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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Optimum backscatter cross section of the ocean as measured by synthetic aperture radars

The interaction of the radar signals from Synthetic Aperture Radar (SAR) and Side Looking Airborne Radar (SLAR) is particularly important for the ocean surface where the radar modulation can yield information about the long ocean wave field. Radar modulation measurements from fixed platforms are made in wavetanks and the open oceans. The surfaces are described in terms of two scale models. The radar modulation is considered to be principally due to: (1) geometrical tilt due to the slope of the long ocean waves, and (2) the straining of the short waves (by hydrodynamic interaction). For application to moving platforms, this modulation needs to be described in terms of a general geometry for both like and cross polarization since the long ocean waves, in general, travel in arbitrary directions. The finite resolution of the radar is considered for tilt modulation with hydrodynamic effects neglected.

Bahar, E.↗
NASA NTRS1984

Synthetic aperture radar target simulator

A simulator for simulating the radar return, or echo, from a target seen by a SAR antenna mounted on a platform moving with respect to the target is described. It includes a first-in first-out memory which has digital information clocked in at a rate related to the frequency of a transmitted radar signal and digital information clocked out with a fixed delay defining range between the SAR and the simulated target, and at a rate related to the frequency of the return signal. An RF input signal having a frequency similar to that utilized by a synthetic aperture array radar is mixed with a local oscillator signal to provide a first baseband signal having a frequency considerably lower than that of the RF input signal.

Zebker, H. A.↗
NASA NTRS1984

Solar Tests of Aperture Plate Materials for Solar Thermal Dish Collectors

If a malfunction occurs in a solar thermal point-focus distributed receiver power plant while a concentrator is pointed at the Sun, motion of the concentrator may stop. As the Sun moves relative to the Earth, the spot of concentrated sunlight then slowly walks off the receiver aperture, across the receiver face plate, and perhaps across adjacent portions of the concentrator. Intense local heating by the concentrated sunlight may damage or destroy these parts. The behavior of various materials under conditions simulating walk-off of a parabolic dish solar collector were evaluated. Each test consisted of exposure to concentrated sunlight at a peak flux density of about 7000 kW/square meter for 15 minutes. Types of materials tested included graphite, silicon carbide, silica, various silicates, alumina, zirconia, aluminum, copper, steel, and polytetrafluoroethylene. The only material that neither cracked nor melted was grade G-90 graphite. Grade CS graphite, a lower cost commercial grade, cracked half-way across, but did not fall apart. Both of these grades are medium-grain extruded graphites. A graphite cloth (graphitized polyacrylonitrile) showed fair performance when tested as a single thin ply; it might be useful as a multi-ply assembly. High purity slipcast silica showed some promise also.

Jaffe, L. D.↗
NASA NTRS1984

Reflector antennas with low sidelobes, low cross polarization, and high aperture efficiency

Techniques are presented for computing the horn near field patterns on the subreflectors and for correcting the phase center errors of the horn pattern by shaping the subreflector surface. The diffraction pattern computations for scanned beams are described. The effects of dish aperture diffraction on pattern bandwidth are investigated. A model antenna consisting of a reflector, shaped subreflector, and corrugated feed horn is described.

Faigen, I. M.↗
NASA NTRS1984

The correction of aberrations computed in the aperture plane of multifrequency microwave radiometer antennas

An analytical/numerical approach to identifying and correcting the aberrations introduced by a general displacement of the feed from the focal point of a single offset paraboloid antenna used in deployable radiometer systems is developed. A 15 meter reflector with 18 meter focal length is assumed for the analysis, which considers far field radiation pattern quality, focal region fields, and aberrations appearing in the aperture plane. The latter are obtained by ray tracing in the transmit mode and are expressed in terms of optical notation. Attention is given to the physical restraints imposed on corrective elements by real microwave systems and to the intermediate near field aspects of the problem in three dimensions. The subject of wave fronts and caustics in the receive mode is introduced for comparative purposes. Several specific examples are given for aberration reduction at eight beamwidths of scan at a frequency of 1.414 GHz.

Schmidt, R. F.↗
NASA NTRS1984

Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions

The method presented for antenna upgrading involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a paraboloid. A pair of offset subreflectors are synthesized which will produce a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method is based on the approximate formulation for an offset dual shaped high gain antenna first presented by Galindo-Israel and Mittra (1977). It is pointed out that in their approximate formulation, the geometrical optics energy was scattered from a subreflector and then from a second large reflector, which reflected a uniform phase distribution. In the offset dual shaped subreflector (DSS) antenna proposed here, the second reflection is from a smaller (sub) reflector, and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. The DSS synthesis is tested on the 210-ft-diameter Cassegrain antenna at Goldstone, California.

