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  • Other Sources  (2,369)
  • COMMUNICATIONS AND RADAR  (2,369)
  • 2000-2004
  • 1985-1989  (1,297)
  • 1980-1984  (1,071)
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  • 1
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    SAR system test and calibration (1986)
    In:  CASI
    Details
    Publication Date: 2004-10-30
    Keywords: COMMUNICATIONS AND RADAR
    Type: V-Gram: Magellan Quarterly Bulletin about Venus and the Radar Mapping Mission, Issue 9; p 10-16
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  • 2
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    Guided waves in cladded composite tubes (1989)
    In:  CASI
    Details
    Publication Date: 2005-11-10
    Description: The objectives of this program are as follows: modelling of dynamics of composite tubular space structure truss members; and utilization of ultrasonic waves as probes for material and defect characterization. This discussion is presented in viewgraph format.
    Keywords: COMMUNICATIONS AND RADAR
    Type: First Annual Symposium. Volume 1: Plenary Session; 17 p
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  • 3
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    Magellan SAR processor design (1986)
    In:  CASI
    Details
    Publication Date: 2004-10-30
    Keywords: COMMUNICATIONS AND RADAR
    Type: V-Gram: Magellan Quarterly Bulletin about Venus and the Radar Mapping Mission, Issue 9; p 22-29
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  • 4
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    Magellan radar system design (1986)
    In:  CASI
    Details
    Publication Date: 2004-10-30
    Keywords: COMMUNICATIONS AND RADAR
    Type: V-Gram: Magellan Quarterly Bulletin about Venus and the Radar Mapping Mission, Issue 9; p 16-22
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  • 5
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    Radar backscatter modelling (1984)
    In:  CASI
    Details
    Publication Date: 2006-04-06
    Description: The terrain analysis software package was restructured and documentation was added. A program was written to test Johnson Space Center's four band scatterometer data for spurious signals data. A catalog of terrain roughness statistics and calibrated four frequency multipolarization scatterometer data is being published to support the maintenance of Death Valley as a radar backscatter calibration test site for all future airborne and spacecraft missions. Test pits were dug through sand covered terrains in the Eastern Sahara to define the depth and character of subsurface interfaces responsible for either backscatter or specular response in SIR-A imagery. Blocky sandstone bedrock surfaces at about 1 m depth were responsible for the brightest SIR-A returns. Irregular very dense CaCO3 cemented sand interfaces were responsible for intermediate grey tones. Ancient river valleys had the weakest response. Reexamination of SEASAT l-band imagery of U.S. deserts continues.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Washington Rept. of Planetary Geology Program, 1983; p 268-269
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  • 6
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    Design optimization of the 34-meter DSN-NCP antennas (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The new NASA Deep Space Network (DSN) 34-m-diameter azimuth-elevation (Az-El) antenna structure is an example of an essentially computer-automated design. In addition to pivotal comptuer Lagrange multiplier design optimization software, much of the associated pre- and post-processing was also performed by computer. The construction of one of these antennas at Goldstone, California, is well advanced and will be completed this summer. A second installation is in progress in Australia. Both atennas will be used primarily for spacecraft tracking and will operate in the 8.5-GHz, 3.5-cm (1.4-in.) wavelength microwave frequency.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Langley Research Center Recent Experiences in Multidisciplinary Analysis and Optimization, Part 2; 16 p
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  • 7
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    Deep space network Mark 4A description (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The general system configuration for the Mark 4A Deep Space Network is described. The arrangement and complement of antennas at the communications complexes and subsystem equipment at the signal processing centers are described. A description of the Network Operations Control Center is also presented.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 255 - 260
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  • 8
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    Choosing channel quantization levels and viterbi decoding for space diversity reception over the additive white Guassian noise channel (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Previous work in the area of choosing channel quantization levels for a additive white Gaussian noise channel composed of one receiver-demodulator is reviewed, and how this applies to the Deep Space Network composed of several receiver-demodulators (space diversity reception) is shown. Viterbi decoding for the resulting quantized channel is discussed.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 90 - 94
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  • 9
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    HEO multimission navigation concept (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: As part of the Network Consolidation Program, the 26-meter Tracking and Communication Subnet was transferred to JPL. Along with this transfer JPL assumed responsibility for tracking and navigation support for Earth orbiter missions normally tracked by the 26-meter sites. The High Earth Orbiter (HEO) Multimission Navigation Facility was formed as a component of the Deep Space Network (DSN) Tracking System for the purpose of supporting Earth orbiter missions and certain classes of deep space missions. This facility has been implemented on a dedicated VAX 11/780 minicomputer within the Network Operations Control Center (NOCC). The primary function of the system is to process radio metric data and estimate the orbit of a spacecraft in near-Earth or deep space environment. The system is capable of processing radio metric data in near-real time and providing the quick turnaround required for Earth orbiter operations. It is also capable of generating precision spacecraft ephemeris for use by the NOCC Support Subsystem and external agencies. This article discusses the implementation and functional operation of the Multimission Navigation Subsystem and describes the support that has been provided for an array of missions.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 261 - 267
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  • 10
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    DSS 13 frequency stability tests performed during May 1985 through March 1986 (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Results of station frequency stability testing performed at DSS 13 (Deep Space Station) during May 1985 through March 1986 are presented. The testing was done on X-band uplink and X- and S-band downlink subsystems as well as on end-to-end systems. The subsystem test data are useful for assessing the frequency stability of various prototype X-band uplink or downlink subsystems for purposes of making design improvements. Information derived from extensive testing at DSS 13 will be useful in the preparation of an X-band Uplink Demonstration Experiment to be conducted at DSS 13, and will also be valuable in the preparations of gravity wave experiments to be conducted at other DSN stations in the future.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 1 - 14
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  • 11
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    DSN frequency and timing system, Mark 4-85 (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: As part of the Deep Space Network (DSN) Mark IVA implementation program, the DSN frequency and timing system is currently undergoing changes. With the implementation of signal processing centers (SPC) at each deep space communications complex (DSCC), major changes to the frequency and timing distribution equipment were necessary. A functional description of the Mark IVA frequency and timing system (FTS) as it exists today and planned capabilities through 1988 is given.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 113-119
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  • 12
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    Use of ground radar to detect reentering debris (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The velocity of the particles is required to identify the type of particles producing the ionization trails. A method of approximating the velocity of a meteor from radar data was developed. The method requires the time between the spacings of the Fresnel interference fringes, the range to the ionization trail, and the wavelength of the radar system. The orbital mechanics of the problem are evaluated, if the particles originate with the shuttle, the orbital mechanics will substantiate the relative position of the particles with the position of the shuttle. A program to determine spacecraft orbital decay due to perturbations is utilized for a preliminary evaluation of the orbital mechanics of the problem. Many assumptions concerning the size, shape, density, etc. of the particles are necessary for the preliminary evaluation. The results do not negate the possibility that the events observed by the radar are reentering particles originating from the shuttle.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Orbital Debris; p 164-169
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  • 13
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    Speech communications in noise (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The physical characteristics of speech, the methods of speech masking measurement, and the effects of noise on speech communication are investigated. Topics include the speech signal and intelligibility, the effects of noise on intelligibility, the articulation index, and various devices for evaluating speech systems.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Physiol., Psychol., and Social Effects of Noise; p 57-110
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  • 14
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    The northern Utah satellite (NUSAT) communications link (1986)
    In:  CASI
    Details
    Publication Date: 2006-06-11
    Description: During the planning stages of the NUSAT satellite, an obvious issue to be discussed was the method of communications to be used. The frequencies would have to be high enough to pass through the atmosphere relatively unattenuated but low enough that antennas and transmission lines would not be so critical in length and properties that unexperienced students would have difficulty with handling them. The frequencies of 450.000 MHz and 137.900 MHz were decided upon and applied for licensing. Representatives of the amateur radio satellite organization, AMSAT, were contacted for ideas. This organization seems to favor AM types of emissions such as CW to control their OSCAR series of satellites and also the current Phase 3 unit. NUSAT personnel felt, however, that there would be merit in the improved signal to noise ratio usually obtained in an FM mode. Doppler shift of the transmitted information on the NUSAT also had to be considered. The final decision was to use Audio Frequency Shift Keying (ASKF) modulated on an FM carrier. In this mode audio tones used would not shift frequency with Doppler.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Goddard Space Flight Center The 1985 Get Away Special Experimenter's Symposium; p 25-32
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  • 15
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    Flight test of a synthetic aperture radar antenna using STEP (1984)
    In:  CASI
    Details
    Publication Date: 2006-04-03
