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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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All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

(abstract) Monitoring the Freeze/Thaw Transitions in Taiga Forests Using ERS-1 SAR

Automated recording stations have been installed at the Bonanza Creek Experimental Forest, a Long Term Ecological Research (LTER) site located near Fairbanks, Alaska, in a forest stand of the Tanana River floodplain underlain by discontinuous permafrost. These stations provide a continuous record of dielectric constant and temperature of tree trunks, and soil moisture and temperature profiles down to the root zone. Along with the weather stations deployed at the same location, these measurements provide a continuous record of the environmental and phenologic conditions of the forest during a complete seasonal cycle. At the same time, ERS-1 SAR imaged the study site repeatedly from space to provide radar backscatter measurements of the forest approximately three times a month. Here, we examine the temporal dynamic of ERS-1 SAR measurements in relation with the changing environmental and phenologic state of the forest canopy and of the forest ground layers during the winter/spring and fall/winter transitions of 1992 and 1993. During these transitions, we examine whether changes in radar backscatter observed by ERS-1 may be related to freezing or thawing of the soil and vegetation in order to determine the start and end of the growing season for the forest. The results of this analysis are used in turn to determine whether similar changes are observed over larger regions. Mosaics of SAR data generated along three different North-South Alaskan ERS-1 transects that intercept with our study site are used in combination with hourly air temperature and daily precipitation rates gathered at airport weather stations by the National Weather Service. Results obtained using ERS-1 data collected from January 1992 to mid-1993 will be discussed.

permafrost temporal dynamics Alaska ecology radar ↗
NASA NTRS1992

Monitoring of environmental conditions in the Alaskan forests using ERS-1 SAR data

Preliminary results from an analysis of the multitemporal radar backscatter signatures of tree species acquired by European Remote Sensing Satellite (ERS-1) synthetic aperture radar (SAR) data are presented. Significant changes in radar backscatter are detected. Correlation of these differences with ground truth observations indicate that these are due to changes in soil and liquid water content as a result of freeze/thaw events. C-band observations acquired by the NASA/Jet Propulsion Laboratory Airborne SAR (JPL AIRSAR) instrument demonstrate the potential of a C-band radar instrument to monitor drought/flood events. The potential of ERS-1 for monitoring phenologic changes in the forest and for classifying tree species is less promising.

Rignot, Eric↗
NASA NTRS1993

Resolution enhancement of spaceborne scatterometer data

Spaceborne wind scatterometers are designed principally to measure radar backscatter from the ocean's surface for the determination of the near-surface wind direction and speed. Although measurements of the radar backscatter are made over land, application of these measurements has been limited primarily to the calibration of the instrument. In this paper we present a method for generating enhanced resolution radar images of the earth's surface using spaceborne scatterometry. The technique is based on a new image reconstruction technique which takes advantage of the spatial overlap in scatterometer measurements made at different times to provide enhanced imaging resolution. We describe the reconstruction algorithm and demonstrate the technique using both simulated and actual Seasat-A Scatterometer (SASS) measurements. The SASS-derived images, which have approximately 4 km resolution, dramatically illustrate the resolution enhancement capability of the new technique. The technique permits utilization of both historic and contemporary scatterometer data for medium-scale monitoring of vegetation and polar ice. We discuss the tradeoff between imaging noise and resolution inherent in the technique.

Long, David G.↗
NASA NTRS1989

Radar interpretation of lava fields as a function of incidence angle - Implications for interpretation of Magellan SAR data on Venus

The capability of a single frequency, single polarization radar to distinguish different lava flows solely on the basis of their relative radar backscatter as measured by scatterometer profiles is addressed. It is found that mantled and unmantled flow surfaces can be separated by their radar backscatter and become more distinct with increasing incidence angle. Pristine pahoehoe surfaces have backscatter coefficients distinctly different from older flows at small angles, whereas the most modified pahoehoe units are identifiable at larger incidence angles. At all latitudes on Venus, it should be possible to distinguish the style of volcanism responsible for volcanic plains emplacement.

Theilig, E.↗
NASA NTRS1992

Greenland observed at high resolution by the Seasat scatterometer

A previously developed method for the reconstruction of surface radar backscatter characteristics is applied to Seasat scatterometer (SASS) data for the study of Greenland's ice sheet. A time series of the radar backscatter images provides an island-wide view of the ice sheet which shows the extent of the summer melt. Medium-scale scatterometer images provide frequent island-wide observations permitting precise measurements of the seasonal extent of the summer melt.

