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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Seasat SAR cross-section modulation by surface winds - GOASEX observations

Seasat radar observations of the Gulf of Alaska by the Synthetic Aperture Radar (SAR) and the Wind Speed Scatterometer (SASS) indicate that L-band backscatter is modulated by wind speed and possibly by wind direction. It is noted that the L-band backscatter is approximately proportional to the square root of surface winds and that upwind/crosswind differences in backscatter can be as large as 10% in the range of these wind speeds. It is suggested that Seasat SAR images, in conjunction with Seasat Scatterometer data may provide fine-scale mapping of ocean winds and directions. The need for further Seasat SAR-SASS comparisons in order to extend the range of wind speeds is stressed.

Thompson, T. W.↗
NASA NTRS1981

A derivation of the statistical characteristics of SAR imagery data

Basic statistical properties of the speckle effect and the associated spatial correlation of SAR image data are discussed. Statistics of SAR sensed measurement and their relationships to the surface mean power reflectivity are derived. The Rayleigh speckle model is reviewed. Applications of the derived statistics to SAR radiometric measures and image processing are considered.

Wu, C.↗
NASA NTRS1980

A digital fast correlation approach to produce SEASAT SAR imagery

This paper describes a digital processing algorithm and its associated system design for producing images from SEASAT Synthetic Aperture Radar (SAR) data. The proposed system uses the fast Fourier transform approach to perform the two-dimensional correlation process. The range migration problem, which is often a major obstacle to efficient processing, can be alleviated by approximating the locus of echoes from a point target by several linear segments. SAR data corresponding to each segment is correlated separately, and the results are coherently summed to produce full-resolution images. This processing approach exhibits high computation efficiency and simple processing control functions. It is particularly attractive for software implementation based on general purpose computer. Results of this implementation and examples of digitally correlated SEASAT SAR imagery are discussed

Wu, C.↗
NASA NTRS1982

Global observations of a SAR arc

A description is presented of the first results from observations of a stable auroral red (SAR) arc with the global auroral imaging instrumentation on board Dynamics Explorer (DE) 1. This instrumentation includes three spin-scan imaging photometers and a data control unit. The last visible-wavelength imaging sequence prior to onset of the geomagnetic storm begins at 0517 UT on October 20, 1981. An auroral substorm is observed in progress two orbits later, and as the substorm subsides and the diffuse aurora retreats poleward a narrow band of 630-nm emissions is observed at latitudes previously occupied by the aurora. This band is detected at 630 nm only, and is identified as the first optical observation of the SAR arc with DE 1. The third orbit of observations commences at 0830 UT on October 21 with the arc remaining visible across the nightside of earth. No visual signature of the SAR arc is detected during the sixth orbit.

Craven, J. D.↗
NASA NTRS1982

Observation of a heated electron population associated with the 6300 A SAR arc emission

Conjunctive observations of the Dynamics Explorer satellite pair and the Pacific Northwest Laboratory scanning photometer unit at Richland, WA have allowed identification of the low energy, heated, electron population associated with the 6300 A Stable Auroral Red (SAR) arc emission. Measurements over the SAR arc on day 296 of 1981 show an enhanced flux in the low altitude 5-12 eV electrons. Analysis of this electron population shows them to be fairly Maxwellian, with temperatures on the order of 9900 + or - 1100 K and an earthward field aligned velocity of approximately 275 + or - 60 km/sec. The electrons are not accompanied by ion precipitation. The ion loss cone is empty at least as high as 6000 km. This fact may cast some doubt as to the role of the ring current ions in the SAR arc production.

Gurgiolo, C.↗
NASA NTRS1982

Evaluation of ERIM optically processed SEASAT SAR data

The results of three studies on the radiometric and geometric properties of optically processed SEASAT SAR imagery are summarized. The accuracy with which the image scale can be predicted based upon a knowledge of the SAR platform and recording system parameters and the processor characteristics was evaluated. The considerations involved in making radiometric measurements from image films, the use of point targets for calibrating the effects of Doppler spectrum shifts on the radiometric calibration of the SAR image data over extended swath lengths was evaluated

Shuchman, R. A.↗
NASA NTRS1982

Analysis of geologic terrain models for determination of optimum SAR sensor configuration and optimum information extraction for exploration of global non-renewable resources. Pilot study: Arkansas Remote Sensing Laboratory, part 1, part 2, and part 3

