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At least 847 records · Page 47

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NASA NTRS1986

Error control techniques for satellite and space communications

The performance anlaysis of NASA's telecommand systems was summarized. It is assumed that the decoded frames are scrambled prior to decoding by the outer code. The average decoded bit error rate are examined which allows the possibility of evaluating many different frame coding options. The capacity and cutoff rate of the outer channel formed by the combination of the actual physical channel and the inner encoder and decoder in a concatenated coding system were analyzed. The best combination of inner and outer codes to use in a concatenated coding system were determined. It was established that in general: (1) it is better not to interleave between the inner and outer codes; and (2) for a fixed overall code rate, it is better to use higher rate inner codes and lower rate outer codes. Inner convolutional codes are considered. The analysis is more difficult in this case because the inner decoder error events do not appear in blocks of fixed length but can be of many different lengths.

Costello, D. J., Jr.↗

NASA NTRS1986

The concept of the Pinhole/Occulter Facility

The Pinhole/Occulter Facility is based on a simple idea for obtaining high angular resolution in astronomical X-ray observations, for example for solar flares at energies 10 keV. The scheme uses a coded aperture device (multiple pinhole camera) with a large separation between the aperture encoder and the detector. Such an imaging device can have an angular resolution much better than 1 arc s if desired. A large structure would also make it possible to have a large external occulter, which would have powerful applications, notably for high-sensitivity observations of the corona in EUV and white light. This capability leads to the definition of the Pinhole/Occulter Facility, which combines both X-ray and coronal observations. The present concept is based on a 35-m deployable boom, erected in the cargo bay of the Space Shuttle and pointed in the solar direction by the Instrument Pointing System of Spacelab.

Hudson, H. S.↗

NASA NTRS1986

Sensitivity analysis for solar plates

Economic evaluation methods and analyses of emerging photovoltaic (PV) technology since 1976 was prepared. This type of analysis was applied to the silicon research portion of the PV Program in order to determine the importance of this research effort in relationship to the successful development of commercial PV systems. All four generic types of PV that use silicon were addressed: crystal ingots grown either by the Czochralski method or an ingot casting method; ribbons pulled directly from molten silicon; an amorphous silicon thin film; and use of high concentration lenses. Three technologies were analyzed: the Union Carbide fluidized bed reactor process, the Hemlock process, and the Union Carbide Komatsu process. The major components of each process were assessed in terms of the costs of capital equipment, labor, materials, and utilities. These assessments were encoded as the probabilities assigned by experts for achieving various cost values or production rates.

Aster, R. W.↗

NASA NTRS1986

Isotopic, petrologic and biogeochemical investigations of banded iron-formations

It is recognized that the first occurrence of banded iron-formations (BIFs) clearly predates biological oxygenation of the atmosphere-hydrosphere system and that their last occurrences extend beyond plausible dates of pervasive biological oxygenation. For this reason, and because enormous quantities of oxidizing power have been sequestered in them, it is widely thought that these massive, but enigmatic, sediments must encode information about the mechanism and timing of the rise of atmospheric O2. By coupling isotopic analyses of iron-formation carbonates with biogeochemical and petrologic investigations, we are studying (1) the mechanism of initial sedimentation of iron; (2) the role of iron in microbially mediated diagenetic processes in fresh iron-formation sediments; and (3) the logical integration of mechanisms of deposition with observed levels of banding. Thus far, it has been shown that (1) carbonates in BIFs of the Hamersley Group of Western Australia are isotopically inhomogenous; (2) the nature and pattern of isotopic ordering is not consistent with a metamorphic origin for the overall depletion of C-13 observed in the carbonates; (3) if biological, the origin of the C-13 depleted carbonate could be either respiratory or fermentative; (4) iron may have been precipitate d as Fe(3+), then reduced to Fe(2+) within the sediment; and (5) sedimentary biogeochemical systems may have been at least partially closed to mass transport of carbonate species.

Hayes, J. M.↗

NASA NTRS1986

Parallel structures in human and computer memory

If one thinks of our experiences as being recorded continuously on film, then human memory can be compared to a film library that is indexed by the contents of the film strips stored in it. Moreover, approximate retrieval cues suffice to retrieve information stored in this library. One recognizes a familiar person in a fuzzy photograph or a familiar tune played on a strange instrument. A computer memory that would allow a computer to recognize patterns and to recall sequences the way humans do is constructed. Such a memory is remarkably similiar in structure to a conventional computer memory and also to the neural circuits in the cortex of the cerebellum of the human brain. It is concluded that the frame problem of artificial intelligence could be solved by the use of such a memory if one were able to encode information about the world properly.

