dry snow zone where terrain elevation is the highest and no snow melt occurs. The lowest elevation boundary of the dry snow zone is known traditionally as the dry snow line. Beneath it is the percolation zone where melting occurs in the summer and water percolates through the snow freezing at depth to form massive ice lenses and ice pipes. At the downslope margin of this zone is the wet snow line. Below it, the wet snow zone corresponds to the lowest elevations where snow remains at the end of the summer. Ablation produces enough meltwater to create areas of snow saturated with water, together with ponds and lakes. The lowest altitude zone of ablation sees enough summer melt to remove all traces of seasonal snow accumulation, such that the surface comprises bare glacier ice.

Rignot, Eric↗
NASA NTRS2013

Updated Model of Radar Backscatter for Rough Lunar Craters

We reexamined our radar scattering model for young, rough craters [1] based on unpublished data from the 1980's [2]. Our model for scattering from the lunar surface is a mixing model consisting of varying amounts of diffuse and specular components as shown in Figure 1. The specular component, which consists of only opposite-sense circular (OC) echoes, results from the mirror-like surface and sub-surface layers that are smooth to a tenth of a radar wavelength for large (>10 wavelengths) areas oriented perpendicular to the radar's line-ofsight. The diffuse component, which has both OC and same sense (SC) circular echoes, is associated with either surface roughness (wavelength-sized rocks) or ice, and is assumed to be uniformly bright, with backscatter being proportional to the cosine of the incidence angle. Only diffuse scattering contributes to the SC echoes.

Moon↗
NASA NTRS1979

Relationship between hurricane surface winds and L-band radar backscatter from the sea surface

High-altitude, airborne, L-band synthetic aperture radar (SAR) data were collected in Hurricane Gloria on 28 and 30 September 1976. The backscattered power levels (proportional to the surface scattering coefficient) averaged over a few square kilometers of surface area were found to vary with surface wind speed and the angle of the wind relative to the radar. Comparisons between the backscatter from the eye and eye-wall regions of the hurricane were made with low-level aircraft wind measurements that were nearly coincident in space and time. The SAR has the potential advantage over other radar types because of its higher spatial resolution. It also appears to have the ability to penetrate rainfall, with a reduction in the echo from the surface. One difference when compared with higher frequency microwave radars is a decrease in the sensitivity of the backscatter to changes in wind speed. This dependence of L-band radar backscatter on surface winds suggests that the winds associated with hurricanes can be measured with airborne or spaceborne radars.

Weissman, D. E.↗
NASA NTRS1994

Structural and stratigraphic features and ERS 1 synthetic aperture radar backscatter characteristics of ice growing on shallow lakes in NW Alaska, winter 1991-1992

Changes in Earth Remote-Sensing Satellite (ERS) 1 C band synthetic aperture radar (SAR) backscatter intensity (sigma(exp 0)) from ice growing on shallow tundra lakes at three locations in NW Alaska are described. Ice core analysis shows that all lakes on the coast at Barrow the ice, whether floating or frozen to the bottom, includes an inclusion-free layer overlying a layer of ice with tubular bubbles oriented parallel to the direction of growth. The clear ice may also be overlain by a discontinuous layer of bubbly snow ice. Backscatter is low (-16 to -22 dB) at the time of initial ice formation, probably due to the specular nature of the upper and lower ice surfaces causing the radar pulse to be reflected away from the radar. As the ice thickens during the autumn, backscatter rises steadily. Once the ice freezes to the lake bottom, regardless of the presence of foward scattering tubular bubbles, low backscatter values of -17 to -18 dB are caused by absorption of the radar signal in the lake bed. For ice that remains afloat all winter the ice-water interface and the tubular bubbles combine, presumably via an incoherent double-bounce mechanism, to cause maximum backscatter values of the order of -6 to -7 dB. The sigma(exp 0) saturates at -6 to -7 dB before maximum ice thickness and tubular bubble content are attained. A simple ice growth model suggests that the layer of ice with tubular bubbles need be only a few centimeters thick midway through the growth season to cause maximum backscatter from floating ice. During the spring thaw a previously unreported backscatter reversal is observed on the floating and grounded portions of the coastal lakes but not on the lakes farther inland. This reversal may be related to the ice surface topography and wetness plus the effects of a longer, cooler melt period by the coast. Time series of backscatter variations from shallow tundra lakes are a record of (1) the development of tubular bubbles in the ice and, by association, changes in the gas content of the underlying water and (2) the freezing of ice to the bottoms of the lakes and therefore lake bathymetry and water availability. SAR is also able to detect the onset of lake ice growth in autumn and the initiation of the spring thaw and thus has potential for monitoring high-altitude lake ice growth and decay processes in relation to climate variability.