Long, D. G.↗
NASA NTRS2000

BOREAS RSS-16 Level-3b DC-8 AIRSAR SY Images

The BOREAS RSS-16 team used satellite and aircraft SAR data in conjunction with various ground measurements to determine the moisture regime of the boreal forest. RSS-16 assisted with the acquisition and ordering of NASA JPL AIRSAR data collected from the NASA DC-8 aircraft. The NASA JPL AIRSAR is a side-looking imaging radar system that utilizes the SAR principle to obtain high-resolution images that represent the radar backscatter of the imaged surface at different frequencies and polarizations. The information contained in each pixel of the AIRSAR data represents the radar backscatter for all possible combinations of horizontal and vertical transmit and receive polarizations (i.e., HH, HV, VH, and VV). Geographically, the data cover portions of the BOREAS SSA and NSA. Temporally, the data were acquired from 12-Aug-1993 to 31-Jul-1995. The level-3b AIRSAR SY data are the JPL synoptic product and contain 3 of the 12 total frequency and polarization combinations that are possible. The data are stored in binary image format files. The data files are available on a CD-ROM (see document number 20010000884), or from the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).

Hall, Forrest G.↗
NASA NTRS1986

Preliminary evaluation of the SIR-B response to soil moisture, surface roughness, and crop canopy cover

Two predawn ascending data-takes by the Shuttle Imaging Radar-B (SIR-B) were used to evaluate the effects of surface roughness, crop canopy, and soil moisture on radar backscatter. The two images, separated by three days, were both obtained at 30-deg local angle of incidence, but with opposite azimuth viewing directions. The imagery was externally calibrated with respect to the radar backscattering coefficient sigma(0) via response to arrays of point and area-extended targets of known radar cross section. Three land-cover classes: (1) corn, (2) corn stubble and plowed bare soil, and (3) disked bare soil, soybeans, soybean stubble, alfalfa, and clover could be readily separated for either observation date on the basis of image tone alone. The dependence of sigma(0) on the surface roughness and canopy brightness inhibits the capability of SIR to globally estimate the near-surface soil moisture from the value of sigma(0) for single date observations, unless the surface roughness or canopy cover conditions are accounted for. However, within given ranges of these conditions, the sigma(0) was found to be highly correlated with the soil moisture.

Dobson, M. C.↗
NASA NTRS1992

High-resolution land/ice imaging using Seasat scatterometer measurements

A new method for obtaining high-resolution images (to 4 km) of the land backscatter from low-resolution Seasat-A scatterometer (SASS) measurements is introduced. The method utilizes the measurement cell overlap in multiple spacecraft passes over the region of interest and signal processing techniques to generate high-resolution images of the radar backscatter. The overlap is exploited to estimate the underlying high-resolution surface radar backscatter characteristics using a robust multivariate image reconstruction algorithm. The algorithm has been designed to operate in the high-noise environment typical of scatterometer measurements. The ultimate resolution obtainable is a function of the number of measurements and the measurement overlap. Sample results based on SASS data are provided.

Long, D. G.↗
NASA NTRS1991

Multi-polarization C-band SAR for soil moisture estimation

Previous studies of synthetic aperture radar (SAR) imagery have shown qualitative relationships between radar backscatter and soil moisture. However, to be able to use these data in operational programs it will be necessary to establish quantitatively how the radar return is related to soil moisture and the effects of surface roughness, soil type, and vegetation cover and growth stage, as a function of frequency and polarization. To this end, a multi-year experiment began in 1990 as a cooperative venture amongst the Canada Center (Agriculture Canada), and the Universities of Guelph, Sherbrooke, Laval, and Waterloo. During 1990, SAR imagery was acquired during two periods (May and Jun.) to correspond to times of minimal and substantial vegetation cover. SAR data were acquired on three days in May and on four days in Jul. to cover different soil moisture conditions. This unique comprehensive data set is used to investigate the relationships between soil moisture and radar backscatter. The experiment and data collected are described, and a preliminary qualitative interpretation of the relationship between soil moisture and image tone is provided.

Brown, R. J.↗
NASA NTRS1974

A theory of wave scatter from an inhomogeneous medium with a slightly rough boundary and its application to sea ice

An analytical theory of electromagnetic wave scattering from an inhomogeneous medium with a slightly rough boundary surface is formulated. The inhomogeneity in the medium is assumed to vary continuously in the vertical direction and to have a small random variation in the horizontal direction. The medium is assumed to consist of two layers. Maxwell's equations are solved by using the small perturbation method together with Fourier transform technique. The resulting differential equations are solved by using WKB and variation of parameter methods. Field amplitudes in each medium are determined by taking boundary conditions into account. The expressions for first order polarized radar backscatter cross-section are obtained. An attempt is made to apply the developed theory to compute sea ice scatter. Numerical calculations are performed for polarized radar backscatter cross-section at two frequencies, 13.3 GHz and 400 MHz. It is shown that WKB method is applicable at both of these frequencies. Theoretical results are compared with the experimental results obtained from NASA Earth Resources Program mission 126. Theoretical results and experimental results are in good agreement.