Computer-generated radar simulations and mathematical geologic terrain models were used to establish the optimum radar sensor operating parameters for geologic research. An initial set of mathematical geologic terrain models was created for three basic landforms and families of simulated radar images were prepared from these models for numerous interacting sensor, platform, and terrain variables. The tradeoffs between the various sensor parameters and the quantity and quality of the extractable geologic data were investigated as well as the development of automated techniques of digital SAR image analysis. Initial work on a texture analysis of SEASAT SAR imagery is reported. Computer-generated radar simulations are shown for combinations of two geologic models and three SAR angles of incidence.

Kaupp, V. H.↗
NASA NTRS1983

Microstrip antennas for SAR applications

Current and future microstrip antenna technology development for Spaceborne Synthetic Aperture Radars (SAR) are summarized. Some of the electrical and mechanical characteristics of previously and presently developed microstrip SAR antennas are shown. The SEASAT, the SIR-A and presently the SIR-B antennas are all designed for operation at L-band with approximately 22 MHz of bandwidth. The antennas have linear polarization with minimum of 20 dB of polarization purity. Both the SEASAT and SIR-A antennas were designed for a fixed pointing angle of 20.5 deg and 47 deg, respectively. However, the SIR-B has the added feature of mechanical beam steering in elevation (range). With the exception of different mechanical characteristics, it is concluded that present spaceborne SAR antennas have only single frequency and single polarization performance. The lack of large spaceborne antennas operating at the higher degree of fabrication tolerance required for a given performance; and larger feed and radiating element losses.

Haddad, H. A.↗
NASA NTRS1983

Mapping and monitoring renewable resources with space SAR

The SEASAT-A SAR and SIR-A imagery was examined to evaluate the quality and type of information that can be extracted and used to monitor renewable resources on Earth. Two tasks were carried out: (1) a land cover classification study which utilized two sets of imagery acquired by the SEASAT-A SAR, one set by SIR-A, and one LANDSAT set (4 bands); and (2) a change detection to examine differences between pairs of SEASAT-A SAR images and relates them to hydrologic and/or agronomic variations in the scene.

Ulaby, F. T.↗
NASA NTRS1983

Acousto-optic/CCD real-time SAR data processor

The SAR processor which uses an acousto-optic device as the input electronic-to-optical transducer and a 2-D CCD image sensor, which is operated in the time-delay-and-integrate (TDI) mode is presented. The CCD serves as the optical detector, and it simultaneously operates as an array of optically addressed correlators. The lines of the focused SAR image form continuously (at the radar PRF) at the final row of the CCD. The principles of operation of this processor, its performance characteristics, the state-of-the-art of the devices used and experimental results are outlined. The methods by which this processor can be made flexible so that it can be dynamically adapted to changing SAR geometries is discussed.

Psaltis, D.↗
NASA NTRS1983

900 km of digital ocean wave spectra from the SEASAT SAR

One of the primary reasons for including the SAR in the complement of ocean microwave instruments on SEASAT was the global ocean wave spectra. An accurate, timely, and global knowledge of the full two dimensional or directional surface wave spectra has great value in both operational wave forecasting and in wave climatology, especially in the Southern Oceans. The sparse sampling obtainable from a single orbiting SAR would by itself be inadequate to reconstruct a global wave field, but when supplemented with auxiliary knowledge of the winds, the SAR might provide essential updates to a global wave model.

Beal, R. C.↗
NASA NTRS1984

Theory and measure of certain image norms in SAR

The principal properties of synthetic aperture radar SAR imagery of point and distributed objects are summarized. Against this background, the response of a SAR (Synthetic Aperture Radar) to the moving surface of the sea is considered. Certain conclusions are drawn as to the mechanism of interaction between microwaves and the sea surface. Focus and speckle spectral tests may be used on selected SAR imagery for areas of the ocean. The fine structure of the sea imagery is sensitive to processor focus and adjustment. The ocean reflectivity mechanism must include point like scatterers of sufficient radar cross section to dominate the return from certain individual resolution elements. Both specular and diffuse scattering mechanisms are observed together, to varying degree. The effect is sea state dependent. Several experiments are proposed based on imaging theory that could assist in the investigation of reflectivity mechanisms.

