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NASA NTRS1990

On-board switching and processing

Findings from NASA's space electronics division's (SED's) advanced systems studies related to future communications satellite services that will require onboard switching and processing technology are reviewed. SED's digital signal switching and processing technology program is reviewed. This program responds to specific systems technology development needs for enabling commercial development of future satellite services. The technologies include: modulators, demodulators, and forward error-correction hardware for space- and ground-based applications; onboard information switching and processing, onboard network control, and health monitoring; and cost-efficient ground terminals. The in-house systems integration, test, and evaluation (SITE) project, which includes a laboratory testbed for evaluating technology in a simulated systems environment, is reviewed.

Harrold, Joseph L.↗

NASA NTRS1990

On complexity of trellis structure of linear block codes

The trellis structure of linear block codes (LBCs) is discussed. The state and branch complexities of a trellis diagram (TD) for a LBC is investigated. The TD with the minimum number of states is said to be minimal. The branch complexity of a minimal TD for a LBC is expressed in terms of the dimensions of specific subcodes of the given code. Then upper and lower bounds are derived on the number of states of a minimal TD for a LBC, and it is shown that a cyclic (or shortened cyclic) code is the worst in terms of the state complexity among the LBCs of the same length and dimension. Furthermore, it is shown that the structural complexity of a minimal TD for a LBC depends on the order of its bit positions. This fact suggests that an appropriate permutation of the bit positions of a code may result in an equivalent code with a much simpler minimal TD. Boolean polynomial representation of codewords of a LBC is also considered. This representation helps in study of the trellis structure of the code. Boolean polynomial representation of a code is applied to construct its minimal TD. Particularly, the construction of minimal trellises for Reed-Muller codes and the extended and permuted binary primitive BCH codes which contain Reed-Muller as subcodes is emphasized. Finally, the structural complexity of minimal trellises for the extended and permuted, and double-error-correcting BCH codes is analyzed and presented. It is shown that these codes have relatively simple trellis structure and hence can be decoded with the Viterbi decoding algorithm.

Lin, Shu↗

NASA NTRS1990

Trellis coded modulation for transmission over fading mobile satellite channel

The combination of trellis coding and multiple phase-shift keyed (MPSK) signaling with asymmetry (nonuniform spacing) to the signal set is disclosed with regard to its suitability for a fading mobile satellite communication channel. For MPSK signaling, introducing nonuniformity in the phase spacing between signal points provides an improvement in performance over that achievable with trellis codes symmetric MPSK signaling, all this without increasing the average or peak power, or changing the bandwidth constraints imposed on the system. Block interleaving may be used to reduce error and pilot tone(s) may be used for improving the error correction performance of the trellis decoder in the presence of channel fading.

Simon, Marvin K.↗

NASA NTRS1990

Predicting thunderstorm evolution using ground-based lightning detection networks

Lightning measurements acquired principally by a ground-based network of magnetic direction finders are used to diagnose and predict the existence, temporal evolution, and decay of thunderstorms over a wide range of space and time scales extending over four orders of magnitude. The non-linear growth and decay of thunderstorms and their accompanying cloud-to-ground lightning activity is described by the three parameter logistic growth model. The growth rate is shown to be a function of the storm size and duration, and the limiting value of the total lightning activity is related to the available energy in the environment. A new technique is described for removing systematic bearing errors from direction finder data where radar echoes are used to constrain site error correction and optimization (best point estimate) algorithms. A nearest neighbor pattern recognition algorithm is employed to cluster the discrete lightning discharges into storm cells and the advantages and limitations of different clustering strategies for storm identification and tracking are examined.

Goodman, Steven J.↗

NASA NTRS1991

Initial studies of array feeds for the 70-meter antenna at 32 GHz

The results of a study to determine the feasibility of using array feed techniques to improve the performance of the 70 m antenna at 32 GHz are presented. Changing from 8.4 to 32 GHz has the potential of increasing the gain by 11.6 dB, but recent measurements indicate that additional losses of from 3 to 7 dB occur at 32 GHz, depending on the elevation angle. Array feeds were proposed to recover some of the losses by compensating for surface distortions that contribute to these losses. Results for both surface distortion compensation and pointing error correction are discussed. These initial studies, however, had one significant restriction: The mechanical finite element model was used to characterize the surface distortions, not the measured distortions from three angle holography data, which would be more representative of the actual antenna. Further work is required to provide for a more accurate estimate of performance that utilizes holography data and, in particular, one that evaluates the performance in the focal plane region of the antenna.