Kanerva, P.↗

NASA NTRS1986

Advanced optical measuring systems for measuring the properties of fluids and structures

Four advanced optical models are reviewed for the measurement of visualization of flow and structural properties. Double-exposure, diffuse-illumination, holographic interferometry can be used for three-dimensional flow visualization. When this method is combined with optical heterodyning, precise measurements of structural displacements or fluid density are possible. Time-average holography is well known as a method for displaying vibrational mode shapes, but it also can be used for flow visualization and flow measurements. Deflectometry is used to measure or visualize the deflection of light rays from collimation. Said deflection occurs because of refraction in a fluid or because of reflection from a tilted surface. The moire technique for deflectometry, when combined with optical heterodyning, permits very precise measurements of these quantities. The rainbow schlieren method of deflectometry allows varying deflection angles to be encoded with colors for visualization.

Decker, A. J.↗

NASA NTRS1985

Voyager-Uranus at our doorstep

Following the encounter of Voyager 2 with Saturn and the subsequent approach, in January 1986, to the planet Uranus, the NASA Deep Space Network (DSN) undertook a reshaping of Voyager's end-to-end data system to provide communication to earth with a sufficient number of images for Uranus characterization. In-flight evolution of the spacecraft and ground system is described, with particular attention given to the aspects of target motion compensation, line enhancement, arraying, and encoding data streams. Solutions to the on-board problems encountered during the flight are presented. Voyager and its principal elements, an actuator for the scan platform, and the coding diagram are among the illustrations included.

Laeser, R. P.↗

NASA NTRS1986

Wavelength-division multiplexed digital optical position transducer

A novel concept for a digital optical position tranducer is presented. This compact and rugged device is electrically passive and requires only a dual-fiber optical link. Wavelength division of a broad-spectrum semiconductor light source is employed to multiplex the channels of a 10-bit digital position encoder. A preliminary design was evaluated and found to have a 10-dB on-off contrast ratio and a 12-dB insertion loss.

Fritsch, K.↗

NASA NTRS1985

Forest discrimination with multipolarization imaging radar

The relations between polarization signatures and biophysical characteristics through a range of different forest environments were investigated using airborne synthetic-aperture (SAR) images acquired at L-band on March 1, 1984 in South Carolina. SAR data acquired in four linear polarization states with 10-m spatial resolution were encoded as color composite images and compared to US Forest Service forest stand data. The most useful correlative forest data were stand basal area, forest age, site condition index, and forest management type. It is found that the multipolarization images discriminate variation in tree density or difference in the amount of understory, but no evidence has been found for discrimination between evergreen and deciduous forest types.

Ford, J. P.↗

NASA NTRS1985

A 250-Mbit/s ring local computer network using 1.3-microns single-mode optical fibers

A 250-Mbit/s three-station fiber-optic ring local computer network was built and successfully demonstrated. A conventional token protocol was employed for bus arbitration to maximize the bus efficiency under high loading conditions, and a non-return-to-zero (NRS) data encoding format was selected for simplicity and maximum utilization of the ECL-circuit bandwidth.

Eng, S. T.↗

NASA NTRS1985

Gain-adaptive vector quantization for medium-rate speech coding

A class of adaptive vector quantizers (VQs) that can dynamically adjust the 'gain' of codevectors according to the input signal level is introduced. The encoder uses a gain estimator to determine a suitable normalization of each input vector prior to VQ coding. The normalized vectors have reduced dynamic range and can then be more efficiently coded. At the receiver, the VQ decoder output is multiplied by the estimated gain. Both forward and backward adaptation are considered and several different gain estimators are compared and evaluated. An approach to optimizing the design of gain estimators is introduced. Some of the more obvious techniques for achieving gain adaptation are substantially less effective than the use of optimized gain estimators. A novel design technique that is needed to generate the appropriate gain-normalized codebook for the vector quantizer is introduced. Experimental results show that a significant gain in segmental SNR can be obtained over nonadaptive VQ with a negligible increase in complexity.

Chen, J.-H.↗

NASA NTRS1986

An entropy maximization problem related to optical communication

In relation to a problem in optical communication, the paper considers the general problem of maximizing the entropy of a stationary radom process that is subject to an average transition cost constraint. By using a recent result of Justesen and Hoholdt, an exact solution to the problem is presented and a class of finite state encoders that give a good approximation to the exact solution is suggested.

