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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Studies of microwave scattering and canopy architecture for boreal forests

Our primary objectives during the last year have been to develop a helicopter-borne radar system for measuring microwave backscatter from vegetation and to use this system to study the characteristics of backscatter from the boreal forest. Our research is aimed at refining the current microwave models and using these improvements for more accurate interpretation of SAR data. SAR data are very useful for monitoring the boreal forest region because of the microwave signal's ability to penetrate clouds and to see at night. Meeting these objectives involves several stages of development. The first stage is the design and implementation of a frequency-modulated continuous-wave (FM-CW) radar system with the capability of measuring backscatter at three frequencies and four polarizations at each frequency. These requirements necessitate a twelve-channel radar system. Using three frequencies is advantageous because it allows us to look at different parts of the canopy. For instance, the lower frequency signal penetrates deeper into the canopy and allows us to see the ground while the high frequency signal is scattered more by the leaves and needles and typically does not penetrate to the floor of the forest. We designed the radar starting with the antenna system. We then developed the intermediate frequency (IF) and radio frequency (RF) sections of the radar. Also, the need to collect data from twelve channels during each flight line presented a complex data acquisition problem that we solved by using a high-speed data acquisition board. After construction, the radar was tested at the lab. We performed extensive testing of the IF and RF systems of the radar during this time. Once we were satisfied with the operation of the radar it was shipped to Canada for use in the second intensive field campaign (IFC-2) from July 16 - August 8, 1994. During IFC-2, we collected backscatter data over the experimental sites in the southern study area (SSA). Additionally, we used a ground-based step-frequency radar to measure the reflection coefficient of the forest floor at the old jack pine (OJP) and young jack pine (YJP) sites. The ground-based radar data have been processed and a frew examples are included in this report. we are currently processing the helicopter-borne radar data.

Lockhart, G. Lance↗
NASA NTRS1975

Methods for correcting microwave scattering and emission measurements for atmospheric effects

The author has identified the following significant results. Algorithms were developed to permit correction of scattering coefficient and brightness temperature for the Skylab S193 Radscat for the effects of cloud attenuation. These algorithms depend upon a measurement of the vertically polarized excess brightness temperature at 50 deg incidence angle. This excess temperature is converted to an equivalent 50 deg attenuation, which may then be used to estimate the horizontally polarized excess brightness temperature and reduced scattering coefficient at 50 deg. For angles other than 50 deg, the correction also requires use of the variation of emissivity with salinity and water temperature.

Komen, M.↗
NASA NTRS1976

Seasonal variations of the microwave scattering properties of deciduous trees as measured in the 1-18 GHz spectral range

The author has identified the following significant results. Employing two FM-CW radar spectrometers, scattering data were acquired from stands of deciduous trees during the spring and autumn. The data suggest that the trees act as a volume scatter target particularly in the 7-18 GHz region. A comparison of data collected in spring and autumn indicates that the radar scattering coefficient, sigma deg, as measured in spring can be substantially larger (as much as 10 dB) than sigma deg as measured in the autumn.

Ulaby, F. T.↗
NASA NTRS1977

Microwave scattering properties of snow fields

Experimental results were presented showing backscatter dependence on frequency, angle of incidence, snow wetness, and frequency modulation. Theoretical studies were made of the inverse scattering problem yielding some preliminary results concerning the determination of the dielectric constant of the snow layer. The experimental results lead to the following conclusions: (1) snow layering affects backscatter; (2) layer response was significant up to 45 degrees of incidence; (3) wetness modifies snow layer effects; and (4) frequency modulation masks the layer response.

Angelakos, D. J.↗
NASA NTRS1977

The theory and measurement of noncoherent microwave scattering parameters

The radar equation for incoherent scenes is derived and scattering coefficients are introduced in a systematic way to account for the complete interaction between the incident wave and the random scene. Intensity (power) and correlation techniques similar to that for coherent targets are proposed to measure all the scattering parameters. The sensitivity of the intensity technique to various practical realizations of the antenna polarization requirements is evaluated by means of computer simulated measurements, conducted with a scattering characteristic similar to that of the sea. It was shown that for scenes satisfying reciprocity one must admit three new cross-correlation scattering coefficients in addition to the commonly measured autocorrelation coefficients.

Claassen, J. P.↗
NASA NTRS1988

Polarimetric microwave scattering from known randomly rough surfaces

Scattering from rough surfaces is reviewed, and artificial randomly rough surfaces are described. Measurements were made from 4 to 18 GHz for the 4 linear polarization states. The target consists of a metalized randomly rough surface constructed from known surface statistical information. The surface was generated using surface generation routines on the computer, and then the data was converted for the machining of the surface on a computer controlled mill. Bistatic and monostatic measurements were acquired at incident angles from 25 to 60 deg. At the 13.5 to 18 GHz frequencies, the effects of backscatter enhancement and depolarization effects can be studied in a regime which does not satisfy Kirchoff or small perturbation assumptions. This allows scattering mechanisms to be introduced into models, which may show missing elements in the more standard formulations. At lower frequency bands such as 4 to 8.5 GHz the scattering behavior was also examined to see the behavior as assumptions of other formulations are satisfied.

