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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 271 records · Page 15
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NASA NTRS1976

Considerations on data compression of synthetic aperture radar images

This paper describes some analytical results relative to the effectiveness of applying data compression techniques for efficient transmission of synthetic aperture radar (SAR) signals and images. A Rayleigh target model is assumed in the analysis. It is also assumed that all surface reflectivity information is of interest and needs to be transmitted. Spectral characteristics of radar echo signals and processed images are analyzed. Analytical results generally indicate that due to the lack of high spatial correlation in the Rayleigh distributed radar surface reflectivity, application of data compression to SAR signals and images under the square difference fidelity criterion may be less effective than its application to images obtained using incoherent illumination. On the other hand, if certain random variations in radar images are considered as undesirable, substantial compression ratio may be achieved by removing such variations.

Wu, C.↗
NASA NTRS1977

Radar speckle reduction in synthetic aperture radar processors by a moving diffuser

Some theoretical and experimental results are presented demonstrating the reduction of synthetic aperture radar speckle when a moving diffuser is placed in the processor image plane and is reimaged with a lens whose resolution is not better than the radar resolution. The resolution of the resulting radar image is determined by the resolution of this output lens.

Jain, A.↗
NASA NTRS1977

Determination of ocean wave heights from synthetic aperture radar imagery

A calculation is presented for the cross-correlation of the radar images obtained by processing the same signal data over different portions of the chirp spectrum bandwidth as a function of the center frequency spacings for these portions. This is shown to be proportional to the square of the product of the characteristic function for ocean wave heights and the pupil function describing the chirp spectrum bandwidth used in the processing. Measurements of this function for ocean wave imagery over the coast of Alaska, the North Atlantic, and Monterey Bay, California, and correlation with the significant wave heights reported from ground truth data indicate that the synthetic aperture radar instrument can be used for providing wave height information in addition to the ocean wave imagery.

Jain, A.↗
NASA NTRS1977

Precipitation mapping with an airborne synthetic aperture imaging radar

Use of an airborne synthetic-aperture radar (SAR) of 3.1-cm wavelength for the detection and mapping of precipitation echoes is described. Difficulties in mapping related to the incoherence of the echoes are discussed; methods of increasing resolution by Doppler filtering are suggested. Observations made with a vertical side-looking SAR system flying above a stratiform storm were found to resemble vertical cross sections obtained with conventional radar showing snow streamers, the bright band and rain. However, the actual section sampled by the SAR after filtering for zero Doppler is shown to depend upon both the relative motion of the particles and the relative winds below the aircraft. Knowledge of the relative winds, deduced from Doppler spectrum variations, and of the particle fall speeds permits the generation of horizontal and vertical cross sections displaced from the aircraft's position.

Atlas, D.↗
NASA NTRS1977

Digital technique for generating synthetic aperture radar images

This paper describes a digital processing method applicable to a synthetic aperture radar, to be carried by the space shuttle or by satellites. The method uses an earth-fixed coordinate system in which corrective procedures are invoked to compensate for errors introduced by the satellite motion, earth curvature, and wavefront curvature. Among the compensations discussed are those of the coordinate system, skewness, roll, pitch, yaw, earth rotation, and others. The application of a Fast Fourier Transform in the numerical processing of the two-dimensional convolution is discussed in detail.

Van De Lindt, W. J.↗
NASA NTRS1977

Digital exploitation of synthetic aperture radar

A digital processing and analysis scheme for use with digitized synthetic aperture radar data was developed. Using data from a four channel system, the imagery is preprocessed using specially designed software and then analyzed using preexisting facilities originally intended for use with MSS type data. Geometric and radiometric correction may be performed if desired, as well as classification analysis, Fast Fourier transform, filtering and level slice and display functions. The system provides low cost output in real time, permitting interactive imagery analysis. System information flow diagrams as well as sample output products are shown.

Wagner, H. L.↗
NASA NTRS1977

Analysis of synthetic aperture radar imagery

Some problems faced in applications of radar measurements in hydrology are: (1) adequate calibration of the radar systems and direct digital data will be required in order that repeatable data can be acquired for hydrologic applications; (2) quantitative hydrologic research on a large scale will be prohibitive with aircraft mounted synthetic aperture radar systems due to the system geometry; (3) spacecraft platforms appear to be the best platforms for radar systems when conducting research over watersheds larger than a few square kilometers; (4) experimental radar systems should be designed to avoid use of radomes; and (5) cross polarized X and L band data seem to discriminate between good and poor hydrologic cover better than like polarized data.

Blanchard, B. J.↗
NASA NTRS1978

Optimal sampling and quantization of synthetic aperture radar signals

Some theoretical and experimental results on optimal sampling and quantization of synthetic aperture radar (SAR) signals are presented. It includes a description of a derived theoretical relationship between the pixel signal to noise ratio of processed SAR images and the number of quantization bits per sampled signal, assuming homogeneous extended targets. With this relationship known, a solution may be realized for the problem of optimal allocation of a fixed data bit-volume (for specified surface area and resolution criterion) between the number of samples and the number of bits per sample. The results indicate that to achieve the best possible image quality for a fixed bit rate and a given resolution criterion, one should quantize individual samples coarsely and thereby maximize the number of multiple looks. The theoretical results are then compared with simulation results obtained by processing aircraft SAR data.

Wu, C.↗
NASA NTRS1978

Synthetic aperture radar in geosynchronous orbit

Radar images of the earth were taken with a synthetic aperture radar (SAR) from geosynchronous orbital ranges by utilizing satellite motion relative to a geostationary position. A suitable satellite motion was obtained by having an orbit plane inclined relative to the equatorial plane and by having an eccentric orbit. Potential applications of these SAR images are topography, water resource management and soil moisture determination. Preliminary calculations show that the United States can be mapped with 100 m resolution cells in about 4 hours. With the use of microwave signals the mapping can be performed day or night, through clouds and during adverse weather.