Mceliece, R. J.↗

NASA NTRS1986

Development of an expert system prototype for determining software functional requirements for command management activities at NASA Goddard

The development of an expert system prototype for software functional requirement determination for NASA Goddard's Command Management System, as part of its process of transforming general requests into specific near-earth satellite commands, is described. The present knowledge base was formulated through interactions with domain experts, and was then linked to the existing Knowledge Engineering Systems (KES) expert system application generator. Steps in the knowledge-base development include problem-oriented attribute hierarchy development, knowledge management approach determination, and knowledge base encoding. The KES Parser and Inspector, in addition to backcasting and analogical mapping, were used to validate the expert system-derived requirements for one of the major functions of a spacecraft, the solar Maximum Mission. Knowledge refinement, evaluation, and implementation procedures of the expert system were then accomplished.

Liebowitz, J.↗

NASA NTRS1986

Catching the whispers from Uranus

Sophisticated telecommunications techniques are described that were used to acquire images of Uranus, its 14 moons and ten narrow rings darker than coal. The images, equal in quality to those transmitted from Saturn several years earlier despite the signal being weaker by 6 dB due to the increased distance, were received from Voyager 2 during its January 24, 1986 flyby of Uranus. Solutions to the problem of the weakening signal were found in modifications to Voyager's image processing system and NASA's ground tracking network. In April 1985, Voyager's prime flight data computer was reconfigured to accept only nonimaging science data, and its backup, only imaging data; the latter was reprogrammed to determine only arithmetic differences between adjacent pixel intensities rather than absolute intensities. By image compression, equivalent imaging information could be sent at lower bit rates. Instead of Golay coding, Reed-Solomon onboard encoding was used. These techniques gained the equivalent of 4-dB in imaging yield. Additional improvements were gained by using earth station antennas in pairs (the Parkes radio telescope and the Canberra ground station antenna). Moves under way to prepare for the Voyager encounter with Neptune in 1989 are described (using additional antennas and arrays, scaling up the Deep Space Network antennas from 64 m to 70 m, etc.) to assure almost Saturn-equivalent pictures despite a further 3.5-dB drop in signal strength.

Bartok, C. D.↗

NASA NTRS1987

Theory of Periodic-Binary-Sequence Generators

Algorithms yield feedback shift registers with maximum regularity. Report provides extensive mathematical treatment of new and previous results related to generation of pseudo-noise binary sequences by feedback shift registers. Generator architectures amenable to efficient implementation in very-large-scale integrated (VLSI) circuits. Report includes literature references to applications of such sequences in random-number generation, radar, VLSI testing, data encryption and decryption, algebraic error-detection and error-correction encoding and decoding, and feedback-shift-register synthesis of sequential machines.

Perlman, M.↗

NASA NTRS1987

Slot-Height Measuring System

Height displayed as function of position. Slot-depth measuring system uses spring-loaded transducer to measure slot depth and wheel-driven rotary optical encoder to measure position along slot. An x-y recorder plots depth vs. position.

Clarke, Alan D.↗

NASA NTRS1987

Algorithm for Fluid Networks

Algorithm performs either transient integration or steady-state relaxation of network that represents internal fluid system. Valid for both single and two-phase flow of any fluid and includes both heat-transfer and work terms. General enough to simulate almost any flow system to almost any degree of detail desired. Encoded into computer program FLUINT, will work in conjunction with SINDA-85 program to enable analysis of pipelines; heating, ventilating, and cooling systems; and almost any other system that involves both fluid flow and heat transfer. Other possible extensions includes effects of flexibilities of containers, high-velocity flow, gravitation, acceleration, and opening and closing valves.

Cullimore, Brent A.↗

NASA NTRS1986

Pulse Code Modulation (PCM) data storage and analysis using a microcomputer

The current widespread use of microcomputers has led to the creation of some very low-cost instrumentation. A Pulse Code Modulation (PCM) storage device/data analyzer -- a peripheral plug-in board especially constructed to enable a personal computer to store and analyze data from a PCM source -- was designed and built for use on the NASA Sounding Rocket Program for PMC encoder configuration and testing. This board and custom-written software turns a computer into a snapshot PCM decommutator which will accept and store many hundreds or thousands of PCM telemetry data frames, then sift through them repeatedly. These data can be converted to any number base and displayed, examined for any bit dropouts or changes (in particular, words or frames), graphically plotted, or statistically analyzed.

Massey, D. E.↗