    Description: To establish confidence in its overall performance, credible information on the synthetic aperture radar antenna's mechanical properties in orbit must be obtained. However, the antenna's size, design, and operating environment make it difficult to simulate operating conditions under 1-g Earth conditions. The Space Technology Experiments Platform (STEP) offers a timely opportunity to mechanically qualify and characterize the antenna design in a representative environment. The proposed experimental configuration would employ a half-system of the full-scale RADARSAT antenna which would be mounted on the STEP platform in the orbiter cargo bay such that it could be deployed and retracted in orbit (as shown in this figure). The antenna would be subjected to typical environmental exposures while an array of targets and sensors on the antenna support structure and reflecting surface are observed and monitored. In particular, the typical environments would include deployment and retraction, dynamic response to vehicle thruster or base exciter inputs, and thermal soak and transient effects upon entering or exiting Earth eclipse. The proposed experiment would also provide generic information on the properties of large space structures in space and on techniques to obtain the desired information.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Langley Research Center STEP Expt. Requirements; p 339-354
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  • 16
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    Distributed synthetic aperture radar simulation (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Synthetic aperture radar (SAR) depends primarily on attainable frequency resolution rather than on large physical size of the antenna array. The distributed architecture concept (DSAR) incorporates active elements (amplifiers) at or near the elemental radiators of the array. Since SAR's are expensive to build and expensive to test, a computer modeling approach is a feasible method of predicting the quality or nature of the SAR image from the proposed system parameters. The goal of this project is to produce a DSAR simulation software package. This report describes the progress made thus far and the work which remains to be done. Extensive work on this project had been done previously by two NASA contractors. The principal task remaining involved the creation of a suitable interface between these programs and the hardware and software available at the Johnson Space Center.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Johnson Space Center NASA/American Society for Engineering Educaqtio; NASA. Johnson Space
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  • 17
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    CV-990 L-band SAR: A calibration experiment (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Calibrated image data is required by most users of synthetic aperture radar (SAR) data particularly those attempting to classify targets based upon their radar backscatter signature as a function of frequency polarization or incidence angle. In this experiment, the backscatter derived by calibrating the NASA/JPL CV-990 L-band SAR, and the backscatter reported from a pass of the NASA/JSC C-130 scatterometer as the two instruments flew over the same site at different times are compared.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA(JPL Aircraft SAR Workshop Proc.; p 17-20
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  • 18
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    Radar system description (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: A brief overview is given of the hardware and software utilized to generate the CV-990 synthetic aperture radar's image products. The on-board hardware is described, then the operational processing software, and finally the image products that are routinely available.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA(JPL Aircraft SAR Workshop Proc.; p 5-12
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  • 19
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    Improved mapping of radio sources from VLBI data by least-square fit (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: A method is described for producing improved mapping of radio sources from Very Long Base Interferometry (VLBI) data. The method described is more direct than existing Fourier methods, is often more accurate, and runs at least as fast. The visibility data is modeled here, as in existing methods, as a function of the unknown brightness distribution and the unknown antenna gains and phases. These unknowns are chosen so that the resulting function values are as near as possible to the observed values. If researchers use the radio mapping source deviation to measure the closeness of this fit to the observed values, they are led to the problem of minimizing a certain function of all the unknown parameters. This minimization problem cannot be solved directly, but it can be attacked by iterative methods which we show converge automatically to the minimum with no user intervention. The resulting brightness distribution will furnish the best fit to the data among all brightness distributions of given resolution.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 199-210
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  • 20
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    A comparison of lightning and nuclear electromagnetic pulse response of tactical shelters (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The internal response (electromagnetic fields and cable responses) of tactical shelters is addressed. Tactical shelters are usually well-shielded systems. Apart from penetrations by signal and power lines, the main leakage paths to the interior are via seams and the environment control unit (ECU) honeycomb filter. The time domain in three-dimensional finite-difference technique is employed to determine the external and internal coupling to a shelter excited by nuclear electromagnetic pulses (NEMP) and attached lightning. The responses of interest are the internal electromagnetic fields and the voltage, current, power, and energy coupled to internal cables. Leakage through the seams and ECU filter is accomplished by their transfer impedances which relate internal electric fields to external current densities. Transfer impedances which were experimentally measured are used in the analysis. The internal numerical results are favorably compared to actual shelter test data under simulated NEMP illumination.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Langley Research Center Intern. Aerospace and Ground Conf. on Lightning and Static Elec.; 12 p
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  • 21
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    Upset susceptibility study employing circuit analysis and digital simulation (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: An approach to predict the susceptibility of digital systems to signal disturbances is described. Electrical disturbances on a digital system's input and output lines can be induced by activities and conditions including static electricity, lightning discharge, electromagnetic interference (EMI), and electromagnetic pulsation (EMP). The electrical signal disturbances employed for the susceptibility study were limited to nondestructive levels, i.e., the system does not sustain partial or total physical damage and reset and/or reload brings the system to an operational status. The front-end transition from the electrical disturbances to the equivalent digital signals was accomplished by computer-aided circuit analysis. The super-sceptre (system for circuit evaluation of transient radiation effects) programs was used. Gate models were developed according to manufacturers' performance specifications and parameters resulting from construction processes characteristic of the technology. Digital simulation at the gate and functional level was employed to determine the impact of the abnormal signals on system performance and to study the propagation characteristics of these signals through the system architecture. Example results are included for an Intel 8080 processor configuration.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Intern. Aerospace and Ground Conf. on Lightning and Static Elec.; 14 p
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  • 22
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    A comparison of lightning and nuclear electromagnetic pulse response of a helicopter (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: A numerical modeling technique is utilized to investigate the response of a UH-60A helicopter to both lightning and nuclear electromagnetic pulses (NEMP). The analytical approach involves the three-dimensional time domain finite-difference solutions of Maxwell's equations. Both the external currents and charges as well as the internal electromagnetic fields and cable responses are computed. Results of the analysis indicate that, in general, the short circuit current on internal cables is larger for lightning, whereas the open-circuit voltages are slightly higher for NEMP. The lightning response is highly dependent upon the rise time of the injected current as was expected. The analysis shows that a coupling levels to cables in a helicopter are 20 to 30 dB larger than those observed in fixed-wing aircraft.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Langley Research Center Intern. Aerospace and Ground Conf. on Lightning and Static Elec.; 7 p
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  • 23
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    Data and results of a laboratory investigation of microprocessor upset caused by simulated lightning-induced analog transients (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Advanced composite aircraft designs include fault-tolerant computer-based digital control systems with thigh reliability requirements for adverse as well as optimum operating environments. Since aircraft penetrate intense electromagnetic fields during thunderstorms, onboard computer systems maya be subjected to field-induced transient voltages and currents resulting in functional error modes which are collectively referred to as digital system upset. A methodology was developed for assessing the upset susceptibility of a computer system onboard an aircraft flying through a lightning environment. Upset error modes in a general-purpose microprocessor were studied via tests which involved the random input of analog transients which model lightning-induced signals onto interface lines of an 8080-based microcomputer from which upset error data were recorded. The application of Markov modeling to upset susceptibility estimation is discussed and a stochastic model development.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Intern. Aerospace and Ground Conf. on Lightning and Static Elec.; 12 p
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  • 24
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    Prediction techniques (1981)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Prediction methods and related propagation results for the evaluation of Earth-space communication paths operating above 10 GHz are presented. Gaseous attenuation, rain, cloud, fog, sand, and dust attenuation, path diversity, signal fluctuations and low angle fading, depolarization effects, bandwidth coherence, and sky noise are considered.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Propagation Effects Handbook for Satellite Systems Design; p 195-348
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  • 25
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    Characteristics of rain and rain systems (1981)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The description and measurement of the macroscopic and microscopic characteristics of rain and rain systems are discussed. The statistical relationships of these characteristics and their effect on polarization and attenuation are considered. Macroscopic characteristics include the size, distribution, and movements of rain cells, the height of melting layers, and the presence of ice crystals. Microscopic characteristics include the size distribution, density, and oblateness of rain drops and ice crystals. The estimation of a major propagation effect, specific attenuation, is described.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Propagation Effects Handbook for Satellite Systems Design; p 5-39
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  • 26
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    Propagation data bases (1981)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Existing data bases accumulated as the result of experiments to gather propagation data on millimeter wave Earth-space links are described. The satellites used are described and results of the significant experiments conducted in the United States are summarized. The data bases consist primarily of cumulative attenuation statistics, though some depolarization measurements are included. Additional summaries of propagation data are cited.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Its Propagation Effects Handbook for Satellite Systems Design; p 155-194
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  • 27