Nance, C. E.↗
NASA NTRS1988

Important changes in microwave scattering properties of young snow-covered sea ice as indicated from dielectric modelling

Winter measurements of the properties of young snow-covered fast ice are input into a mixture model to calculate the complex dielectric constant. Large changes in the electromagnetic properties of the medium occur over the 46 day period of measurements. This early evolution of the surface is shown to have an important impact upon predicted radar backscattering properties at GHz frequencies. This has consequences for the unambiguous identification of young ice forms from synthetic aperture or side-looking airborne radar images.

Drinkwater, M. R.↗
NASA NTRS1989

Microwave scattering models and basic experiments

Progress is summarized which has been made in four areas of study: (1) scattering model development for sparsely populated media, such as a forested area; (2) scattering model development for dense media, such as a sea ice medium or a snow covered terrain; (3) model development for randomly rough surfaces; and (4) design and conduct of basic scattering and attenuation experiments suitable for the verification of theoretical models.

Fung, Adrian K.↗
NASA NTRS1992

Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space

The electromagnetic bias is an error present in radar altimetry of the ocean due to the non-uniform reflection from wave troughs and crests. A study of the electromagnetic bias became necessary to permit error reduction in mean sea level measurements of satellite radar altimeters. Satellite radar altimeters have been used to find the upper and lower bounds for the electromagnetic bias. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to predict the electromagnetic bias. The predicted electromagnetic bias will be compared to measurements at C and Ku bands.

Arnold, David↗
NASA NTRS1992

Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space

The electromagnetic (EM) bias 'epsilon' is an error present in radar altimetry of the ocean due to the nonuniform reflection from wave troughs and crests. The EM bias is defined as the difference between the mean reflecting surface and the mean sea surface. A knowledge of the EM bias is necessary to permit error reduction in mean sea level measurements by satellite radar altimeters. Direct measurements of the EM bias were made from a Shell Offshore oil production platform in the Gulf of Mexico for a six month period during 1989 and 1990. Measurements of the EM bias were made at 5 and 14 Ghz. During the EM bias experiments by Melville et al., a wire wave gauge was used to obtain the modulation of the high frequency waves by the low frequency waves. It became apparent that the EM bias was primarily caused by the modulation of the short waves. This was reported by Arnold et al. The EM bias is explained using physical optics scattering and an empirical model for the short wave modulation. Measurements of the short wave modulation using a wire wave gauge demonstrated a linear dependence of the normalized bias on the short wave modulation strength, M. The theory accurately predicts this dependence by the relation epsilon = -alphaMH sub 1/3. The wind speed dependence of the normalized bias is explained by the dependence of the short wave modulation strength on the wind speed. While other effects such as long wave tilt and curvature will have an effect on the bias, the primary cause of the bias is shown to be due to the short wave modulation. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to estimate the EM bias. The estimated EM bias will be compared to measurements at C and Ku bands.

Arnold, David↗
NASA NTRS1992

Microwave scattering and emission properties of large impact craters on the surface of Venus

Many of the impact craters on Venus imaged by the Magellan synthetic aperture radar (SAR) have interior floors with oblique incidence angle backscatter cross sections 2 to 16 times (3 dB to 12 dB) greater than the average scattering properties of the planet's surface. Such high backscatter cross sections are indicative of a high degree of wavelength-scale surface roughness and/or a high intrinsic reflectivity of the material forming the crater floors. Fifty-three of these (radar) bright floored craters are associated with 93 percent of the parabolic-shaped radar-dark features found in the Magellan SAR and emissivity data, features that are thought to be among the youngest on the surface of Venus. It was suggested by Campbell et al. that either the bright floors of the parabolic feature parent craters are indicative of a young impact and the floor properties are modified with time to a lower backscatter cross section or that they result from some property of the surface or subsurface material at the point of impact or from the properties of the impacting object. As a continuation of earlier work we have examined all craters with diameters greater than 30 km (except 6 that were outside the available data) so both the backscatter cross section and emissivity of the crater floors could be estimated from the Magellan data.

Stacy, N. J. S.↗
NASA NTRS1992

C-band microwave scattering from small balsam fir

An experiment to examine the C-band backscattering characteristics of conifer trees was conducted using a truck-mounted scatterometer. Small (1 m tall) balsam fir (Abies balsamea) were arranged at various equidistant spacings on a platform to present canopies of varying density to the radar. C-band backscattering measurements of a range of canopy densities were acquired under different polarizations and incidence angles. The measured backscattering coefficient from the tree canopies increased with increasing biomass, but approached a maximum at a LAI of 2.5 and fresh biomass of 3.3 kg/sq m. A backscatter model was implemented using measured canopy attributes and showed close agreement with scatterometer measurements over the range of canopy densities. Model results indicated that branches were the prime scatterers of the radar while needles were found to only slightly attenuate the radar signal.

