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At least 199 records · Page 11

The consistent comparison problem in N-version software

The authors have identified a difficulty in the implementation of N-version programming. The problem, called the consistent comparison problem, arises for applications in which decisions are based on the results of comparing finite-precision numbers. It is shown that when versions make comparisons involving the results of finite-precision calculations, it is impossible to guarantee the consistency of their results. It is therefore possible that correct versions may arrive at completely different outputs for an application that does not apparently have multiple correct solutions. If this problem is not dealt with explicitly, an N-version system may be unable to reach consensus even when none of its component versions falls.

Brilliant, Susan S.↗

On the average rate of growth in sunspot number and the size of the sunspot cycle

The average rate of growth in sunspot number over selected time intervals and the maximum average value as they both relate to the size of the cycle are examined, in order to predict the size of cycle 22. The predictions are compared with those of Wilson (1990) to determine whether a consensus is apparent. The average rate of growth during the ascending portion of the sunspot cycle, defined as the difference in smoothed sunspot number values between elapsed time t and sunspot minimum divided by t, is shown to correlate with the size of the sunspot cycle, especially for t greater or equal to 18 months. The maximum value of the average rate of growth is also shown to highly correlate (r = 0.98) with the size of the cycle. Using 4.5 as the maximum value of the average rate of growth, a lower limit for R(M) is estimated. The results show that the findings are consistent with the previous single variate predictions for R(M) for cycle 22.

Wilson, Robert M.↗

Superaromatics: The key to a unified cosmic dust theory

The theory of Superaromatics, the key to a unified cosmic dust theory, was constructed by analyzing several thousand astronomical features covering every major aspect of astrophysics and astrochemistry relating to dust. To insure consistency between disciplines, the logical structure of the conclusions in each field was checked rather than accepting the current consensus. No substantial contradictory features are known to the author. The analysis falls into seven major parts: (1) kinetics of grain formation and destruction; (2) optical spectra of the interstellar medium (ISM); (3) meteorite interplanetary dust particle (IPD) chemistry; (4) structure and chemistry of the interstellar medium arising from surface catalysis; (6) dynamics of circumstellar and interstellar dust clouds, including galactic morphology; and (7) the chemistry and physics of previously unidentified compounds. Only tentative conclusions are presented here. The principle conclusion is that quantum mechanics as it is normally formulated is incomplete. The probable cause is that it is formulated with complex numbers rather than the more fundamental quaternion system. The manifestation in astrochemistry is that the most stable compounds are superaromatic and exotic enough to confound most classical analysis.

Manuel, Lawrence R.↗

Statistical methods and neural network approaches for classification of data from multiple sources

Statistical methods for classification of data from multiple data sources are investigated and compared to neural network models. A problem with using conventional multivariate statistical approaches for classification of data of multiple types is in general that a multivariate distribution cannot be assumed for the classes in the data sources. Another common problem with statistical classification methods is that the data sources are not equally reliable. This means that the data sources need to be weighted according to their reliability but most statistical classification methods do not have a mechanism for this. This research focuses on statistical methods which can overcome these problems: a method of statistical multisource analysis and consensus theory. Reliability measures for weighting the data sources in these methods are suggested and investigated. Secondly, this research focuses on neural network models. The neural networks are distribution free since no prior knowledge of the statistical distribution of the data is needed. This is an obvious advantage over most statistical classification methods. The neural networks also automatically take care of the problem involving how much weight each data source should have. On the other hand, their training process is iterative and can take a very long time. Methods to speed up the training procedure are introduced and investigated. Experimental results of classification using both neural network models and statistical methods are given, and the approaches are compared based on these results.

Benediktsson, Jon Atli↗

Integration of the TDWR and LLWAS wind shear detection system

Operational demonstrations of a prototype TDWR/LLWAS (Terminal Doppler Weather Radar/Low Level Wind shear Alarm System) integrated wind shear detection system were conducted. The integration of wind shear detection systems is needed to provide end-users with a single, consensus source of information. A properly implemented integrated system provides wind shear warnings of a higher quality than stand-alone LLWAS or TDWR systems. The algorithmic concept used to generate the TDWR/LLWAS integrated products and several case studies are discussed, indicating the viability and potential of integrated wind shear detection systems. Implications for integrating ground and airborne wind shear detection systems are briefly examined.

Cornman, Larry↗

NASA-LaRc Flight-Critical Digital Systems Technology Workshop

The outcome is documented of a Flight-Critical Digital Systems Technology Workshop held at NASA-Langley December 13 to 15 1988. The purpose of the workshop was to elicit the aerospace industry's view of the issues which must be addressed for the practical realization of flight-critical digital systems. The workshop was divided into three parts: an overview session; three half-day meetings of seven working groups addressing aeronautical and space requirements, system design for validation, failure modes, system modeling, reliable software, and flight test; and a half-day summary of the research issues presented by the working group chairmen. Issues that generated the most consensus across the workshop were: (1) the lack of effective design and validation methods with support tools to enable engineering of highly-integrated, flight-critical digital systems, and (2) the lack of high quality laboratory and field data on system failures especially due to electromagnetic environment (EME).