Jeffries, M. O.↗
NASA NTRS1983

The relationship between strength of turbulence and backscattering radar power at HF and VHF

The formulae relating turbulence and other atmospheric parameters to backscattered power for radar observations are reviewed. Emphasis is on the case of scatter from turbulent irregularities which have scales corresponding to the range of isotropic, inertial range turbulence. The applicability of this assumption is discussed. A formula is introduced for the mesosphere which relates ionospheric electron densities to backscattered power.

Hocking, W. K.↗
NASA NTRS1995

Relating Radar Backscatter to Boreal Forest Canopy Parameters

During the first intensive field campaign for the Boreal Ecosystem Atmospheric Study (BOREAS) in August of 1993, AIRSAR data were acquired over the entire BOREAS study area. On of the objectives of the AIRSAR deployment was to examine the sensitivity of the radar signal to vegetation type and biomass distribution in the boreal forest and to develop algorithms for inferring vegetation parameters. During the experiment a set of ground measurements were also made to support the AIRSAR data analysis. The dominant stands in the study area consist of black spruce, young jack pine, old jack pine, and aspen. These stands represent a wide range of biomass and canopy architectural variations which can be distinguished in SAR images.

radar backscatter↗
NASA NTRS1983

C-band measurements of radar backscatter from ice project summary report

The ability to measure the radar scattering coefficient of ice with a helicopter or surface spectrometer was extended into the 4-8 GHz spectral region. The scattering coefficient was measured at Mould Bay, N.W.T., over a frequency range from 4 to 18 GHz for both summer and fall conditions. Scatter from fresh water ice in the St. Lawrence River and from numerous seasonal sea-ice types along the coast of Newfoundland were also measured. The C-band (near 5 GHz) scattering cross section for different types of ice shows poorer contrast than the scattering coefficient at higher frequencies, but better contrast than the negligible value found at L-band (1.5 GHz). At frequencies above 4 GHz the contrast in scattering coefficient between the different ice types is much less in summer than in other seasons; at most times of year the scattering is much stronger from multiyear than from other ice types, but in early summer it is actually slightly weaker than that from first year ice.

Onstott, R. G.↗
NASA NTRS1989

Experimental investigation of the dependence of radar backscattering on wind speed, wind stress and wave height

During summer 1988, radar measurements were performed in conjunction with detailed environmental observations on Lake Washington at the University of Washington Sand Point field station. Radar data were collected at 5.3 and 10 GHz for incidence angles between 30 and 60 deg with VV-polarization. The environmental measurements included wind speed and direction, large-wave heights, the high-frequency portion of the wave spectrum, humidity, and air and water temperatures. The small-scale wave spectrum was measured using a resistance wire gauge. The results show that backscatter increased with wind speed as expected. However, little difference was observed in the scattering coefficient for upwind and crosswind directions. The results also indicated an increase in the amplitude of small waves with friction velocity.

Gogineni, S. P.↗
NASA NTRS1982

Radar backscattering from a rough rotating triaxial ellipsoid with applications to the geodesy of small asteroids

Radar geodesy of the asteroid 433 Eros was reported by Jurgens and Goldstein (1976). Their measurements were based on the spectral properties of a rough rotating triaxial ellipsoid. This paper presents the theory by which theoretical spectra based on a backscattering model of the form of the nth power of the cosine of theta, or any reasonable backscattering model, can be computed. Some general properties of these spectra are demonstrated, and simple measurements of the apparent radar cross section, center frequency, and effective bandwidth as a function of time are shown to be useful in determining the sizes of the semi-major axes, the exponent n in the scattering model, the backscatter efficiency of the surface material, the rotation period, and the declination of the observer above or below the rotation equator. Some of these parameters are correlated, and multiple estimation may be difficult unless the observations span a period sufficiently long so as to present a wide range of declinations of the observer. However, the departure from a spherical shape aids in separating the scattering properties from the target dimensions and facilitates the simultaneous estimation of all free parameters.

Jurgens, R. F.↗