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    A new SPS antenna design to reduce grating lobes (1982)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: In the Solar Power Satellite system, the antenna's misalignment will produce well defined grating lobes. These gratings lobe peaks occur every 440 km and are potentially hazardous to the environment. One way to suppress these peaks is to phase control every power module. The cost due to the increase in receiving electronics and processors, however, could prove to be prohibitive. A new design of the antenna involving the addition of two broad gaps, one along the x axis and another along the y axis is proposed. The gap distance is exactly one half of the distance between the center of two neighboring subarrays. Calculation of far field radiation patterns shows that the design reduces grating lobe peaks without sacrificing power in the main lobe.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The 1981 NASA(ASEE Summer Fac. Fellowship Program; 14 p
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  • 28
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    An overview of several rain and attenuation models (1981)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The key features of the Rice-Holmberg, Dutton-Dougherty, Global, Lin, and Piecewise Uniform Rain Rate models for estimation of cumulative attenuation statistics on Earth-space millimeter paths are summarized. The models provide either rain rate statistics or attenuation statistics, which can be related by use of the specific attenuation and the effective path length relation. The effective path length is defined. The advantages and disadvantages of each model for specific applications are discussed. The Global model is recommended as the design tool for predicting propagation effects in Earth-space communication systems.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Propagation Effects Handbook for Satellite Systems Design; p 41-101
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  • 29
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    Application of propagation predictions to Earth/space telecommunications system design (1981)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The corresponding between a given propagation phenomenon and system performance is considered. Propagation data are related to system performance parameters, allowing the systems engineer to perform the analyses determining how well requirements are met by a given system design, and enabling the systems engineer to modify that design if necessary. The various ways of specifying performance criteria for different kinds of systems are discussed, and a general procedure for system design is presented and demonstrated.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Propagation Effects Handbook for Satellite Systems Design; p 349-404
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  • 30
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    Depolarization on Earth-space paths (1981)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Sources of depolarization effects on the propagation paths of orthogonally-polarized information channels are considered. The main sources of depolarization at millimeter wave frequencies are hydrometeor absorption and scattering in the troposphere. Terms are defined. Mathematical formulations for the effects of the propagation medium characteristics and antenna performance on signals in dual polarization Earth-space links are presented. Techniques for modeling rain and ice depolarization are discussed.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Propagation Effects Handbook for Satellite Systems Design; p 103-154
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  • 31
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    Real-time synthetic aperture radar processing (1984)
    In:  CASI
    Details
    Publication Date: 2006-04-09
    Description: Real-time acousto-optic SAR processors are described and experimentally demonstrated. SAR imaging is performed in one of the architectures by applying the signal to an acousto-optic device and correlating it with chirp signals recorded on an optical transparency by time integration on a CCD detector. In a different implementation, the imaging is preformed by interfering the light beams diffracted from two separate acousto-optic devices, one modulated the radar signal and the second by the reference chirp waveform.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Langley Research Center Opt. Inform. Process. for Aerospace Appl. 2; p 199-213
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  • 32
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    Microwave reflector characterization using simple instruments in EVA (1984)
    In:  CASI
    Details
    Publication Date: 2006-04-03
    Description: An antenna with rigid panels which can be measured under ground conditions, carried to space in a packaged condition, deployed into a form similar to the Earth-measured one, measured under space conditions, restowed, and brought back to Earth so that the original measurements can be verified is the type being proposed for this experiment. The antenna chosen will be measured under ground conditions, carried aloft, deployed into its antenna shape, lifted by the remote manipulator system to a position where it can be sighted by two astronauts at the two theodolites, and held there until the surface characterization can be completed. An alternate method would be to use photogrammetry and take pictures of the surface from the payload handling station. After the surface characterization is completed, the antenna will be folded and restowed into the Shuttle bay for return to Earth. The surface characterization will be repeated on Earth after its return for verification both of the original measurement taken on Earth and the measurement taken in space.
    Keywords: COMMUNICATIONS AND RADAR
    Type: STEP Expt. Requirements; p 333-338
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  • 33
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    Adaptive microwave reflector (1984)
    In:  CASI
    Details
    Publication Date: 2006-04-03
    Description: As an interim step in going to the 100-meter reflector that was evaluated, a 5-meter reflector is proposed to test the electrostatic concept under space conditions. Some of the issues which require the space environment for evaluation are the following questions: Can deployment of a box ring structure with a thin film reflector attached be manually deployed? In the absence of humidity, can a 0.3-mil aluminized Kapton film reflector be formed by the electrostatic process suitable for antenna applications? Can the photogrammetric process be used to evaluate the reflector surface with pictures taken from the payload handling station? Can the space charging effect be evaluated with the 5-meter reflector attached to the Shuttle? Does the outgassing of moisture from 0.3-mil Kapton film affect its reflector capability? A box ring truss support structure and an automatic sequence deployment system are discussed.
    Keywords: COMMUNICATIONS AND RADAR
    Type: STEP Expt. Requirements; p 325-331
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  • 34
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    Passive microwave remote sensing of the asteroids using the VLA: Continuation (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The structure and composition parameters for a selected set of the main belt asteroids were inferred by employing microwave remote sensing techniques developed for Earth observations. Precise flux density measurements made with the Very Large Array (VLA) and the 11 m mm-wave antenna of the National Radio Astronomy Observatory are used to define the microwave continuum spectra of these asteroids. These spectra were inverted in order to estimate the near-surface bulk properties (radii, roughness, composition) independent of previous optical or infrared spectroscopy.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA, Washington Reports of Planetary Astronomy, 1985; p 133-134
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  • 35
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    The 1- to 4-K refrigeration techniques for cooling masers on a beam waveguide antenna (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The status of technology is reported for various 1- to 4-K commercially available refrigeration systems capable of producing 1.5-K refrigeration to cool masers and superconducting cavity oscillators on the proposed beam waveguide antenna. The design requirements for the refrigeration system and the cryostat are presented. A continuously operating evaporation refrigerator that uses capillary tubing to provide a continuous, self-regulating flow of helium at approximately 1.5 K has been selected as the first refrigerator design for the beam waveguide antenna.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 56 - 65
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  • 36
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    New short-time alignment technique for 70-meter antenna surface panels (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: With severely limited field modification time for upgrading the 64-m antenna to 70-m diameter, a new shorter time method for aligning the surface panels of the main reflector was needed. For each target on the surface panel, both distance (or range) and elevation angle measurements are made. A new technique for setting the surface panels at zenith look has been devised. This article describes the software required to convert the computed target distortions obtained from the JPL-IDEAS structural analysis computer program (defining the gravity load change from a 45-deg elevation angle to zenith look) into the theodolite reading at zenith look. The technique results in a perfectly shaped reflector at the 45-deg rigging elevation, with acceptable surface error tolerance.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 15 - 25
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  • 37
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    Array feed synthesis for correction of reflector distortion and Vernier Beamsteering (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: An algorithmic procedure for the synthesis of planar array feeds for paraboloidal reflectors is described which simultaneously provides electronic correction of systematic reflector surface distortions as well as a Vernier electronic beamsteering capability. Simple rules of thumb for the optimum choice of planar array feed configuration (i.e., number and type of elements) are derived from a parametric study made using the synthesis procedure. A number of f/D ratios and distortion models were examined that are typical of large paraboloidal reflectors. Numerical results are presented showing that, for the range of distortion models considered, good on-axis gain restoration can be achieved with as few as seven elements. For beamsteering to +/- 1 beamwidth (BW), 19 elements are required. For arrays with either 7 or 19 elements, the results indicate that the use of high-aperture-efficiency elements (e.g., disk-on-rod and short backfire) in the array yields higher system gain than can be obtained with elements having lower aperture efficiency (e.g., open-ended waveguides). With 37 elements, excellent gain and beamsteering performance to +/- 1.5 BW are obtained independent of the assumed effective aperture of the array element. An approximate expression is derived for the focal-plane field distribution of the distorted reflector. Contour plots of the focal-plane fields are also presented for various distortion and beam scan angle cases. The results obtained show the effectiveness of the array feed approach.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 43 - 55
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  • 38
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    Comparison of the calibration of ionospheric delay in VLBI data by the methods of dual frequency and Faraday rotation (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: When both S-band and X-band data are recorded for a signal which has passed through the ionosphere, it is possible to calculate the ionospheric contribution to signal delay. In Very Long Baseline Interferometry (VLBI) this method is used to calibrate the ionosphere. In the absence of dual frequency data, the ionospheric content measured by Faraday rotation, using a signal from a geostationary satellite, is mapped to the VLBI observing direction. The purpose here is to compare the ionospheric delay obtained by these two methods. The principal conclusions are: (1) the correlation between delays obtained by these two methods is weak; (2) in mapping Faraday rotation measurements to the VLBI observing direction, a simple mapping algorithm which accounts only for changes in hour angle and elevation angle is better than a more elaborate algorithm which includes solar and geomagnetic effects; (3) fluctuations in the difference in total electron content as seen by two antennas defining a baseline limit the application of Faraday rotation data to VLBI.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 11-23
