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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19

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NASA NTRS1980

Shuttle orbiter Ku-band radar/communications system design evaluation. Ku band bent-pipe channel performance evaluation

Because of difficulties with the bit detector of the SPA mode 1 channel 3 input port, a new bit synchronizer was required. The two prime candidate designs are described and analyzed. The selected design is a modification of one which utilizes a phase frequency detector to track the received data clock frequency and a mid-bit transition point sample detector to generate a bit timing error (phase error) signal to control the relative phase between the local clock and the local data stream. The model used to calculate the effects of cable attenuation and rise time degradation is discussed.

Holmes, J. K.↗

NASA NTRS1980

Stereosat - A new astrodynamics challenge

The paper investigates the astrodynamics requirements for a spacecraft which produces stereo images of the land masses of the earth for use in geoscience investigations, cartography and earth resource exploration. It describes a candidate Stereosat imaging product, presents criteria for evaluating potential image quality and usability, and assesses the impact of various spacecraft and astrodynamic factors on image quality. Several challenges are identified and discussed, namely (1) securing adequate image quality for pixel level elevation measurements, (2) determining absolute location of imaged phenomena in remote regions, (3) satisfying the long-term orbit prediction requirements, and (4) transmitting accurate orbit data along with the image data stream. Meeting these challenges will greatly enhance operational facility.

Driver, J. M.↗

NASA NTRS1979

Performance of unbalanced QPSK in the presence of noisy reference and crosstalk

The problem of transmitting two telemetry data streams having different rates and different powers using unbalanced quadriphase shift keying (UQPSK) signaling is considered. It is noted that the presence of a noisy carrier phase reference causes a degradation in detection performance in coherent communications systems and that imperfect carrier synchronization not only attenuates the main demodulated signal voltage in UQPSK but also produces interchannel interference (crosstalk) which degrades the performance still further. Exact analytical expressions for symbol error probability of UQPSK in the presence of noise phase reference are derived.

Divsalar, D.↗

NASA NTRS1981

A bit transition density encoder for the Space Shuttle 2 MHz data channel

As a result of several unique factors, the 2 MHz data channel which carries science data from the various experiments aboard the Space Shuttle through the Tracking and Data Relay Satellite System (TDRSS) lacks sufficient bit transition density to satisfy the bit synchronizer requirements at the ground station. It is noted that six encoding techniques were examined, and all but one, a PN cover sequence (Reset Bit Scrambler) were dismissed because of incompatibility with system constraints. The rationale behind the particular PN sequence chosen, the way it was truncated, and the new sequence properties are presented. The properties of the raw data stream are analyzed, as are the encoded sequence properties, including transition density. Diagrams of the encoder and decoder are included.

Schoggen, W. O.↗

NASA NTRS1980

Performance of convolutionally coded unbalanced QPSK systems

An evaluation is presented of the performance of three representative convolutionally coded unbalanced quadri-phase-shift-keying (UQPSK) systems in the presence of noisy carrier reference and crosstalk. The use of a coded UQPSK system for transmitting two telemetry data streams with different rates and different powers has been proposed for the Venus Orbiting Imaging Radar mission. Analytical expressions for bit error rates in the presence of a noisy carrier phase reference are derived for three representative cases: (1) I and Q channels are coded independently; (2) I channel is coded, Q channel is uncoded; and (3) I and Q channels are coded by a common 1/2 code. For rate 1/2 convolutional codes, QPSK modulation can be used to reduce the bandwidth requirement.

Divsalar, D.↗

NASA NTRS1983

Node Synchronization for the Viterbi Decoder

At very low signal to noise ratios such as those that Voyager 2 will encounter at Uranus, the performance of the Reed-Solomon/Viterbi concatenated coding system could be seriously degraded by loss of node synchronization by the Viterbi decoder. This problem is described and now it can be almost completely avoided with a simple outboard hardware ""node synchronizer''. This device makes statistical decisions about node sync based on the hard quantized undecoded data stream. In a worst case Voyager-like environment, our method will detect and correct a true loss of node sync (thought to be a very rate event) within several hundred bits; many of these false alarms for our technique is on the order several years.