Cramer, P. W.↗

NASA NTRS1990

Automatic Speech Recognition in Air Traffic Control: a Human Factors Perspective

The introduction of Automatic Speech Recognition (ASR) technology into the Air Traffic Control (ATC) system has the potential to improve overall safety and efficiency. However, because ASR technology is inherently a part of the man-machine interface between the user and the system, the human factors issues involved must be addressed. Here, some of the human factors problems are identified and related methods of investigation are presented. Research at M.I.T.'s Flight Transportation Laboratory is being conducted from a human factors perspective, focusing on intelligent parser design, presentation of feedback, error correction strategy design, and optimal choice of input modalities.

Karlsson, Joakim↗

NASA NTRS1991

Testing interconnected VLSI circuits in the Big Viterbi Decoder

The Big Viterbi Decoder (BVD) is a powerful error-correcting hardware device for the Deep Space Network (DSN), in support of the Galileo and Comet Rendezvous Asteroid Flyby (CRAF)/Cassini Missions. Recently, a prototype was completed and run successfully at 400,000 or more decoded bits per second. This prototype is a complex digital system whose core arithmetic unit consists of 256 identical very large scale integration (VLSI) gate-array chips, 16 on each of 16 identical boards which are connected through a 28-layer, printed-circuit backplane using 4416 wires. Special techniques were developed for debugging, testing, and locating faults inside individual chips, on boards, and within the entire decoder. The methods are based upon hierarchical structure in the decoder, and require that chips or boards be wired themselves as Viterbi decoders. The basic procedure consists of sending a small set of known, very noisy channel symbols through a decoder, and matching observables against values computed by a software simulation. Also, tests were devised for finding open and short-circuited wires which connect VLSI chips on the boards and through the backplane.

Onyszchuk, I. M.↗

NASA NTRS1990

Telemetry advances in data compression and channel coding

Addressed in this paper is the dependence of telecommunication channel, forward error correcting coding and source data compression coding on integrated circuit technology. Emphasis is placed on real time high speed Reed Solomon (RS) decoding using full custom VLSI technology. Performance curves of NASA's standard channel coder and a proposed standard lossless data compression coder are presented.

Miller, Warner H.↗

NASA NTRS1990

Worldwide differential GPS for Space Shuttle landing operations

Worldwide differential Global Positioning System (WWDGPS) is viewed as an effective method of offering continuous high-quality navigation worldwide. The concept utilizes a network with as few as 33 ground stations to observe most of the error sources of GPS and provide error corrections to users on a worldwide basis. The WWDGPS real-time GPS tracking concept promises a threefold or fourfold improvement in accuracy for authorized dual-frequency users, and in addition maintains an accurate and current ionosphere model for single-frequency users. A real-time global tracking network also has the potential to reverse declarations of poor health on marginal satellites, increasing the number of satellites in the constellation and lessening the probability of GPS navigation outage. For Space Shuttle operations, the use of WWDGPS-aided P-code equipment promises performance equal to or better than other current landing guidance systems in terms of accuracy and reliability. This performance comes at significantly less cost to NASA, which will participate as a customer in a system designed as a commercial operation serving the global civil navigation community.

Loomis, Peter V. W.↗

NASA NTRS1991

A fuzzy logic based spacecraft controller for six degree of freedom control and performance results

The development philosophy of the fuzzy logic controller is explained, details of the rules and membership functions used are given, and the early results of testing of the control system for a representative range of scenarios are reported. The fuzzy attitude controller was found capable of performing all rotational maneuvers, including rate hold and rate maneuvers. It handles all orbital perturbations very efficiently and is very responsive in correcting errors.

Lea, Robert N.↗

NASA NTRS1991

Total recall in distributive associative memories

Iterative error correction of asymptotically large associative memories is equivalent to a one-step learning rule. This rule is the inverse of the activation function of the memory. Spectral representations of nonlinear activation functions are used to obtain the inverse in closed form for Sparse Distributed Memory, Selected-Coordinate Design, and Radial Basis Functions.

Danforth, Douglas G.↗

NASA NTRS1992

Investigation of Bandwidth-Efficient Coding and Modulation Techniques

The necessary technology was studied to improve the bandwidth efficiency of the space-to-ground communications network using the current capabilities of that network as a baseline. The study was aimed at making space payloads, for example the Hubble Space Telescope, more capable without the need to completely redesign the link. Particular emphasis was placed on the following concepts: (1) what the requirements are which are necessary to convert an existing standard 4-ary phase shift keying communications link to one that can support, as a minimum, 8-ary phase shift keying with error corrections applied; and (2) to determine the feasibility of using the existing equipment configurations with additional signal processing equipment to realize the higher order modulation and coding schemes.

Osborne, William P.↗

NASA NTRS1991

Some partial-unit-memory convolutional codes

The results of a study on a class of error correcting codes called partial unit memory (PUM) codes are presented. This class of codes, though not entirely new, has until now remained relatively unexplored. The possibility of using the well developed theory of block codes to construct a large family of promising PUM codes is shown. The performance of several specific PUM codes are compared with that of the Voyager standard (2, 1, 6) convolutional code. It was found that these codes can outperform the Voyager code with little or no increase in decoder complexity. This suggests that there may very well be PUM codes that can be used for deep space telemetry that offer both increased performance and decreased implementational complexity over current coding systems.