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  • 39
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    SETI radio spectrum surveillance system (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The SETI Radio Spectrum Surveillance System (SRSSS) will provide a data base for assessing the radio frequency interference (RFI) environment for SETI and minimizing RFI disruptions during the search. The system's hardware and software are described and the sensitivity of the system is discussed.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 173-184
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  • 40
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    Radar investigation of asteroids (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The number of radar detected asteroids has climbed from 6 to 40 (27 mainbelt plus 13 near-Earth). The dual-circular-polarization radar sample now comprises more than 1% of the numbered asteroids. Radar results for mainbelt asteroids furnish the first available information on the nature of these objects at macroscopic scales. At least one object (2 Pallas) and probably many others are extraordinarily smooth at centimeter-to-meter scales but are extremely rough at some scale between several meters and many kilometers. Pallas has essentially no small-scale structure within the uppermost several meters of the regolith, but the rms slope of this regolith exceeds 20 deg., much larger than typical lunar values (approx. 7 deg.). The origin of these slopes could be the hypervelocity impact cratering process, whose manifestations are likely to be different on low-gravity, low-radius-of-curvature objects from those on the terrestrial planets. The range of mainbelt asteroid radar albedoes is very broad and implies big variations in regolith porosity or metal concentration, or both. The highest albedo estimate, for 16 Psyche, is consistent with a surface having porosities typical of lunar soil and a composition nearly completely metallic. Therefore, Psyche might be the collisionally stripped core of a differentiated small plant, and might resemble mineralogically the parent bodies of iron meteorites.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA, Washington Reports of Planetary Astronomy, 1985; p 118-119
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  • 41
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    Goldstone solar system radar (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Information is provided about physical nature planetary surfaces and their topography as well as dynamical properties such as orbits and spin states using ground based radar as a remote sensing tool. Accessible targets are the terrestrial planets: the Earth's Moon, Mercury, Venus and Mars, the outer planets rings and major moons, and many transient objects such as asteroids and comets. Data acquisition utilizes the unique facilities of the Goldstone Deep Space Network, occasionally the Arecibo radar, and proposed use of the VLA (very large array).
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA, Washington Reports of Planetary Astronomy, 1985; p 5-7
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  • 42
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    The Arecibo S-band radar program (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Reduction to images of the 1983 monostatic Venus mapping data was completed. Analysis of the images, including detailed comparisons with Pioneer Venus data and Soviet Venera 15/16 images, continued. Feature locations in Arecibo Venus radar images produced between 1975 and 1983 were used to make a new determination of the rotation vector. This result is of general interest but was also required for planning and cartographic purposes for the Magellan mission. A new set of observations of Venus were made in March/April 1985, aimed at imaging low northern and high southern latitudes, including the south polar region, at 1 to 2km resolution. A limited number of ranging observations were made to Mercury and Venus. Ten asteroids were detected increasing the number studied with the Arecibo system to almost 40. A system to allow high time resolution, as well as high Doppler frequency resolution, studies of small asteroids or comets which make close approaches to the Earth was implemented. A radar detection of Halley's Comet was obtained when it approached to within 0.62 AU of the Earth in November/December of last year. It is still uncertain whether the echo is a reflection from the nucleus or from a cloud of cm-sized particles in the vicinity of the nucleus.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA, Washington Reports of Planetary Astronomy, 1985; p 3-4
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  • 43
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    Cramer-Rao bounds for signal-to-noise ratio and combiner weight estimation (1986)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Cramer-Rao lower bounds on estimator variance are calculated for arbitrary unbiased estimates of signal-to-noise ratio and combiner weight parameters. Estimates are assumed to be based on a discrete set of observables obtained by matched filtering of a biphase modulated signal. The bounds are developed first for a problem model based on one observable per channel symbol period, and then extended to a more general problem in which subperiod observables are also available.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 124 - 130
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  • 44
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    A method to dramatically improve subcarrier tracking (1986)
    In:  CASI
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    Publication Date: 2006-02-14
    Description: A method is presented for achieving a dramatic improvement in phase tracking of square wave subcarriers or other square waves. The method is to set the amplitude of the phase quadrature reference signal to zero except near the zero crossings of the input signal. Without changing the loop bandwidth, the variance of the phase error can be reduced to approximately W sigma(sub 0)(2), were sigma (sub 0)(2) is the phase error variance without windowing, and W is the fraction of cycle in which the reference signal has a nonzero value. Simulation results confirm the analysis and establish minimum W versus signal-to-noise ratio. Typically, the window can be made so narrow as to achieve a phase error variance of 1.5 sigma(sub 0)(4).
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 103 - 110
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  • 45
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    A new method for frame synchronization (1986)
    In:  CASI
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    Publication Date: 2006-02-14
    Description: A frame synchronization method based on an examination of the shortest bit length containing all error bursts is introduced. It is shown that the new method is more reliable and efficient than the commonly used one based on counting the number of errors for the convolutionally coded channel.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report (date]; p 111 - 123
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  • 46
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    Propagation and radiation characteristics of a multimode corrugated waveguide feedhorn (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: A prototype of the multimode corrugated feedhorn which will be used in the 400 kW CW Ka-band radar system is described. A rough design is done using coupled mode theory and standard corrugated waveguide modes. A more exact analysis using mode matching techniques is then used which takes into account the effect of a finite number of corrugations per wavelength and determines the modes which are reflected from the device. A prototype feedhorn has been constructed and measured. These experimental results are then compared to the theoretical predictions which agree satisfactorily closely.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 57-67
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  • 47
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    A VLBI survey at 2.29 GHz (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The Deep Space Network (DSN) is establishing a high-accuracy VLBI celestial reference frame. The results of a search for suitable radio sources to be used in constructing this frame are given. The VLBI observations using DSN baselines at 2.29 GHz with fringe spacings of about 3 milliarcseconds have been performed on 1398 radio sources spread over the entire sky. Of those, 917 sources were detected including 93% of the identifed BL Lacertae objects, 86% of Quasars and 36% of galaxies. The resulting catalog of compact radio sources is also useful for various astrophysical studies and in the formation of VLBI celestial reference frames.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 1-10
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  • 48
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    A new 70-meter antenna quadripod with reduced RF blockage (1985)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The new subreflector mount (quadripod) for the 64-meter to 70-meter antenna extension project was the result of many trial designs aimed at reducing radio frequency spherical and plane wave blockage and minimizing structural weight while satisfying strength and natural frequency requirements. An optimum design emerged which has a gain improvement of 0.32 dB over the present 64-meter design. Some of the trial designs made and the final optimum configuration selected are described.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommun. and Data Acquisition Progr. Rept.; p 24-30
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  • 49
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    Effect of space environment on space-based radar phased-array antenna (A0133) (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: Kapton polyimide film was selected as the baseline material for the Grumman spce based radar (SBR) concept. To gain the requisite confidence for long-term service durability, it is desirable to subject material specimens as well as a portion of the SBR antenna directly to the combined space environment and compare property degradation to that caused by laboratory simulation. The overall objective of this program is to evauate the effect of the space environment on polymeric materials currently being considered for the Grumman SBR Phased-Array Antenna. Degradation mechanisms caused by thermal cycling, ultraviolet and charged-particle irradiation, applied load, and high-voltage plasma interaction will be evaluated. The experiment occupies a 6-in.-deep end corner tray located on the space end of the Long Duration Exposure Facility and consists of both passive and active parts. The passive part addresses the effect of environment and stress on the dimensional stability spliced and continuous Kapton, both plain and reinforced. The active part will study the interaction of high voltage and low-Earth-orbit plasma.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Langley Research Center Long Duration Exposure Facility (LDEF); p 21-23
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  • 50
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    Airborne Doppler radar flight experiments for the detection of microbursts (1989)
    In:  CASI
    Details
    Publication Date: 2011-10-14
    Description: In the interest of aviation safety, NASA and the FAA are jointly conducting research to determine the applicability of airborne, coherent Doppler radar techniques to detect early microburst in wind shear conditions during aircraft takeoff and landing. Researchers have developed a computer model of the radar which predicts its response when viewing a simulated microburst against the simulated clutter background of an airport, the so-called radar microburst ground clutter model. Studies employing this model revealed that Doppler radar can accurately detect microburst ahead of the aircraft in time for pilot evasive response, but flight experiments will be required for complete performance evaluation of the system. An experimental X band radar is being developed for future flight experiments to verify the simulation modeling results. A description of the experimental radar, recording equipment, and its installation on the NASA 515 aircraft is presented. The flight experiments to be conducted are also described.