Lorden, G.↗

NASA NTRS1983

Laboratory system for demonstrating spacecraft processing of multispectral image data

The various elements of the Information Adaptive System (IAS) program are discussed along with the design features, approaches, and algorithms employed within the system. This program was initiated by NASA in order to develop an adaptable data processing system which will accommodate high speed multispectral image data in the real-time spacecraft environment. The development of this system is examined, including the test and support equipment required to evaluate and demonstrate the operational performance of the system in terms of the applicability of the design to high speed onboard processing. In addition to a high throughput rate capability, this system will be flexible, programmable, and capable of adapting its operation to external commands and characteristics of the data stream.

Meredith, B. D.↗

NASA NTRS1983

Design and performance of the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument

Design and performance data are analyzed for the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument, which has been developed for the NASA Earth Radiation Budget Satellite (ERBS). SAGE II is designed to monitor globally, from 70 degrees S to 70 degrees N latitude, the vertical distribution of stratospheric aerosols, ozone, water vapor, and nitrogen dioxide by measuring the extinction of solar radiation through the earth's atmosphere during the ERBS observatory solar occultations. A flat scanning mirror reflects solar radiation into a Cassegrain type telescope, which forms a solar image on the entrance slit of a grating spectrometer. The instantaneous-field-of-view of the SAGE II is scanned along the vertical solar diameter by the elevation scan mirror. The optical system is contained within an azimuth gimbal which tracks the solar radiometric centroid during the data event, while the spectrometer isolates seven spectral wavelengths from 0.385-1.02 micrometers. The seven channels of the spectrometer use silicon photodiode dedectors operated in the photovoltaic mode. The detector outputs are multiplexed into a serial data stream for readout by the ERBS telemetry system, with each output being sampled 64 times per second and digitized to 12 bit resolution.

Zaun, N. H.↗

NASA NTRS1982

Imaging spectrometer - An advanced multispectral imaging concept

The concept of an imaging spectrometer, which is being studied as a potential Space Shuttle experiment, is evaluated as a 'push-broom' imager that includes a spectrometer to disperse each line of imaging information into its spectral components. Using this instrument, the dispersed energy falls upon a two-dimensional focal plane array that detects both spatial and spectral information. As the line field of view is advanced over the earth by the motion of the spacecraft, the focal plane is read out constantly, which produces 'push-broom' images at multiple wavelengths. Ground instantaneous fields of view of 10 m in the visual and 20 m in the infrared are provided by the system, at a spectral resolution of 20 nm over the range from 0.4-2.5 microns. The system utilizes a triple-pass Schmidt optical system with a mosaic focal plane. A subset of the data stream is selected and encoded for transmission by the use of onboard processing.

Wellman, J. B.↗

NASA NTRS1985

Convolutionally-Coded Unbalanced QPSK Systems

Report discusses error-rate performance for three convolutionallycoded unbalanced quadriphase-shift-keying (UQPSK) communication systems with noisy carriers that introduce crosstalk. Systems analyzed unbalanced in sense that each transmits two data streams with different bit rates and (in some cases) different powers.

Divsalar, D.↗

NASA NTRS1984

Effects of NRZ-M Modulation on Convolutional Codes Performance

Non-return-to-zero mark (NRZ-M) modulation is often used to resolve data sense in suppressed carrier telemetry systems because such systems are subject to half cycle slips that result in complementing the encoded data stream. The performance of coded telemetry systems with NRZ-M is sensitive to the order in which the various operations are done. A system that demodulates the NRZ-M waveform and then decodes performs differently from a system that does the decoding first. The performance of the NASA standard (7, 1/2) convolutional codes is determined for several systems using NRZ-M. Several different demodulation schemes for NRZ-M are considered. It is shown that, even for the best soft-decision method examined, there is a 2.7 dB loss at a decoded bit error rate of 0.005 if the NRZ-M demodulation occurs before rather than after Viterbi decoding.

Deutsch, L. J.↗

NASA NTRS1984

The effect of radiation on the IRAS all-sky survey

The Infrared Astronomical Satellite (IRAS) is in a sun synchronous, 'near' polar orbit at an altitude of 900 km. The primary objective of IRAS is related to the conduction of an all-sky survey in the wavelength range from 8 microns to 120 microns. The present investigation is concerned with three components of the radiation environment encountered by IRAS, taking into account the high energy protons in the South Atlantic Anomaly, high energy electrons in the horns of the Van Allen belts, and cosmic rays. The effect of radiation on the returned data stream is studied, and attention is given to the steps which were taken to minimize the impact of radiation on the completeness of the survey.

