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  • Other Sources  (1,583)
  • COMMUNICATIONS AND RADAR  (1,583)
  • 2000-2004
  • 1990-1994  (1,583)
  • 1
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    Recalibration of the Viyager PRA antenna for polarization sense measurement (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The Voyager Planetary Radio Astronomy (PRA) antenna and receiver system provides an indication of the sense of elliptical or circular polarization of radiation that is not correct for all directions of incidence. The true sense could be determined for all directions if accurate calibration data were available. It was not feasible to make the calibration before the Voyagers were launched. Lecacheux & Ortega-Molina (1987), however, were able to derive such calibration data from planetary radio observations made in flight. They expressed their results in terms of the tilt of a plane (the E-plane) that divides the incident ray directions for which the indicated polarization sense is correct from those directions for which the indicated sense is reversed. We demonstrate that there are certain directions for which this calibration is itself in error, and that the surface dividing the two sets of incident rays is more complex than a tilted plane. We are able to make a crude approximation to the true surface from the limited data available.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 281; 3; p. 945-954
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  • 2
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    Mitigation of multipath in DGPS ground reference stations (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Multipath represents one of the most serious threats to accuracy in Differential GPS (DGPS). The theoretical foundations of multipath are presented and several multipath mitigation techniques are reviewed. Special emphasis is placed on signal diffraction methods. Reflections enter peaks and nulls in the distorted antenna pattern in a random manner. This, coupled with the oscillations in relative phase (with respect to the direct signal), results in a multipath error signature which is noise-like. Since it is noise-like, it may be significantly reduced by averaging the code phase measurements against the more stable carrier-phase measurements. Exploitation of this effect to reduce multipath error represents a significant increase in accuracy for real time DGPS. Experiments have been performed which verify these conclusions.
    Keywords: COMMUNICATIONS AND RADAR
    Type: In: Institute of Navigation, National Technical Meeting, San Diego, CA, Jan. 27-29, 1992, Proceedings (A93-21176 06-04); p. 105-114.
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  • 3
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    Data compression of stereopairs (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: This paper addresses the problem of stereopair data compression. Two fundamentally different techniques for compressing stereopairs are discussed. The first technique, called disparity-compensated transform-domain predictive coding, attempts to minimize the mean-square error between the original stereopair and the compressed stereopair. The second technique, called mixed-resolution coding, is a psychophysically justified technique that exploits known facts about human stereovision to code stereopairs in a subjectively acceptable manner. A method for assessing the quality of compressed stereopairs is also presented. It involves measuring the ability of an observer to perceive depth in coded stereopairs. It is found that observers generally perceived objects to be further away in compressed stereopairs than they did in originals. Finally, the problem of coding stereopairs is considered from a rate-distortion perspective. It is proved that the rate distortion limit for coding stereopairs cannot in general be achieved by a coder that first codes and decodes the right picture sequence independently of the left picture sequence, and then codes and decodes the left picture sequence given the decoded right picture sequence.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); 40; 4 Ap; 684-696
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  • 4
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    Rain rate statistics and fade distributions at 20 and 30 GHz derived from a network of rain gauges in the Mid-Atlantic coast over a five year period (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: A network of ten tipping bucket rain gauges located within a grid 70 km north-south and 47 km east-west in the Mid-Atlantic coast of the United States was used to analyze rain rate and modeled slant path attenuation distributions at 20 and 30 GHz. It was shown that, for realistic fade margins at 20 GHz and above, the variable integration times results are adequate to estimate slant path attenuations using models which require 1 min averages. Crane's Global Model was used to derive fade distributions at 20 and 30 GHz.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 40; 11; p. 1408-1415.
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  • 5
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    A modal radar cross section of thin-wire targets via the singularity expansion method (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: A modal radar cross section (RCS) of arbitrary wire scatterers is constructed in terms of SEM parameters. Numerical results are presented for both straight and L-shaped wire targets and are compared to computations performed in the frequency domain using the method of moments.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 40; 10; p. 1256-1260.