    Keywords: COMMUNICATIONS AND RADAR
    Type: AGARD, High Resolution Air- and Spaceborne Radar; 14 p
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  • 51
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    Calibration of quadpolarization SAR data using backscatter statistics (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A new technique is described for calibration of complex multipolarization SAR imagery. Scatterer reciprocity and lack of correlation between like- and cross-polarized radar echoes for natural targets are used to remove cross-polarized contamination in the radar data channels without the use of known ground targets. If known targets are available, all data channels can be calibrated relative to one another and absolutely as well. The method is verified with airborne SAR data.
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    Magellan SAR processing algorithm and H/W design (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The SAR (synthetic-aperture radar) data-processing algorithm to be used for the Magellan mission is described. Radar system design, SAR data characteristics, and hardware (H/W) constraints, which are critical to the processing algorithm design, are highlighted. Data flow and the H/W architecture are given to show the real-time data processing capability. Simulation results obtained from processing the synthetic point-target echos are presented to demonstrate the performance of the processing algorithm.
    Keywords: COMMUNICATIONS AND RADAR
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  • 53
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    A square multiflare horn with 1-megawatt CW power-handling capability (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The analysis and low-power testing of a square multiflare horn designed for 1-megawatt CW operation is described. Design considerations for the five-port input section are discussed. The required aperture modes are determined from radiation pattern considerations, and analysis of the multiflare section is carried out using mode matching. Measurements demonstrate that a circularly symmetric beam is produced with relatively low sidelobes.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Microwave and Optical Technology Letters (ISSN 0895-2477); 2; 400-404
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  • 54
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    Experimental study of the cross-polarization characteristics of rectangular microstrip antennas (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The cross polarization characteristics of rectantular patch antennas are studied experimentally. Data are presented showing the dependence of the copolarization to cross-polarization ratio on the aspect ratio in both the E and H planes. Three substrate thicknesses are included and the variation with resonant frequency is examined.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Microwave and Optical Technology Letters (ISSN 0895-2477); 2; 247-249
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  • 55
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    Interpolation solutions for the problem of synthesis of dual-shaped offset reflector antennas (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Synthesis of dual-shaped offset reflector antennas to control the exit aperture distribution of amplitude and phase has received considerable attention in recent years. For a given feed illumination and desired aperture field distribution, an exact formulation of the problem of simultaneously synthesizing the shapes of the sub and main reflectors was presented recently by Galindo-Israel et al. (1987) in terms of a set of nonlinear first-order differential equations. In this paper, a numerical approach to solving these equations is discussed which circumvents some of the difficulties encountered by Galindo-Israel et al., particularly for small values of theta.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Microwave and Optical Technology Letters (ISSN 0895-2477); 2; 43-47
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  • 56
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    A technique to calibrate polarimetric radar images using only image parameters and trihedral corner reflectors (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The technique described uses the radar return from natural targets and at least one trihedral corner reflector to calibrate compressed polarimetric radar data for relative amplitude, relative phase, absolute amplitude, and system crosstalk. The crosstalk method is based on the theoretical result that, for natural targets with azimuthal symmetry, the copolarized and cross-polarized components of the scattering matrix are uncorrelated. This method does not require any external calibration targets to be deployed. Since compressed data are used, it is necessary to model the transmitting and receiving systems as reciprocal. The method is used to estimate the crosstalk parameters of the NASA/JPL aircraft for different types of terrain and for two frequencies. For the C-band systems the crosstalk is less than -20 dB for all ranges in the images. The crosstalk of the L-band system is a function of range, however, and may be as poor as -10 dB in the near range, leading to a noticeable distortion of the polarization signatures.
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  • 57
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    Doppler centroid estimation ambiguity for synthetic aperture radars (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A technique for estimation of the Doppler centroid of an SAR in the presence of large uncertainty in antenna boresight pointing is described. Also investigated is the image degradation resulting from data processing that uses an ambiguous centroid. Two approaches for resolving ambiguities in Doppler centroid estimation (DCE) are presented: the range cross-correlation technique and the multiple-PRF (pulse repetition frequency) technique. Because other design factors control the PRF selection for SAR, a generalized algorithm is derived for PRFs not containing a common divisor. An example using the SIR-C parameters illustrates that this algorithm is capable of resolving the C-band DCE ambiguities for antenna pointing uncertainties of about 2-3 deg.
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  • 58
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    Polarization filtering of SAR data (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A theoretical analysis of polarization filtering for the bistatic case is developed for optimum discrimination between two types of targets. The resulting method is half analytical and half numerical. Because it is based on the Stokes matrix representation, the targets of interest can be extended targets. The scattered field from such targets is partially polarized. This method is then applied to the monostatic case with numerical examples relying on the JPL (Jet Propulsion Laboratory) full-polarimetric L-band radar data. A matched filter to maximize the power ratio between urban and natural targets is developed. The results show that the same filter is optimal for both ocean and forest targets as natural targets.
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  • 59
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    Squint mode SAR processing algorithms (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The unique characteristics of a spaceborne SAR (synthetic aperture radar) operating in a squint mode include large range walk and large variation in the Doppler centroid as a function of range. A pointing control technique to reduce the Doppler drift and a new processing algorithm to accommodate large range walk are presented. Simulations of the new algorithm for squint angles up to 20 deg and look angles up to 44 deg for the Earth Observing System (Eos) L-band SAR configuration demonstrate that it is capable of maintaining the resolution broadening within 20 percent and the ISLR within a fraction of a decibel of the theoretical value.
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  • 60
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    Optimal Doppler centroid estimation for SAR data from a quasi-homogeneous source (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This correspondence briefly describes two Doppler centroid estimation (DCE) algorithms, provides a performance summary for these algorithms, and presents the experimental results. These algorithms include that of Li et al. (1985) and a newly developed one that is optimized for quasi-homogeneous sources. The performance enhancement achieved by the optimal DCE algorithm is clearly demonstrated by the experimental results.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); GE-24; 1022-102
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    Miniature voice output communication aid (1982)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA Technol. Appl. Team; p 20
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  • 62
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    Use of speckle for determining the response characteristics of Doppler imaging radars (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: An optical model is developed for imaging optical radars such as the SAR on Seasat and the Shuttle Imaging Radar (SIR-B) by analyzing the Doppler shift of individual speckles in the image. The signal received at the spacecraft is treated in terms of a Fresnel-Kirchhoff integration over all backscattered radiation within a Huygen aperture at the earth. Account is taken of the movement of the spacecraft along the orbital path between emission and reception. The individual points are described by integration of the point source amplitude with a Green's function scattering kernel. Doppler data at each point furnishes the coordinates for visual representations. A Rayleigh-Poisson model of the surface scattering characteristics is used with Monte Carlo methods to generate simulations of Doppler radar speckle that compare well with Seasat SAR data SIR-B data.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Optical Engineering (ISSN 0091-3286); 25; 772-779
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    16-QAM and trellis-coded 16-QAM on nonlinear channels (1985)
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    Publication Date: 2011-08-19
    Description: The use of 16-QAM on bandlimited nonlinear satellite channels, in uncoded and trellis-coded form, for bandwidth efficient modulation (ideally 4 bps/Hz) is discussed. To avoid severe degradation due to AM/AM and AM/PM distortion, or to avoid the need for large back-off, predistorting the 16-QAM constellation at the modulator is considered. Performance for varying back-off settings for uncoded and coded 16-QAM is simulated, and it is found that the four-state trellis code has a coding gain of about 8 dB, larger than expected based on linear channel analysis, while the 16-state code gains only marginally beyond this.