Meissner, C. W., Jr.↗

Heavy-lift launch vehicle options for future space exploration initiatives

A review of present heavy-lift launch vehicles (HLLV) capable of placing heavy payloads in earth orbit is presented and the basis for an emerging consensus that an HLLV will be required in the near future is discussed. Some of the factors for the policies governing the roles and requirements for these vehicles in the future, such as cost, technology development, and lead time to first use are addressed. Potential Space Station Freedom application is discussed as well as application to the proposed initiatives for human exploration of Mars and the moon.

Branscome, Darrell R.↗

United States studies in orbital debris - Prevention and mitigation

Debris in space has become an issue that has commanded considerable interest in recent years as society has become both more dependent upon space based systems, and more aware of its dependence. After many years of study the United States Space Policy of February 1988 directed that all sectors of the U.S. community minimize space debris. Other space organizations have adopted similar policies. Among the study activities leading to the policy and to subsequent implementing directives were discussions with the ESA, NASDA, and other space operating agencies. The policy derived from technical consensus on the nature of the issues and upon the courses of action available to mitigate the problem, but there remains the concern as to the adequacy of the data to define cost effective strategies. There are now in place mechanisms to continue technical discussions in more formal terms.

Loftus, Joseph P., Jr.↗

A reply from earth? - A proposed approach to developing a message from humankind to extraterrestrial intelligence after we detect them

The question of the formulation of a proper response to the detection of an extraterrestrial civilization is considered. It is proposed that an international agreement or declaration of principles establishing procedures enabling international participation in the making of such decisions be developed and that the SETI Committee of the International Academy of Aeronautics, in cooperation with other interested bodies, draft an agreement or declaration of principles that would set up these procedures; and that the draft be presented to the United Nations for consideration through the Committee on Peaceful Uses of Outer Space. A suggested outline of the agreement or declaration is presented and proposes that a response to the detection of extraterrestrial intelligence should be on behalf of all humankind; that this decision should be made by an appropriately representative international body; and that the content of the reply should reflect an international consensus.

Michaud, Michael↗

A design approach for ultrareliable real-time systems

A design approach developed over the past few years to formalize redundancy management and validation is described. Redundant elements are partitioned into individual fault-containment regions (FCRs). An FCR is a collection of components that operates correctly regardless of any arbitrary logical or electrical fault outside the region. Conversely, a fault in an FCR cannot cause hardware outside the region to fail. The outputs of all channels are required to agree bit-for-bit under no-fault conditions (exact bitwise consensus). Synchronization, input agreement, and input validity conditions are discussed. The Advanced Information Processing System (AIPS), which is a fault-tolerant distributed architecture based on this approach, is described. A brief overview of recent applications of these systems and current research is presented.

Lala, Jaynarayan H.↗

The Space Station decision - Politics, bureaucracy, and the making of public policy

The lack of consensus that dominates the conception of major scientific and technological programs is demonstrated via a comparison of the decisions to build the Space Station and the Space Transportation System, and the decision to go to the moon. It is argued that the way political reality conditions administrative behavior in NASA is shown by the decision to promote international cooperation prior to program approval. It is concluded that so long as NASA remains a government agency, its officials will struggle to learn how to balance professional accountability with political reality.

Mccurdy, Howard E.↗

Selling the Space Telescope - The interpenetration of science, technology, and politics

Attention is given to the politics of initiating the Space Telescope program and to the manner in which the coalition, or working consensus, for the Telescope was assembled, in particular, the role played by astronomers. It is contended that what ensued was a case study in the influence of government patronage on a large-scale scientific and technological program. It is concluded that while a politically feasible Space Telescope did result, in the selling process the Telescope had been both oversold and underfunded.

Smith, Robert W.↗

Jetting and the origin of tektites

The scientific consensus is that tektites were produced by impacts on the Earth, but the exact mechanism by impacts might form tektites is still unclear. The most widely cited mechanism is jetting, which results from the extremely high pressures generated at the intersection of two bodies whose surfaces converge obliquely at high speed. Theory of jetting for thin plates is extended to the case of the impact of the sphere onto a half-space. The calculations are done for the impact of a silicate sphere onto a silicate target for impact speeds of 15, 20, and 25 km/sec, spanning the range of reasonable impact speeds for asteroids. The angle of impact is varied from 0 to 75 deg. The mass jetted, the jet velocity, projectile fraction in the jet, azimuthal distribution of the jet, and the phase of the jetted material are calculated as functions of time. The total mass jetted and the overall mass-averages of jet velocity, etc. are also calculated.