Parashar, S. K.↗
NASA NTRS1981

Microwave response of snow

Truck-mounted microwave sensors were used to acquire backscattering coefficient and apparent temperature data for snow-covered terrain at 19 frequencies between 1 and 18 GHz and at 35.6 GHz. Passive microwave measurements were made at 10.7, 37, and 94 GHz. The radar backscattering coefficient and radiometric emissivity were evaluated as a function of snow depth, wetness, surface roughness, and other snow parameters. It is shown that volume scattering makes the radar backscattering coefficient to increase and the radiometric emissivity to decrease with increasing snow depth until the snow layer appears electromagnetically semiinfinite in extent. The presence of liquid water in the snowpack results in increased attenuation and reduction in scattering which leads to less backscatter and more emission by the snow volume. It is concluded that microwave sensors have the potential capability for remotely monitoring both the snow equivalent and wetness of snowpacks.

Ulaby, F. T.↗
NASA NTRS1993

Synergy between optical and microwave remote sensing to derive soil and vegetation parameters from MAC Europe 1991 Experiment

The ability of remote sensing for monitoring vegetation density and soil moisture for agricultural applications is extensively studied. In optical bands, vegetation indices (NDVI, WDVI) in visible and near infrared reflectances are related to biophysical quantities as the leaf area index, the biomass. In active microwave bands, the quantitative assessment of crop parameters and soil moisture over agricultural areas by radar multiconfiguration algorithms remains prospective. Furthermore the main results are mostly validated on small test sites, but have still to be demonstrated in an operational way at a regional scale. In this study, a large data set of radar backscattering has been achieved at a regional scale on a French pilot watershed, the Orgeval, along two growing seasons in 1988 and 1989 (mainly wheat and corn). The radar backscattering was provided by the airborne scatterometer ERASME, designed at CRPE, (C and X bands and HH and VV polarizations). Empirical relationships to estimate water crop and soil moisture over wheat in CHH band under actual field conditions and at a watershed scale are investigated. Therefore, the algorithms developed in CHH band are applied for mapping the surface conditions over wheat fields using the AIRSAR and TMS images collected during the MAC EUROPE 1991 experiment. The synergy between optical and microwave bands is analyzed.

Taconet, O.↗
NASA NTRS2012

Estimating Sea Surface Salinity and Wind Using Combined Passive and Active L-Band Microwave Observations

Several L-band microwave radiometer and radar missions have been, or will be, operating in space for land and ocean observations. These include the NASA Aquarius mission and the Soil Moisture Active Passive (SMAP) mission, both of which use combined passive/ active L-band instruments. Aquarius s passive/active L-band microwave sensor has been designed to map the salinity field at the surface of the ocean from space. SMAP s primary objectives are for soil moisture and freeze/thaw detection, but it will operate continuously over the ocean, and hence will have significant potential for ocean surface research. In this innovation, an algorithm has been developed to retrieve simultaneously ocean surface salinity and wind from combined passive/active L-band microwave observations of sea surfaces. The algorithm takes advantage of the differing response of brightness temperatures and radar backscatter to salinity, wind speed, and direction, thus minimizing the least squares error (LSE) measure, which signifies the difference between measurements and model functions of brightness temperatures and radar backscatter. The algorithm uses the conjugate gradient method to search for the local minima of the LSE. Three LSE measures with different measurement combinations have been tested. The first LSE measure uses passive microwave data only with retrieval errors reaching 1 to 2 psu (practical salinity units) for salinity, and 1 to 2 m/s for wind speed. The second LSE measure uses both passive and active microwave data for vertical and horizontal polarizations. The addition of active microwave data significantly improves the retrieval accuracy by about a factor of five. To mitigate the impact of Faraday rotation on satellite observations, the third LSE measure uses measurement combinations invariant under the Faraday rotation. For Aquarius, the expected RMS SSS (sea surface salinity) error will be less than about 0.2 psu for low winds, and increases to 0.3 psu at 25 m/s wind speed for warm waters (25 C). To achieve the required 0.2 psu accuracy, the impact of sea surface roughness (e.g. wind-generated ripples) on the observed brightness temperature has to be corrected to better than one tenth of a degree Kelvin. With this algorithm, the accuracy of retrieved wind speed will be high, varying from a few tenths to 0.6 m/s. The expected direction accuracy is also excellent (less than 10 ) for mid to high winds, but degrades for lower speeds (less than 7 m/s).