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    2400 bit/s DMSK modem for mobile satellite service (1985)
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    Publication Date: 2011-08-19
    Description: The letter describes the implementation of a differentially coherent receiver suitable for bursty signal transmission over fading channels. To meet the severe system phase response requirement, this implementation is conducted at baseband. Means of frequency correction (AFC) is also shown. This receiver is simple and can easily be fabricated using VLSI technology.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Electronics Letters (ISSN 0013-5194); 21; 965
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  • 65
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    An optimized bandwidth efficient demand assigned protocol for integrated Mobile Satellite Services (1985)
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    Publication Date: 2011-08-19
    Description: This paper describes the design of a demand assigned protocol based on bandwidth efficiency for integrated services of a Mobile Satellite System (MSS). An MSS provides data (closed-ended) and voice (open-ended) communications services for a large number of mobile users dispersed over a wide geographical area. Each mobile requests its desired service through a designated set of channels to a network management center. Either pure or slotted ALOHA random access scheme can be used to make connection requests, while data and voice communications are demand assigned. All channels have equal bandwidth and can be adaptively used for reservation requests, data connections or voice connections to maximize the bandwidth utilization. In this paper, perfect communications channels are assumed. It has been shown that, for the case considered, using the slotted ALOHA scheme for making connection requests can save about 30 percent on the total number of channels over using the pure ALHOA scheme.
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    Application of a VLSI vector quantization processor to real-time speech coding (1986)
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    Publication Date: 2011-08-19
    Description: Attention is given to a working vector quantization processor for speech coding that is based on a first-generation VLSI chip which efficiently performs the pattern-matching operation needed for the codebook search process (CPS). Using this chip, the CPS architecture has been successfully incorporated into a compact, single-board Vector PCM implementation operating at 7-18 kbits/sec. A real time Adaptive Vector Predictive Coder system using the CPS has also been implemented.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Journal on Selected Areas in Communications (ISSN 0733-8716); SAC-4; 112-124
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  • 67
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    Geostationary communications platform payload concepts (1986)
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    Publication Date: 2011-08-19
    Description: The types of operational communications payloads that could be appropriate for a large geostationary facility in the middle to late 1990s are examined. A description of each type of payload is given, and various key issues and economic factors are discussed. A comparison with existing satellites indicates that large platforms may offer cost advantages.
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  • 68
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    A method to dramatically improve subcarrier tracking (1988)
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    Publication Date: 2011-08-19
    Description: A method is presented for achieving a dramatic improvement in phase tracking of square wave subcarriers or other square waves. The method is to set the amplitude of the phase quadrature reference signal to zero except near the zero crossings of the input signal. Without changing the loop bandwidth, the variance of the phase error can be reduced to approximately W sigma(sub 0)(2), where sigma (sub 0)(2) is the phase error variance without windowing, and W is the fraction of cycle in which the reference signal has a nonzero value. Simulation results confirm the analysis and establish minimum W versus signal-to-noise ratio. Typically, the window can be made so narrow as to achieve a phase error variance of 1.5 sigma(sub 0)(4).
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); 36; 238-243
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  • 69
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    Speckle noise reduction of 1-look SAR imagery (1987)
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    Publication Date: 2011-08-19
    Description: Speckle noise is inherent to synthetic aperture radar (SAR) imagery. Since the degradation of the image due to this noise results in uncertainties in the interpretation of the scene and in a loss of apparent resolution, it is desirable to filter the image to reduce this noise. In this paper, an adaptive algorithm based on the calculation of the local statistics around a pixel is applied to 1-look SAR imagery. The filter adapts to the nonstationarity of the image statistics since the size of the blocks is very small compared to that of the image. The performance of the filter is measured in terms of the equivalent number of looks (ENL) of the filtered image and the resulting resolution degradation. The results are compared to those obtained from different techniques applied to similar data. The local adaptive filter (LAF) significantly increases the ENL of the final image. The associated loss of resolution is also lower than that for other commonly used speckle reduction techniques.
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  • 70
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    On the optimum polarizations of incoherently reflected waves (1987)
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    Publication Date: 2011-08-19
    Description: The Stokes scattering operator is noted to be the most useful characterization of incoherent scattering in radar imaging; the polarization that would yield an optimum amount of power received from the scatterer is obtained by assuming a knowledge of the Stokes scattering operator instead of the 2x2 scattering matrix with complex elements. It is thereby possible to find the optimum polarizations for the case in which the scatterers can only be fully characterized by their Stokes scattering operator, and the case in which the scatterer can be fully characterized by the complex 2x2 scattering matrix. It is shown that the optimum polarizations reported in the literature form the solution for a subset of a more general class of problems, so that six optimum polarizations can exist for incoherent scattering.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-35; 818-825
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  • 71
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    Low-loss offset feeds for electrically large symmetric dual-reflector antennas (1987)
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    Publication Date: 2011-08-19
    Description: Circularly symmetric, dual reflector, high gain antenna systems often require feeds placed off the system's axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise with a high gain/low noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Casegrainian or the dual shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a planned 70-meter, dual shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-35; 745-755
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  • 72
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    Analysis of a dual-wavelength surface reference radar technique (1987)
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    Publication Date: 2011-08-19
    Description: The use of the single-wavelength surface reference technique (SRT), the standard dual-wavelength technique (DWT), and dual-wavelength surface reference technique (DSRT) for measuring precipitation from space is analyzed, and the results are compared. The characteristics of the three methods are described, and the surface return powers during clear air conditions are measured. It is observed that the rain rate estimates obtained with the DWT and DSRT are identical in form, and the surface reflectance can be derived from the estimates by replacing the rain reflectivity statistics with those of the backscattering cross section of the surface. The data reveal that the DSRT is more applicable than the SRT when the wavelength correlation in the backscattering cross section of the surface is high or when the mean values of the backscattering cross section at the two wavelengths are nearly equal.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); GE-25; 456-471
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  • 73
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    Word and frame synchronization with verification for PPM optical communications (1986)
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    Publication Date: 2011-08-19
    Description: A method for obtaining word and frame synchronization in pulse position modulated optical communication systems is described. The method uses a short sync sequence inserted at the beginning of each data frame and a verification procedure to distinguish between inserted and randomly occurring sequences at the receiver. This results in an easy to implement sync system which provides reliable synchronization even at high symbol error rates. Results are given for the application of this approach to a highly energy efficient 256-ary PPM test system.
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  • 74
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    A synchronization technique for optical PPM signals (1986)
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    Publication Date: 2011-08-19
    Description: A technique for maintaining synchronization between optical PPM (pulse-position modulation) pulses and a receiver clock by means of a delay-tracking loop is described and analyzed. The tracking loop is driven by a doubly stochastic Poisson process that contains information about the location of the desired slot boundaries. The slot boundaries are subject to slowly varying random delays that are ultimately tracked by the loop. The concept of fractional rms delay error is introduced to quantify the effects of signal and background induced shot noise on the performance of the delay-tracking loop.
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  • 75
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    Simple method for designing or analyzing an optical communication link (1986)
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    Publication Date: 2011-08-19
    Description: A simple method is described for determining the performance of a free space optical communication link. The method can be used either in the system design (synthesis) mode, or in the performance evaluation (analysis) mode. Although restricted to photo counting based detection (e.g., photomultiplier tube or equivalent) of pulse position modulated signals, the method is still sufficiently general to accommodate space-based, as well as ground-based, reception.
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  • 76
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    4800 B/S speech compression techniques for mobile satellite systems (1986)
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    Publication Date: 2011-08-19
    Description: This paper will discuss three 4800 bps digital speech compression techniques currently being investigated for application in the mobile satellite service. These three techniques, vector adaptive predictive coding, vector excitation coding, and the self excited vocoder, are the most promising among a number of techniques being developed to possibly provide near-toll-quality speech compression while still keeping the bit-rate low enough for a power and bandwidth limited satellite service.
    Keywords: COMMUNICATIONS AND RADAR
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  • 77
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    Multiple trellis coded modulation (MTCM) (1986)
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    Publication Date: 2011-08-19
    Description: A new trellis coded modulation technique, referred to as multiple trellis coded modulation wherein more than one channel symbol per trellis branch is transmitted and demonstrated. Simple two state trellis codes for symmetric MPSK and AM modulations, which can achieve 3 dB gain over uncoded modulation at very high signal-to-noise ratios without bandwidth expansion and without reduction in information bit rate have been found. The gain of the new trellis codes with respect to previously reported two state trellis codes is between 1 to 2 dB at very high signal-to-noise ratios, depending on the number of bits per hertz transmitted. These gains are achieved with no additional cost in complexity; while indeed additional computations per branch are needed for the multiple trellis coding scheme. This concept can be extended to higher number of states and other type of modulations; i.e., quadrature amplitude modulation.