Raney, R. K.↗
NASA NTRS1983

Radiometric calibration of Seasat SAR imagery

A goal of future synthetic aperture radar (SAR) system design is to yield the radiometrically accurate imagery necessary for use in classification problems. To accomplish this, a reliable calibration technique must be developed. Many such techniques have been proposed (e.g., Stiles and Holtzman, 1982); however, until now, none have been tested on spaceborne SAR digital data. In this paper, a method of significantly increasing the reproducibility of several independent Seasat SAR data sets of a test site using an automated correction procedure is described.

Croft, C.↗
NASA NTRS1984

A fast, programmable hardware architecture for the processing of spaceborne SAR data

The development of high-throughput SAR processors (HTSPs) for the spaceborne SARs being planned by NASA, ESA, DFVLR, NASDA, and the Canadian Radarsat Project is discussed. The basic parameters and data-processing requirements of the SARs are listed in tables, and the principal problems are identified as real-operations rates in excess of 2 x 10 to the 9th/sec, I/O rates in excess of 8 x 10 to the 6th samples/sec, and control computation loads (as for range cell migration correction) as high as 1.4 x 10 to the 6th instructions/sec. A number of possible HTSP architectures are reviewed; host/array-processor (H/AP) and distributed-control/data-path (DCDP) architectures are examined in detail and illustrated with block diagrams; and a cost/speed comparison of these two architectures is presented. The H/AP approach is found to be adequate and economical for speeds below 1/200 of real time, while DCDP is more cost-effective above 1/50 of real time.

Bennett, J. R.↗
NASA NTRS1983

The potential of SAR directional spectra in operational wave forecasting

Digitally processed Seasat SAR ocean wave imagery, when digitally transformed, smoothed, and corrected for known instrument biases, can accurately track multiple (at least three) wave systems in the open ocean, across major currents, and into shallow water. Furthermore, the relative backscatterer of the digitally processed SAR image, when sampled within a single pass and along a constant range, responds to the local wind through a simple power law relationship. The results indicate that SAR spectra, if obtained from a low altitude satellite, could be an important supplement to global winds and non-directional wave heights obtained through other methods.

Beal, R. C.↗
NASA NTRS1984

A fundamental model and efficient inference for SAR ocean imagery

Employing a synthetic aperture radar (SAR) imaging model based on fundamental models of nonlinear hydrodynamics, electromagnetic scattering from a two-scale surface, and SAR imaging of a time-variant scene, the optimal (minimum mean-square error) estimates of the parameters of a sinusoidal, long gravity wave, and the short gravity wave ensemble are found in an efficient recursive form and their performance evaluated, generally by numerical simulation, in a one-dimensional stationary version. An application is made to Seasat-SAR complex imagery.

Harger, R. O.↗
NASA NTRS1984

Geologic interpretation of Seasat SAR imagery near the Rio Lacantum, Mexico

A mosaic of the Seasat Synthetic Aperture Radar (SAR) optically processed images over Central America is presented. A SAR image of the Rio Lacantum area (southeastern Mexico) has been digitally processed and its interpretation is presented. The region is characterized by low relief and a dense vegetation canopy. Surface is believed to be indicative of subsurface structural features. The Seasat-SAR system had a steep imaging geometry (incidence angle 23 + or - 3 deg off-nadir) which is favorable for detection of subtle topographic variations. Subtle textural features in the image corresponding to surface topography were enhanced by image processing techniques. A structural and lithologic interpretation of the processed images is presented. Lineaments oriented NE-SW dominate and intersect broad folds trending NW-SE. Distinctive karst topography characterizes one high relief area

Rebillard, PH.↗
NASA NTRS1985

Inversion problems in SAR imaging

Attention is given to two classes of SAR imaging inversion problems: fine resolution imagery of stationary scenes, and the imaging of a moving surface. Absolute scattering coefficient values are obtained from the output image by means of reference reflectors. The inversion of SAR data obtained from a moving surface requires consideration of motion effects in addition to those for the fixed scatterer; systematic displacements on a scale greater than the resolution result in azimuth shifts in the mapping of the scene onto the SAR image.

Larson, R. W.↗