Wolff, D. M.↗

NASA NTRS1984

Node synchronization for the Viterbi decoder

At very low signal to noise ratios such as those that Voyager 2 will encounter at Uranus, the performance of the Reed-Solomon/Viterbi concatenated coding system could be seriously degraded by loss of node synchronization by the Viterbi decoder. This problem is described and now it can be almost completely avoided with a simple outboard hardware 'node synchronizer'. This device makes statistical decisions about node sync based on the hard quantized undecoded data stream. In a worst case Voyager-like environment, our method will detect and correct a true loss of node sync (thought to be a very rare event) within several hundred bits; many of these false alarms for our technique is on the order several years. Previously announced in STAR as N83-28028

Lorden, G.↗

NASA NTRS1986

Systolic VLSI Reed-Solomon Decoder

Decoder for digital communications provides high-speed, pipelined ReedSolomon (RS) error-correction decoding of data streams. Principal new feature of proposed decoder is modification of Euclid greatest-common-divisor algorithm to avoid need for time-consuming computations of inverse of certain Galois-field quantities. Decoder architecture suitable for implementation on very-large-scale integrated (VLSI) chips with negative-channel metaloxide/silicon circuitry.

Shao, H. M.↗

NASA NTRS1986

Picture-Element Comparator

Proposed circuit, called "neighborhood comparison operator," compares data from neighboring picture elements (pixels) to find peaks, ridges, and valleys in picture data. Circuit also able to expand or shrink pixel regions. Circuit concept developed for image-processing computers. Circuit handles data stream of 12-bit pixels rather than conventional 8 or 16 bits. Consist entirely of standard logic chips.

Gennery, D. B.↗

NASA NTRS1985

Study of application of space telescope science operations software for SIRTF use

The design and development of the Space Telescope Science Operations Ground System (ST SOGS) was evaluated to compile a history of lessons learned that would benefit NASA's Space Infrared Telescope Facility (SIRTF). Forty-nine specific recommendations resulted and were categorized as follows: (1) requirements: a discussion of the content, timeliness and proper allocation of the system and segment requirements and the resulting impact on SOGS development; (2) science instruments: a consideration of the impact of the Science Instrument design and data streams on SOGS software; and (3) contract phasing: an analysis of the impact of beginning the various ST program segments at different times. Approximately half of the software design and source code might be useable for SIRTF. Transportability of this software requires, at minimum, a compatible DEC VAX-based architecture and VMS operating system, system support software similar to that developed for SOGS, and continued evolution of the SIRTF operations concept and requirements such that they remain compatible with ST SOGS operation.

Dignam, F.↗

NASA NTRS1983

An imaging spectrometer experiment for the Shuttle

An imaging spectrometer experiment concept for earth remote sensing, developed as part of NASA's Multispectral Linear Array program, will map a series of test sites at high spatial and spectral resolution from the vantage point of the Shuttle payload bay. The instrument covers the spectral range from 0.4 to 2.5 micrometers with a sampling interval of 10 nanometers in the visible and near infrared (to 1.0 micrometer) and 20 nanometers in the short wavelength infrared (1.0 to 2.5 micrometers). Resolution corresponding to a ground instantaneous field of view (pixel size) of 30 meters is provided over a swath width of 12 kilometers. On-board data editing is utilized to select a subset of the data stream for transmission to the ground. The instrument utilizes silicon and mercury cadmium telluride area array detectors. Pointing mirrors are included to permit specific test sites to be imaged from the Shuttle orbit.

Wellman, J. B.↗

NASA NTRS1986

Usuda Deep Space Center support for ICE

The planning, implementation and operations that took place to enable the Usuda, Japan, Deep Space Center to support the International Cometary Explorer (ICE) mission are summarized. The results show that even on very short notification our two countries can provide mutual support to help ensure mission success. The data recovery at the Usuda Deep Space Center contributed significantly to providing the required continuity of the experimental data stream at the encounter of the Comet Giacobini-Zinner.

Goodwin, J. P.↗