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  • 78
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    ACTS experiments program (1986)
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    Publication Date: 2011-08-19
    Description: NASA's Advanced Communications Technology Satellite which will flight test the advanced technologies associated with a Ka-band multibeam antenna, onboard signal processing and switching, and laser communications is described. The ACTS Experiment Program includes flight system technology experiments, ground system technology experiments, network control, propagation experiments, and end-to-end system experiments. Operational communications modes employing the baseband processor and microwave switch matrix are presented as well as the antenna coverage pattern.
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  • 79
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    Transmitter pointing loss calculation for free-space optical communications link analyses (1987)
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    Publication Date: 2011-08-19
    Description: In calculating the performance of free-space optical communications links, the transmitter pointing loss is one of the two most important factors. It is shown in this paper that the traditional formula for the instantaneous pointing loss (i.e., for the transmitter telescope far-field beam pattern) is quite inaccurate. A more accurate and practical approximation is developed in which the pointing loss is calculated using a Taylor series approximation. The four-term series is shown to be accurate to 0.1 dB for the theta angles not greater than 0.9 lambda/D (wavelength/telescope diameter).
    Keywords: COMMUNICATIONS AND RADAR
    Type: Applied Optics (ISSN 0003-6935); 26; 2055-205
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  • 80
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    Estimating the arrival times of photon-limited laser pulses in the presence of shot and speckle noise (1987)
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    Publication Date: 2011-08-19
    Description: Maximum-likelihood (ML) receivers are frequently used to optimize the timing performance of laser-ranging and laser-altimetry systems in the presence of shot and speckle noise. Monte Carlo method was used to examine ML-receiver performance with return signals in the 10-5000-photoelectron (pe) range. The simulations were performed for shot noise only and for shot and speckle noise. The results agree with previous theory for signal strengths greater than about 100 pe's but show that the theory can significantly underestimate timing errors for weaker received signals. Sharp high-bandwidth features in the detected signals are shown to improve timing performance only if their signal levels are greater than 4-5 pe's.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Optical Society of America, Journal, A: Optics and Image Science (ISSN 0740-3232); 4; 1080-108
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  • 81
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    Nonuniform sampling techniques for antenna applications (1987)
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    Publication Date: 2011-08-19
    Description: A two-dimensional sampling technique, which can employ irregularly spaced samples (amplitude and phase) in order to generate the complete far-field patterns is presented. The technique implements a matrix inversion algorithm, which depends only on the nonuniform sampled data point locations and with no dependence on the actual field values at these points. A powerful simulation algorithm is presented to allow a real-life simulation of many reflector/feed configurations and to determine the usefulness of the nonuniform sampling technique for the copolar and cross-polar patterns. Additionally, an overlapped window concept and a generalized error simulation model are discussed to identify the stability of the technique for recovering the field data among the nonuniform sampled data. Numerical results are tailored for the pattern reconstruction of a 20-m offset reflector antenna operating at L-band. This reflector is planned to be used in a proposed measurement concept of large antenna aboard the Space Shuttle, whereby it would be almost impractical to accurately control the movement of the Shuttle with respect to the RF source in prescribed directions in order to generate uniform sampled points. Also, application of the nonuniform sampling technique to patterns obtained using near-field measured data is demonstrated. Finally, results of an actual far-field measurement are presented for the construction of patterns of a reflector antenna from a set of nonuniformly distributed measured amplitude and phase data.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-35; 268-279
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  • 82
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    Combined coding and delay-throughput analysis for fading channels of mobile satellite communications (1986)
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    Publication Date: 2011-08-19
    Description: This paper presents the analysis of using the punctured convolutional code with Viterbi decoding to improve communications reliability. The punctured code rate is optimized so that the average delay is minimized. The coding gain in terms of the message delay is also defined. Since using punctured convolutional code with interleaving is still inadequate to combat the severe fading for short packets, the use of multiple copies of assignment and acknowledgment packets is suggested. The performance on the average end-to-end delay of this protocol is analyzed. It is shown that a replication of three copies for both assignment packets and acknowledgment packets is optimum for the cases considered.
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    Laboratory measurements and a theoretical analysis of the TCT fading channel radio system (1987)
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    Publication Date: 2011-08-19
    Description: This paper documents the laboratory and theoretical performance of a pilot-aided digital radio system. The technique considered transmits a midband pilot tone to improve the receiver data detection performance in a multipath fading channel and is referred to as the tone calibrated technique (TCT). The performance of a 860 MHz prototype system carrying 2.4 kbit/s data under Rician fading conditions is reported. Both experimental and analytical results show that the error floor experienced with nonpilot-aided transmission methods is effectively removed by the TCT scheme, resulting in significant performance gains at high signal-to-noise ratio values. The paper also examines the TCT system performance under typical operating conditions and presents a new analysis of the TCT theoretical error probability.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); COM-35; 172-180
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  • 84
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    Channel simulation to facilitate mobile-satellite communications research (1987)
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    Publication Date: 2011-08-19
    Description: The mobile-satellite-service channel simulator, which is a facility for an end-to-end hardware simulation of mobile satellite communications links is discussed. Propagation effects, Doppler, interference, band limiting, satellite nonlinearity, and thermal noise have been incorporated into the simulator. The propagation environment in which the simulator needs to operate and the architecture of the simulator are described. The simulator is composed of: a mobile/fixed transmitter, interference transmitters, a propagation path simulator, a spacecraft, and a fixed/mobile receiver. Data from application experiments conducted with the channel simulator are presented; the noise converison technique to evaluate interference effects, the error floor phenomenon of digital multipath fading links, and the fade margin associated with a noncoherent receiver are examined. Diagrams of the simulator are provided.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); COM-35; 47-56
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  • 85
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    Subreflector extension for improved efficiencies in Cassegrain antennas - GTD/PO analysis (1986)
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    Publication Date: 2011-08-19
    Description: Both offset and symmetric Cassegrain reflector antennas are used in satellite and ground communication systems. It is known that the subreflector diffraction can degrade the performance of these reflectors. A geometrical theory of diffraction/physical optics analysis technique is used to investigate the effects of the extended subreflector, beyond its optical rim, on the reflector efficiency and far-field patterns. Representative numerical results are shown for an offset Cassegrain reflector antenna with different feed illumination tapers and subreflector extensions. It is observed that for subreflector extensions as small as one wavelength, noticeable improvements in the overall efficiencies can be expected. Useful design data are generated for the efficiency curves and far-field patterns.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-34; 1266-126
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  • 86
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    Rate 5/6 Trellis-coded 8-PSK (1986)
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    Publication Date: 2011-08-19
    Description: Some new short constraint length codes are described for satellite channels which are constant-envelope and which have spectral and energy efficiencies intermediate to other recent coded techniques, namely r = 2/3 coded 8-PSK and r = 3/4 coded 16-PSK. Two intervals of 8-PSK transmission are used to form a modulation base for r = 5/6 Trellis codes. A particularly attractive design is an eight-state code, producing 6.2 dB gain on the Gaussian channel over uncoded 8-PSK, yet sacrificing only 16 percent in spectral efficiency.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); Com-34; 1045-104
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    Further results on rate 1/N convolutional code constructions with minimum required SNR criterion (1985)
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    Publication Date: 2011-08-19
    Description: New good (K, 1/N) convolutional codes for 8 or = to K or = to 13 and 2 or = to N or = to 8 were found and tabulated which require minimum signal-to-noise ratio (SNR) for given desired bit error rates (BER) with Viterbi decoding. The transfer function bound was used for the BER evaluations. These low-rate codes are expected to have a number of applications, especially for systems having large bandwidth-bit time products such as deep space and spread spectrum communication systems.
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  • 88
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    Land vehicle antennas for satellite mobile communications (1985)
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    Publication Date: 2011-08-19
    Description: Antenna designs applicable to future satellite mobile vehicle communications are examined. Microstrip disk, quadrifilar helix, cylindrical microstrip, and inverted V and U crossed-dipole low gain antennas (3-5 dBic) that provide omnidirectional coverage are described. Diagrams of medium gain antenna (9-12 dBic) concepts are presented; the antennas are classified into three types: (1) electronically steered with digital phase shifters; (2) electronically switched with switchable power divider/combiner; and (3) mechanically steered with motor. The operating characteristics of a conformal antenna with electronic beam steering and a nonconformal design with mechanical steering are evaluated with respect to isolation levels in a multiple satellite system. Vehicle antenna pointing systems and antenna system costs are investigated.