Galindo-Israel, V.↗
NASA NTRS1983

A comparative study on synthetic aperture radar and optical imagery of ocean waves

The ocean imaging capability of synthetic aperture radar (SAR) is examined in terms of an intercomparison between simultaneous, spatially collocated images of a coastal ocean scene, as recorded by aerial photography and aircraft-borne SAR. Based on a detailed study of these images, this paper examines various local features of oceanographic interest as they are imaged by the two methods. This provides information not available in the usual comparisons of normalized SAR and single point buoy spectra. In addition, aspects of the intensity wavenumber spectra of the two images are examined and discussed. An interesting and surprising conclusion arising from the comparison study is the ability of the SAR to image accurately non-uniform (possibly nonlinear) wave features which are imaged by the aerial photography. This result should have important implications on the development of SAR imaging models and the interpretation of the effect of ocean surface motion on the SAR images.

Banner, M. L.↗
NASA NTRS1984

Digital processing considerations for extraction of ocean wave image spectra from raw synthetic aperture radar data

The digital processing requirements of several algorithms for extracting the spectrum of a detected synthetic aperture radar (SAR) image from the raw SAR data are described and compared. The most efficient algorithms for image spectrum extraction from raw SAR data appear to be those containing an intermediate image formation step. It is shown that a recently developed compact formulation of the image spectrum in terms of the raw data is computationally inefficient when evaluated directly, in comparison with the classical method where matched-filter image formation is an intermediate result. It is also shown that a proposed indirect procedure for digitally implementing the same compact formulation is somewhat more efficient than the classical matched-filtering approach. However, this indirect procedure includes the image formation process as part of the total algorithm. Indeed, the computational savings afforded by the indirect implementation are identical to those obtained in SAR image formation processing when the matched-filtering algorithm is replaced by the well-known 'dechirp-Fourier transform' technique. Furthermore, corrections to account for slant-to-ground range conversion, spherical earth, etc., are often best implemented in the image domain, making intermediate image formation a valuable processing feature.

Lahaie, I. J.↗
NASA NTRS1984

Large-aperture interferometer with local reference beam

A large-aperture interferometer was devised by adding a local-reference-beam-generating optical system to a schlieren system. Two versions of the interferometer are demonstrated, one employing 12.7 cm (5 in.) diameter schlieren optics, the other employing 30.48 cm (12 in.) diameter parabolic mirrors in an off-axis system. In the latter configuration a cylindrical lens is introduced near the light source to correct for astigmatism. A zone plate is a satisfactory decollimating element in the reference-beam arm of the interferometer. Attempts to increase the flux and uniformity of irradiance in the reference beam by using a diffuser are discussed. Previously announced in STAR as N83-13979

Howes, W. L.↗
NASA NTRS1984

A method for detection of deformations in large phase array antennas for spaceborne synthetic aperture radars

A method to measure deformations in large, planar phased array antennas used in spaceborne synthetic aperture radars (SAR) is described. This method has the advantages that the deformation measurements can be performed anywhere in orbit and that no additional sensor is required. Indirect test results using the SEASAT SAR data indicate that this technique can detect deformations of small fractions of a wavelength.

Li, F. K.↗
NASA NTRS1986

Designing Eccentric Aperture Optical Systems

Computer program aids in design of eccentric aperture optical systems by predicting vector aberration that occurs in optical system having tilted or decentered optical elements. Computer program incorporates vector theory of aberrations.

Rogers, J. R.↗
NASA NTRS1984

The investigation of selected oceanographic applications of spaceborne synthetic-aperture radar

Synthetic aperture radar images obtained from Seasat and SIR-A showed that a number of oceanographic features were imaged in considerable detail, like internal waves, large ocean waves, bathymetric features, eddies, and slicks. the imaging mechanisms however, are not well understood, and for both SEASAT and SIR-A there are few supporting sea surface measurements to assist in the study of these imaging mechanisms. The SIR-B will conduct three separate experiments to provide a better understanding of the use of spaceborne SAR for imaging: (1) internal waves; (2) ocean surface waves, and (3) shallow water bathymetry. These experiments are chosen because they lead to possible applications for microwave remote sensing of the ocean surface and give a better understanding of the microwave/sea surface imaging mechanism.

Keyte, G. E.↗
NASA NTRS1984

Information extraction and transmission techniques for spaceborne synthetic aperture radar images

Information extraction and transmission techniques for synthetic aperture radar (SAR) imagery were investigated. Four interrelated problems were addressed. An optimal tonal SAR image classification algorithm was developed and evaluated. A data compression technique was developed for SAR imagery which is simple and provides a 5:1 compression with acceptable image quality. An optimal textural edge detector was developed. Several SAR image enhancement algorithms have been proposed. The effectiveness of each algorithm was compared quantitatively.