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  • 6
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    Multiple-access phased array antenna simulator for a digital beam-forming system investigation (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Future versions of data relay satellite systems are currently being planned by NASA. Being given consideration for implementation are on-board digital beamforming techniques which will allow multiple users to simultaneously access a single S-band phased array antenna system. To investigate the potential performance of such a system, a laboratory simulator has been developed at NASA's Lewis Research Center. This paper describes the system simulator, and in particular, the requirements, design and performance of a key subsystem, the phased array antenna simulator, which provides realistic inputs to the digital processor including multiple signals, noise, and nonlinearities.
    Keywords: COMMUNICATIONS AND RADAR
    Type: International Journal of Satellite Communications (ISSN 0737-2884); 10; 5; p. 293-297.
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  • 7
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    94 GHz integrated horn monopulse antennas (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: A monolithic azimuthal monopulse antenna for 94 GHz applications has been developed. The structure consists of a single dipole suspended in one plane of an integrated horn cavity to obtain the sum pattern, and an antiparallel pair of dipoles suspended in a different plane of the same horn cavity to achieve the difference pattern. Pattern measurements of microwave models and on the millimeter-wave antennas show good agreement with theory and exhibit symmetry with a sharp -30 dB null at broadside for the difference antenna. Microwave model measurements show input impedances close to 50 Ohms, with greater than -25 dB isolation between sum and difference antennas across a 10 percent bandwidth.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 40; 8; p. 981-984.
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  • 8
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    Ka-band feed arrays for spacecraft reflector antennas with limited scan capability - An overview (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: JPL and NASA are in the process of developing ground and spacecraft antenna systems at Ka-band frequencies for future deep space applications. The use of Ka-band (32-GHz) down communication will result in smaller ground and spacecraft antennas and associated equipment, and will provide larger bandwidths necessary for very high data rate communication and radio navigation. In this article, the use of a small phased array as a feed for a reflector antenna system with limited scan capability is addressed. Different feed and antenna configurations, as well as array architectures, are examined. Some theoretical an experimental parameters of a particular breadboard feed array developed by JPL and the University of Massachusetts are presented. Guidelines for the future direction of this effort are provided.
    Keywords: COMMUNICATIONS AND RADAR
    Type: In: 1992 IEEE Aerospace Applications Conference, Snowmass, CO, Feb. 2-7, 1992, Digest (A93-14676 03-32); p. 57-66.
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  • 9
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    Site diversity for satellite earth terminals and measurements at 28 GHz (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: This paper reviews site diversity concepts and models and presents the results of a site diversity experiment, in which radiometers were used to measure attenuation on earth-space paths, performed at Columbus, Ohio, for a period of one year. Attenuation statistics are presented first in the form of the joint probability density function of attenuation at the two sites. From these are derived the cumulative two-dimensional distribution of attenuation at both sites, the cumulative distribution of attenuation at each site, the effective cumulative distribution of attenuation for a diversity system that always switches to the better of two signals, and the effective cumulative distribution of attenuation for a system with maximal-ratio signal combining. The switched-system diversity gain is compared with Hodge's predictive model, and the diversity improvement is compared with Boithias' model. Fade-duration and interfade-interval statistics are also presented for each site and for systems that employ switching and maximal-ratio combining. Rain-rate and interrainfall interval statistics are also included.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE, Proceedings (ISSN 0018-9219); 81; 6; p. 897-904.
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  • 10
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    New approaches in interferometric SAR data processing (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: It is well established that interferometric synthetic aperture radar (SAR) images can be inverted to perform surface elevation mapping. Among the factors critical to the mapping accuracy are registration of the interfering SAR images and phase unwrapping. A novel registration algorithm is presented that determines the registration parameters through optimization. A new figure of merit is proposed that evaluates the registration result during the optimization. The phase unwrapping problem is approached through a new method involving fringe line detection. The algorithms are tested with two SEASAT SAR images of terrain near Yellowstone National Park. These images were collected on Seasat orbits 1334 and 1420, which were very close together in space, i.e., less than 100 m. The resultant elevation map is compared with the USGS digital terrain elevation model.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); 30; 3 Ma
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