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  • 89
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    Tropospheric calibration in radio interferometry (1984)
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    Publication Date: 2011-08-19
    Description: A new tropospheric mapping function is derived which is more accurate than previous mapping functions above elevations of 4 deg. The error due to the given analytic aproximation is estimated to be less than 0.2 percent for elevation angles larger than 6 deg (less than 0.4 cm at 6 deg). The mathematical expansion used in the derivation is valid for any laterally homogeneous atmospheric model of refractivity. The new mapping function, computer-generated ray tracing tables, and other mapping functions are compared.
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  • 90
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    Reflector sidelobe degradation due to random surface errors (1986)
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    Publication Date: 2011-08-19
    Description: It is well known that the sidelobe structure of a reflector antenna is highly susceptible to random surface errors, and that in most applications it is not adequate to investigate only the average behavior of the antenna. In this study, an attempt is made to determine the probability distribution of the sidelobe level of a reflector antenna subject to some random surface errors. Specifically, the random pattern function is considered and its sidelobe level studied using the level-upcrossing theory. Both the degradation of the maximum sidelobe and the degradation of the sidelobe region with respect to an International Radio Consultative Committee (CCIR) sidelobe envelope are obtained. The theoretical results are found in excellent agreement with those obtained by Monte Carlo simulations. Finally, some useful tolerance charts are presented.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-34; 164-172
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  • 91
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    Model verifies design of mobile data modem (1986)
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    Publication Date: 2011-08-19
    Description: It has been proposed to use differential minimum shift keying (DMSK) modems in spacecraft-based mobile communications systems. For an employment of these modems, it is necessary that the transmitted carrier frequency be known prior to signal detection. In addition, the time needed by the receiver to lock onto the carrier frequency must be minimized. The present article is concerned with a DMSK modem developed for the Mobile Satellite Service. This device demonstrated fast acquisition time and good performance in the presence of fading. However, certain problems arose in initial attempts to study the acquisition behavior of the AFC loop through breadboard techniques. The development of a software model of the AFC loop is discussed, taking into account two cases which were plotted using the model. Attention is given to a demonstration of the viability of the modem by an approach involving modeling and analysis of the frequency synchronizer.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Microwaves & RF (ISSN 0745-2993); 25; 81
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  • 92
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    Results of a three-year 11.6 GHz low-angle propagation experiment using the SIRIO satellite (1986)
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    Publication Date: 2011-08-19
    Description: The results of three years of attenuation and depolarization measurements at 11.6 GHz on a 10.7 deg elevation angle downlink from the SIRIO spacecraft are summarized. Cumulative distributions and worst-month statistics for rain rate, attenuation, and cross-polarization discrimination (XPD) are presented. Beacon attenuation and radiometric attenuation are compared, and the statistics of fade durations and of rapid XPD changes are discussed.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-34; 58-65
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  • 93
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    Performance of a class of parallel spread-spectrum code acquisition schemes in the presence of data modulation (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: An acquisition procedure for data-modulated direct-sequence spread-spectrum systems is investigated. The correlation time is partitioned into subintervals, and the integration results in these subintervals are noncoherently combined for detection. The tradeoff between noncoherent combining loss and data modulation degradation guides the optimum choice of the number of subintervals. Two forms of data modulation are considered, namely, the alternate-data and random-data cases. The parallel acquisition schemes discussed allow multiple code-phase offsets to be examined at each test. The circular state diagram approach is used to analyze the performances of these schemes. The theory presented is valid for a class of such parallel schemes.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); 36; 596-604
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  • 94
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    Jacobi-Bessel analysis of reflector antennas with elliptical apertures (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Although many reflector antennas possess circular projected apertures, there are recent satellite and ground antenna applications for which it is desirable to employ reflectors with elliptical apertures. Here a modification of the Jacobi-Bessel expansion is presented for the diffraction analysis of reflectors with elliptical apertures. A comparative study is also performed between this modified Jacobi-Bessel algorithm and the one which uses the Jacobi-Bessel expansion over a circumscribing circular region. Numerical results are presented for offset reflectors with elliptical and circular apertures and the improved convergence properties of the modified algorithm are highlighted.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-35; 1070-107
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  • 95
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    Evaluating roughness models of radar backscatter (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Three radar backscatter roughness models were assessed using soil moisture data collected by the Space Shuttle flight 41G SIR-B SAR in an intensively farmed area. The SIR-B data swath included a large number of bare, dry fields with a large variety of surface roughnesses. The small perturbation model gives the best results, particularly when fields with a definite periodic row structure were omitted. The standard deviation of surface heights appears to be a good measure of relative roughness conditions, but the correlation length is not a good descriptor of the surface, and does not seem to be related in any way to the measured backscatter.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); GE-25; 709-713
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  • 96
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    Performance analysis of replication ALOHA for fading mobile communications channels (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper describes an ALOHA random access protocol for fading communications channels. A two-state Markov model is used for the channel error process to account for the channel fading memory. The ALOHA protocol is modified to send multiple contiguous copies of a message at each transmission attempt. Both pure and slotted ALOHA channels are considered. The analysis is applicable to fading environments where the channel memory is short compared to the propagation delay. It is shown that smaller delay may be achieved using replications and, in noisy conditions, can also improve throughput.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); COM-34; 1256-125
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  • 97
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    Roundoff noise analysis for digital signal power processors using Welch's power spectrum estimation (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The noise due to finite-word-length effects is analyzed for digital-signal power processors using Welch's power-spectrum estimation technique to measure the power of Gaussian random signals over a frequency band of interest. The input of the digital signal processor contains a finite-length time interval in which the true Gaussian signal is contaminated by Gaussian noise. The roundoff noise-to-signal ratio in the measurement of the signal power is derived, and computer simulations which validate the analytical results are presented. These results can be used in tradeoff studies of hardware design, such as the number of bits required at each processing stage. The results presented in this paper are currently being used in the design of a digital Doppler processor (Chi et al., 1986) for a radar scatterometer.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Acoustics, Speech, and Signal Processing (ISSN 0096-3518); ASSP-35; 784-795
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  • 98
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    Fraunhofer filters to reduce solar background for optical communications (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A wavelength that lies within a spectral interval of reduced solar emission (a Fraunhofer line) can carry optical communications with reduced interference from direct or reflected background sunlight. Suitable Fraunhofer lines are located within the tuning range of good candidate lasers. The laser should be tunable dynamically to track Doppler shifts in the sunlight incident on any solar system body that may appear in the background as viewed by the receiver. A Fraunhofer filter used with a direct-detection receiver should be tuned to match the Doppler shifts of the source and background. The required tuning calculated here for various situations is also required if, instead, one uses a heterodyne receiver with limited post-detection bandwidth.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Optical Engineering (ISSN 0091-3286); 28; 963-968
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  • 99
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    Image understanding and interpretation in microwave diversity imaging (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The authors investigate microwave imaging of metallic objects using a diversity method and interpret and predict the reconstructed image from an approach based on analysis of the scattering mechanism and a back-projection algorithm used in image retrieval. The connection between the various scattering mechanisms and the reconstructed images is discussed, what the images represent is interpreted, and a prediction is made as to what the image will look like over given spectral and angular windows. A brief description is given of the microwave diversity imaging system and the formulation of the microwave diversity imaging based on the physical optics approximation. The scattering mechanism of a complex shaped metallic object is then briefly reviewed and an alternate approach to interpreting the reconstructed image based on the understanding of the scattering mechanism and the reconstruction algorithm is given. Several numerical and experimental examples are included to support this interpretation approach.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 37; 1048-105
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  • 100
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    A study of three techniques used in the diffraction analysis of shaped dual-reflector antennas (1989)
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    Publication Date: 2011-08-19
    Description: An examination is presented of three techniques used for the efficient computation of fields diffracted by a subreflector that has been shaped by geometrical optics synthesis. It is found that these techniques, which are based on the geometrical theory of diffraction (GTD), produce errors in the computed fields that are specific to shaped reflectors. These errors are examined for a reflector system shaped to produce maximum gain from a tapered feed illumination. The discrepancies are directly related to the caustic being located near an observation point of the GTD calculations. The errors found are localized, and they increase in magnitude as the caustic approaches the main reflector. In a general offset geometry, the location of the caustic may be located arbitrarily close to the main reflector given a prescribed output aperture distribution. For the specific case considered here-the common situation of shaping to produce maximum gain-the caustic is located near the edge of the main reflector and on the reflection shadow boundary. A local correction is derived which creates a uniform solution through the caustic and across the reflection shadow boundary. Away from this point the calculation receeds to the standard GTD solution.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 37; 979-983
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