Frost, V. S.↗
NASA NTRS1983

Analysis of data acquired by synthetic aperture radar and LANDSAT Multispectral Scanner over Kershaw County, South Carolina, during the summer season

Data acquired by synthetic aperture radar (SAR) and LANDSAT multispectral scanner (MSS) were processed and analyzed to derive forest-related resources inventory information. The SAR data were acquired by using the NASA aircraft X-band SAR with linear (HH, VV) and cross (HV, VH) polarizations and the SEASAT L-band SAR. After data processing and data quality examination, the three polarization (HH, HV, and VV) data from the aircraft X-band SAR were used in conjunction with LANDSAT MSS for multisensor data classification. The results of accuracy evaluation for the SAR, MSS and SAR/MSS data using supervised classification show that the SAR-only data set contains low classification accuracy for several land cover classes. However, the SAR/MSS data show that significant improvement in classification accuracy is obtained for all eight land cover classes. These results suggest the usefulness of using combined SAR/MSS data for forest-related cover mapping. The SAR data also detect several small special surface features that are not detectable by MSS data.

Wu, S. T.↗
NASA NTRS1985

Method of measuring sea surface water temperature with a satellite including wideband passive synthetic-aperture multichannel receiver

A wideband passive synthetic-aperture multichannel receiver with an antenna is mounted on a satellite which travels in an orbit above the Earth passing over large bodies of water, e.g., the Atlantic Ocean. The antenna is scanned to receive signals over a wide frequency band from each incremental surface area (pixel) of the water which are related to the pixel's sea temperature. The received signals are fed to several channels which are tuned to separate selected frequencies. Their outputs are fed to a processor with a memory for storage. As the antenna points to pixels within a calibration area around a buoy of known coordinates, signals are likewise received and stored. Exactly measured sea temperature is received from the buoy. After passing over several calibration areas, a forward stepwise regression analysis is performed to produce an expression which selects the significant from the insignificant channels and assigns weights (coefficients) to them. The expression is used to determine the sea temperature at each pixel based on the signals received therefrom. Wind temperature, pressure, and wind speed at each pixel can also be calculated.

Stacey, J. M.↗
NASA NTRS1984

Astrometric plates obtained at the primary focus of large aperture reflectors

Coma, astigmatism, and great differences in stellar magnitudes between photographed object and reference stars constitute the main sources of errors in measuring positional plates. These three sources of error can easily be eliminated by the method used at the Klet Observatory for obtaining precise observations of faint objects. The astrometric plates are taken by the method of two diaphragms. The first diaphragm, with a small central aperture; is located in front of the photographic plate. The second diaphragm is situated in front of the mirror. By a very short (of the order of tens of seconds) exposure a sufficient number of reference stars can be obtained throughout the entire plate. The stars are very well defined to the very edge of the plate and are easy to measure. Moreover, this method makes it possible to use plates of larger dimensions than usual so that it is always possible to find the necessary reference stars.

Markos, A.↗
NASA NTRS1985

Method and apparatus for Delta Kappa synthetic aperture radar measurement of ocean current

A synthetic aperture radar (SAR) employed for delta k measurement of ocean current from a spacecraft without the need for a narrow beam and long observation times. The SAR signal is compressed to provide image data for different sections of the chirp band width, equivalent to frequencies and a common area for the separate image fields is selected. The image for the selected area at each frequency is deconvolved to obtain the image signals for the different frequencies and the same area. A product of pairs of signals is formed, Fourier transformed and squared. The spectrum thus obtained from different areas for the same pair of frequencies are added to provide an improved signal to noise ratio. The shift of the peak from the center of the spectrum is measured and compared to the expected shift due to the phase velocity of the Bragg scattering wave. Any difference is a measure of current velocity v sub o (delta k).

Jain, A.↗
NASA NTRS1985

Hexagonal uniformly redundant arrays for coded-aperture imaging

Uniformly redundant arrays are used in coded-aperture imaging, a technique for forming images without mirrors or lenses. The URAs constructed on hexagonal lattices, are outlined. Details are presented for the construction of a special class of URAs, the skew-Hadamard URAs, which have the following properties: (1) nearly half open and half closed (2) antisymmetric upon rotation by 180 deg except for the central cell and its repetitions. Some of the skew-Hadamard URAs constructed on a hexagonal lattice have additional symmetries. These special URAs that have a hexagonal unit pattern, and are antisymmetric upon rotation by 60 deg, called hexagonal uniformly redundant arrays (HURAs). The HURAs are particularly suited to gamma-ray imaging in high background situations. In a high background situation the best sensitivity is obtained with a half open and half closed mask. The hexagonal symmetry of an HURA is more appropriate for a round position-sensitive detector or a closed-packed array of detectors than a rectangular symmetry.

Finger, M. H.↗