Yueh, Simon H.↗
NASA NTRS1993

Estimation of biophysical properties of upland Sitka spruce (Picea sitchensis) plantations

It is widely accepted that estimates of forest above-ground biomass are required as inputs to forest ecosystem models, and that SAR data have the potential to provide such information. This study describes relationships between polarimetric radar backscatter and key biophysical properties of a coniferous plantation in upland central Wales, U.K. Over the test site, topography was relatively complex and was expected to influence the amount of radar backscatter.

Green, Robert M.↗
NASA NTRS1974

Investigation of radar discrimination of sea ice

The ability of radar to discriminate sea ice types and their thickness was studied. Radar backscatter measurements at 400 MHz (multi-polarization) and 13.3 GHz (VV polarization) obtained from NASA Earth Resources Aircraft Program Mission 126 were analyzed in detail. The scatterometer data were separated into seven categories of sea ice according to age and thickness as interpreted from stereo aerial photographs. The variations of radar backscatter cross-section with sea ice thickness at various angles are presented at the two frequencies. There is a reversal of angular character of radar return from sea ice less than 18 cm thick at the two frequencies. Multi-year ice (sea ice greater than 180 cm thick) gives strongest return at 13.3 GHz. First-year ice (30 cm to 90 cm thick) gives strongest return at 400 MHz. Open water can be differentiated at both the frequencies. Four-polarization 16.5 GHz radar imagery was also obtained. Open water and three categories of sea ice can be identified on the images. The results of the imagery analysis are consistent with the radar scatterometer results.

Parashar, S. K.↗
NASA NTRS2007

Simulation Studies of the Effect of Forest Spatial Structure on InSAR Signature

The height of scattering phase retrieved from InSAR data is considered being correlated with the tree height and the spatial structure of the forest stand. Though some researchers have used simple backscattering models to estimate tree height from the height of scattering center, the effect of forest spatial structure on InSAR data is not well understood yet. A three-dimensional coherent radar backscattering model for forest canopies based on realistic three-dimensional scene was used to investigate the effect in this paper. The realistic spatial structure of forest canopies was established either by field measurements (stem map) or through use of forest growth model. Field measurements or a forest growth model parameterized using local environmental parameters provides information of forest species composition and tree sizes in certain growth phases. A fractal tree model (L-system) was used to simulate individual 3- D tree structure of different ages or heights. Trees were positioned in a stand in certain patterns resulting in a 3-D medium of discrete scatterers. The radar coherent backscatter model took the 3-D forest scene as input and simulates the coherent radar backscattering signature. Interferometric SAR images of 3D scenes were simulated and heights of scattering phase centers were estimated from the simulated InSAR data. The effects of tree height, crown cover, crown depth, and the spatial distribution patterns of trees on the scattering phase center were analyzed. The results will be presented in the paper.

Sun, Guoqing↗
NASA NTRS2011

Backscatter Measurements Over Vegetation by Ground-Based Microwave Radars

In the study of radar backscattering from vegetated terrain, it is important to understand how the electromagnetic wave interacts with vegetation and the underlying ground. In this paper, an expression of backscattering from a vegetation canopy in the case of spherical wave illumination is derived. Such an expression might apply to the practical case of a ground-based scatterometer overlooking vegetation. The relative importance of the beamwidth as well as the platform height on backscattering from vegetated terrain is studied. Preliminary results indicate that the discrepancy with plane wave illumination can be rather significant, and therefore should not be overlooked.

Kurum, Mehmet↗
NASA NTRS1995

Development of an L-, C-, and X-band radar for backscattering studies over vegetation

With the recent surge of interest in global change, the impact of different ecosystems on the Earth's carbon budget has become the focus of many scientific studies. Studies have been launched by NASA and other agencies to address this issue. One such study is the Boreal Ecosystem-Atmosphere Study (BOREAS). BOREAS focuses on the boreal ecosystem in Northern Canada. As a part of the BOREAS study, we have developed a helicopter-borne three-band radar system for measuring the scattering coefficient of various stands within the boreal forest. During the summer of 1994 the radar was used at the southern study area (SSA) in Saskatchewan over the young jack pine (YJP), old jack pine (OJP), old black spruce (OBS) and old aspen (OA) sites. The data collected will be used to study the interaction of microwaves with forest canopy. By making use of three different frequency bands the contribution to the backscatter from each of the layers within the canopy can be determined. Using the knowledge gained from these studies, we will develop algorithms to enable more accurate interpretation of SAR images of the boreal region. This report describes in detail the development of the L-, C- and X-band radar system. The first section provides background information and explains the objectives of the boreal forest experiment. The second section describes the design and implementation of the radar system. All of the subsystems of the radar are explained in this section. Next, problems that were encountered during system testing and the summer experiments are discussed. System performance and results are then presented followed by a section on conclusions and further work.

Lockhart, G. Lance↗