Abdel-Ghaffar, K.↗

NASA NTRS1991

Systematic methods for knowledge acquisition and expert system development

Nine cooperating rule-based systems, collectively called AUTOCREW, were designed to automate functions and decisions associated with a combat aircraft's subsystem. The organization of tasks within each system is described; performance metrics were developed to evaluate the workload of each rule base, and to assess the cooperation between the rule-bases. Each AUTOCREW subsystem is composed of several expert systems that perform specific tasks. AUTOCREW's NAVIGATOR was analyzed in detail to understand the difficulties involved in designing the system and to identify tools and methodologies that ease development. The NAVIGATOR determines optimal navigation strategies from a set of available sensors. A Navigation Sensor Management (NSM) expert system was systematically designed from Kalman filter covariance data; four ground-based, a satellite-based, and two on-board INS-aiding sensors were modeled and simulated to aid an INS. The NSM Expert was developed using the Analysis of Variance (ANOVA) and the ID3 algorithm. Navigation strategy selection is based on an RSS position error decision metric, which is computed from the covariance data. Results show that the NSM Expert predicts position error correctly between 45 and 100 percent of the time for a specified navaid configuration and aircraft trajectory. The NSM Expert adapts to new situations, and provides reasonable estimates of hybrid performance. The systematic nature of the ANOVA/ID3 method makes it broadly applicable to expert system design when experimental or simulation data is available.

Belkin, Brenda L.↗

NASA NTRS1992

An 8-PSK TDMA uplink modulation and coding system

The combination of 8-phase shift keying (8PSK) modulation and greater than 2 bits/sec/Hz drove the design of the Nyquist filter to one specified to have a rolloff factor of 0.2. This filter when built and tested was found to produce too much intersymbol interference and was abandoned for a design with a rolloff factor of 0.4. The preamble is limited to 100 bit periods of the uncoded bit period of 5 ns for a maximum preamble length of 500 ns or 40 8PSK symbol times at 12.5 ns per symbol. For 8PSK modulation, the required maximum degradation of 1 dB in -20 dB cochannel interference (CCI) drove the requirement for forward error correction coding. In this contract, the funding was not sufficient to develop the proposed codec so the codec was limited to a paper design during the preliminary design phase. The mechanization of the demodulator is digital, starting from the output of the analog to digital converters which quantize the outputs of the quadrature phase detectors. This approach is amenable to an application specific integrated circuit (ASIC) replacement in the next phase of development.

Ames, S. A.↗

NASA NTRS1991

Maximum aposteriori joint source/channel coding

A maximum aposteriori probability (MAP) approach to joint source/channel coder design is presented in this paper. This method attempts to explore a technique for designing joint source/channel codes, rather than ways of distributing bits between source coders and channel coders. For a nonideal source coder, MAP arguments are used to design a decoder which takes advantage of redundancy in the source coder output to perform error correction. Once the decoder is obtained, it is analyzed with the purpose of obtaining 'desirable properties' of the channel input sequence for improving overall system performance. Finally, an encoder design which incorporates these properties is proposed.

Sayood, Khalid↗

NASA NTRS1990

Advanced communications payload for mobile applications

An advanced satellite payload is proposed for single hop linking of mobile terminals of all classes as well as Very Small Aperture Terminal's (VSAT's). It relies on an intensive use of communications on-board processing and beam hopping for efficient link design to maximize capacity and a large satellite antenna aperture and high satellite transmitter power to minimize the cost of the ground terminals. Intersatellite links are used to improve the link quality and for high capacity relay. Power budgets are presented for links between the satellite and mobile, VSAT, and hub terminals. Defeating the effects of shadowing and fading requires the use of differentially coherent demodulation, concatenated forward error correction coding, and interleaving, all on a single link basis.

Ames, S. A.↗

NASA NTRS1992

Nonlinear, nonbinary cyclic group codes

New cyclic group codes of length 2(exp m) - 1 over (m - j)-bit symbols are introduced. These codes can be systematically encoded and decoded algebraically. The code rates are very close to Reed-Solomon (RS) codes and are much better than Bose-Chaudhuri-Hocquenghem (BCH) codes (a former alternative). The binary (m - j)-tuples are identified with a subgroup of the binary m-tuples which represents the field GF(2 exp m). Encoding is systematic and involves a two-stage procedure consisting of the usual linear feedback register (using the division or check polynomial) and a small table lookup. For low rates, a second shift-register encoding operation may be invoked. Decoding uses the RS error-correcting procedures for the m-tuple codes for m = 4, 5, and 6.

Solomon, G.↗