Tomiyasu, K.↗
NASA NTRS1978

Random sampling adaptively focusing synthetic aperture radar

A high resolution narrow angle of view imaging radar system is considered that employs an airborne synthetic aperture of 600 meters operating at X-band to produce a beamwidth of approximately 0.05 mr. This system differs from a conventional SAR in that only a smaller number of wavefront samples, spaced randomly over the aperture are processed, and adaptive beamforming with open loop scanning is used. As a result, the processing requirements are reduced to within the capability of present day small computer technology, and the tolerance on flight stability is loosened by about 100:1. The system is described and initial analysis and evaluation results are presented.

Powers, E. N.↗
NASA NTRS1978

The SEASAT-A synthetic aperture radar design and implementation

The SEASAT-A synthetic aperture imaging radar system is the first imaging radar system intended to be used as a scientific instrument designed for orbital use. The requirement of the radar system is to generate continuous radar imagery with a 100 kilometer swath with 25 meter resolution from an orbital altitude of 800 kilometers. These requirements impose unique system design problems and a description of the implementation is given. The end-to-end system is described, including interactions of the spacecraft, antenna, sensor, telemetry link, recording subsystem, and data processor. Some of the factors leading to the selection of critical system parameters are listed. The expected error sources leading to degradation of image quality are reported as well as estimate given of the expected performance from data obtained during a ground testing of the completed subsystems.

Jordan, R. L.↗
NASA NTRS1976

Seasat-A Synthetic Aperture Radar - Radar system implementation

The Synthetic Aperture Radar (SAR) onboard the Seasat-A satellite will conduct a number of experiments involving deep ocean waves, coastal wave patterns, polar ice and land forms. The SAR will have a 25 m by 25 m resolution over a swath of 100 km width centered about 300 km to the right of the spacecraft track. The SAR's high data rate limits operations to times when Seasat-A is in view of a few ground stations with special SAR receiving equipment. However, the SAR will collect much useful data about deep ocean and coastal waves in the Atlantic and Pacific Oceans; about ice in the Northwest Atlantic, in the Great Lakes and off the coast of Alaska; and about land over much of the United States and Canada.

Thompson, T. W.↗
NASA NTRS1978

Synthetic atmospheric transmittance spectra near 15 and 4.3 microns

Synthetic monochromatic atmospheric transmittance spectra are presented for infrared intervals in the vicinity of the 15 and 4.3 microns CO2 bands. The intervals are nominally 20 kaysers in width, and the spectra partition the intervals 560, 780 kayser and 2180, 2400 kayser, respectively. The spectra are for a vertical atmospheric path. A given spectrum is presented at that pressure for which the mean weighting function is a maximum. In order to indicate the molecular origin of the features in a given spectrum, separate spectra are also presented for those species which make center line contributions to the absorption over the interval. The transmittance model for the calculations is described. In addition to the representation of spectral features for intervals which comprise these two CO2 bands, the spectra afford a utility in the design of high precision satellite temperature radiometers.

Susskind, J.↗
NASA NTRS1978

Focusing effects in the synthetic aperture radar imaging of ocean waves

The paper derives the properties of the image obtained for an ocean wave whose cross section and surface profile are functions, representing the wave phenomena, whose exact properties are determined by the ocean wave surface properties, for ocean wavelength, height, and orbital frequency. The effect of defocusing of the wave image due to its temporal motion is calculated, and both the resolution of the radar system if no focus compensation is provided in the processor and the necessary distance the azimuth telescope has to be moved to provide diffraction-limited imaging are derived. These results are illustrated for data obtained by synthetic aperture radar during Hurricane Gloria on September 30, 1976, and by ERIM radar over Marineland, Florida, on December 15, 1975.

Jain, A.↗
NASA NTRS1977

Open-Cycle Gas Turbine/Steam Turbine Combined Cycles with synthetic fuels from coal

The Open-Cycle Gas Turbine/Steam Turbine Combined Cycle can be an effective energy conversion system for converting coal to electricity. The intermediate step in this energy conversion process is to convert the coal into a fuel acceptable to a gas turbine. This can be accomplished by producing a synthetic gas or liquid, and by removing, in the fuel conversion step, the elements in the fuel that would be harmful to the environment if combusted. In this paper, two open-cycle gas turbine combined systems are evaluated: one employing an integrated low-Btu gasifier, and one utilizing a semi-clean liquid fuel. A consistent technical/economic information base is developed for these two systems, and is compared with a reference steam plant burning coal directly in a conventional furnace.

Shah, R. P.↗
NASA NTRS1978

Concentration-response data on toxicity of pyrolysis gases from six synthetic polymers

Concentration-response data are presented on the toxic effects of the pyrolysis gases from six synthetic polymers on Swiss Webster male mice, using a toxicity screening test method. The polymers consisted of one sample each of polycarbonate and polystyrene, and two samples each of polyethylene and acrylonitrile-butadiene-styrene (ABS). All six samples were evaluated using the rising temperature program, and three samples were evaluated using the fixed temperature program. The pyrolysis gases from polycarbonate, polyethylene, and ABS appeared to exhibit the concentration-response relationships commonly encountered in toxicology. The polystyrene sample gave variable results; probably due to poor reproducibility of the essentially anaerobic pyrolysis. Carbon monoxide seemed to be the principal toxicant in the pyrolysis gases from polycarbonate and polyethylene, but did not appear to be the principal toxicant in the pyrolysis gases from ABS.

Hilado, C. J.↗