Vickery, A. M.↗

Archaebacterial rhodopsin sequences: Implications for evolution

It was proposed over 10 years ago that the archaebacteria represent a separate kingdom which diverged very early from the eubacteria and eukaryotes. It follows that investigations of archaebacterial characteristics might reveal features of early evolution. So far, two genes, one for bacteriorhodopsin and another for halorhodopsin, both from Halobacterium halobium, have been sequenced. We cloned and sequenced the gene coding for the polypeptide of another one of these rhodopsins, a halorhodopsin in Natronobacterium pharaonis. Peptide sequencing of cyanogen bromide fragments, and immuno-reactions of the protein and synthetic peptides derived from the C-terminal gene sequence, confirmed that the open reading frame was the structural gene for the pharaonis halorhodopsin polypeptide. The flanking DNA sequences of this gene, as well as those of other bacterial rhodopsins, were compared to previously proposed archaebacterial consensus sequences. In pairwise comparisons of the open reading frame with DNA sequences for bacterio-opsin and halo-opsin from Halobacterium halobium, silent divergences were calculated. These indicate very considerable evolutionary distance between each pair of genes, even in the dame organism. In spite of this, three protein sequences show extensive similarities, indicating strong selective pressures.

Lanyi, J. K.↗

Final STS-35 Columbia descent BET products and results for LaRC OEX investigations

Final STS-35 'Columbia' descent Best Estimate Trajectory (BET) products have been developed for Langley Research Center (LaRC) Orbiter Experiments (OEX) investigations. Included are the reconstructed inertial trajectory profile; the Extended BET, which combines the inertial data and, in this instance, the National Weather Service atmospheric information obtained via Johnson Space Center; and the Aerodynamic BET. The inertial BET utilized Inertial Measurement Unit 1 (IMU1) dynamic measurements for deterministic propagation during the ENTREE estimation process. The final estimate was based on the considerable ground based C-band tracking coverage available as well as Tracking Data and Relay Satellite System (TDRSS) Doppler data, a unique use of the latter for endo-atmospheric flight determinations. The actual estimate required simultaneous solutions for the spacecraft position and velocity, spacecraft attitude, and six IMU parameters - three gyro biases and three accelerometer scale factor correction terms. The anchor epoch for this analysis was 19,200 Greenwich Mean Time (GMT) seconds which corresponds to an initial Shuttle altitude of approximately 513 kft. The atmospheric data incorporated were evaluated based on Shuttle derived considerations as well as comparisons with other models. The AEROBET was developed based on the Extended BET, the measured spacecraft configuration information, final mass properties, and the final Orbiter preoperation databook. The latter was updated based on aerodynamic consensus incrementals derived by the latest published FAD. The rectified predictions were compared versus the flight computed values and the resultant differences were correlated versus ensemble results for twenty-two previous STS entry flights.

Oakes, Kevin F.↗

The study of PDF turbulence models in combustion

The accurate prediction of turbulent combustion is still beyond reach for today's computation techniques. It is the consensus of the combustion profession that the predictions of chemically reacting flow were poor if conventional turbulence models were used. The main difficulty lies in the fact that the reaction rate is highly nonlinear, and the use of averaged temperature, pressure, and density produces excessively large errors. The probability density function (PDF) method is the only alternative at the present time that uses local instant values of the temperature, density, etc. in predicting chemical reaction rate, and thus it is the only viable approach for turbulent combustion calculations.

Hsu, Andrew T.↗

Collected notes from the Benchmarks and Metrics Workshop

In recent years there has been a proliferation of proposals in the artificial intelligence (AI) literature for integrated agent architectures. Each architecture offers an approach to the general problem of constructing an integrated agent. Unfortunately, the ways in which one architecture might be considered better than another are not always clear. There has been a growing realization that many of the positive and negative aspects of an architecture become apparent only when experimental evaluation is performed and that to progress as a discipline, we must develop rigorous experimental methods. In addition to the intrinsic intellectual interest of experimentation, rigorous performance evaluation of systems is also a crucial practical concern to our research sponsors. DARPA, NASA, and AFOSR (among others) are actively searching for better ways of experimentally evaluating alternative approaches to building intelligent agents. One tool for experimental evaluation involves testing systems on benchmark tasks in order to assess their relative performance. As part of a joint DARPA and NASA funded project, NASA-Ames and Teleos Research are carrying out a research effort to establish a set of benchmark tasks and evaluation metrics by which the performance of agent architectures may be determined. As part of this project, we held a workshop on Benchmarks and Metrics at the NASA Ames Research Center on June 25, 1990. The objective of the workshop was to foster early discussion on this important topic. We did not achieve a consensus, nor did we expect to. Collected here is some of the information that was exchanged at the workshop. Given here is an outline of the workshop, a list of the participants, notes taken on the white-board during open discussions, position papers/notes from some participants, and copies of slides used in the presentations.

Drummond, Mark E.↗

A consensual neural network

A neural network architecture called a consensual neural network (CNN) is proposed for the classification of data from multiple sources. Its relation to hierarchical and ensemble neural networks is discussed. CNN is based on the statistical consensus theory and uses nonlinearly transformed input data. The input data are transformed several times, and the different transformed data are applied as if they were independent inputs. The independent inputs are classified using stage neural networks and outputs from the stage networks are then weighted and combined to make a decision. Experimental results based on remote-sensing data and geographic data are given.

Benediktsson, J. A.↗