Ranson, K. J.↗
NASA NTRS1993

An experimental study of microwave scattering from rain- and wind-roughened seas

This paper investigates radar cross-section (RCS) characteristics of rain- and wind-roughened sea-surfaces. We conducted experiments in laboratory wind-wave tanks using artificial rain. The study includes light rain rates, light wind speeds, and combinations of these. A 36 Ghz scatterometer was operated at 30 deg incidence angle and with vertical polarization. RCS data were obtained not only with the scatterometer pointing up-wind but also as a function of azimuthal angle. We use a scatterometer rain and wind model SRWM-1, which relates the total average RCS in storms to the sum of the average RCS due to rain plus the average RCS due to wind. Implications of the study for operational monitoring of wind in rainy oceanic areas by satellite-borne instruments is discussed.

Bliven, L. F.↗
NASA NTRS1993

Microwave Scattering Model for Grass Blade Structures

In this paper, the electromagnetic scattering solution for a grass blade with complex cross-section geometry is considered. It is assumed that the blade cross section is electrically small, but its length is large compared to the incident wavelength. In a recent study it has been shown that the scattering solution for such problems, in the form of a polarizability tensor, can be obtained using the low-frequency approximation in conjunction with the method of moments. In addition, the study shows that the relationship between the polarizability tensor of a dielectric cylinder and its dielectric constant can be approximated by a simple algebraic expression. The results of this study are used to show that this algebraic approximation is valid also for cylinders with cross sections the shape of grass blades, providing that proper values am selected for each of three constants appearing in the expression. These constants are dependent on cylinder shape, and if the relationship between the constants and the three parameters describing a grass blade shape can be determined, an algebraic approximation relating polarizability tensor to blade shape, as well as dielectric constant, can be formed. Since the elements of the polarizability tensor are dependent on only these parameters, this algebraic approximation can replace the cumbersome method of moments model. A conjugate gradient method is then implemented to correctly determine the three constants of the algebraic approximation for each blade shape. A third-order polynomial fit to the data is then determined for each constant, thus providing a complete analytic replacement to the numerical (moment method) scattering model. Comparisons of this approximation to the numerical model show an average error of less than 3%.

Stiles, James M.↗
NASA NTRS1997

Snow Crystal Shape and Microwave Scattering

The radiometric calibration of SeaWiFS data includes a correction for the temperature dependence of the individual detector sensitivities. The detector temperature are measured by temperature sensors mounted on instrument focal planes. Processing of the temperature sensor output by an onboard instrument computer introduces a nonlinear response into the temperature data. This paper describes the calibration of the temperature sensor output and the computation of the temperature corrections for the radiometric calibration of the instrument.

Foster, J. L.↗
DOE OSTI2025

Development and preliminary results of 270 GHz microwave forward scattering diagnostic system on the experimental advanced superconducting tokamak (EAST)

To measure localized (intermediate, high) poloidal wavenumber electron density fluctuations, a tangential millimeter-wave collective scattering system has been designed and successfully developed on the experimental advanced superconducting tokamak (EAST). This innovative system employs a 270 GHz mm-wave probe beam, emitted from the mid-plane of Port K and directed towards Port P after passing through a combination of two in-vessel mirrors. Here these two ports are located on the EAST device’s midplane and are 110° apart. The downward scattered signals pass through two in-vessel mirrors, and exit through the P window. The received multi-channel separated scattering signals are arranged along the poloidal direction. This optical arrangement makes the monitored turbulent wave number is poloidal dominant and up to 40 cm −1 . The tangential microwave scattering scheme enables this diagnostic to monitor local density fluctuations, with typically a radial spatial resolution △ R = 5 cm, and a wavenumber resolution of △ k = 0.4 cm −1 . The scattering system underwent comprehensive laboratory testing in 2023, with installation finalized in 2024. The system features steerable launch and receiver optics, allowing remote control to position the scattering volume from normalized radius ρ –0 to the pedestal region on a shot-by-shot basis. Effective experimental data, characterized by distinct off-center spectral peaks, have been successfully obtained in neutral beam-heated plasmas on EAST. The beam tracing forward modeling has been employed for experimental data interpretation. The advanced millimeter-wave scattering system provides fluctuation measurement capability from ion temperature gradient to electron temperature gradient scale on low field side. Preliminary experimental density fluctuation data have been successfully obtained in neutral beam-heated plasmas on EAST. The localized measurement will be used for kinetic turbulence transport numerical simulation validation.

high-k collective scattering↗