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At least 1,567 records · Page 87

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NASA NTRS1991

Ionospheric storm effects at subauroral latitudes - A case study

An attempt is made to classify ionospheric storm effects at subauroral latitudes according to their presumed origin. The storm of December 7/8, 1982, serves as an example. It is investigated using ionosonde, electron content, and DE 2 satellite data. The following effects are distinguished: (1) positive storm effects caused by traveling atmospheric disturbances, (2) positive storm effects caused by changes in the large-scale thermospheric wind circulation, (3) positive storm effects caused by the expansion of the polar ionization enhancement, (4) negative storm effects caused by perturbations of the neutral gas composition, and (5) negative storm effects caused by the equatorward displacement of the trough region.

Proelss, G. W.↗

NASA NTRS1990

A case study in nonconformance and performance trend analysis

As part of NASA's effort to develop an agency-wide approach to trend analysis, a pilot nonconformance and performance trending analysis study was conducted on the Space Shuttle auxiliary power unit (APU). The purpose of the study was to (1) demonstrate that nonconformance analysis can be used to identify repeating failures of a specific item (and the associated failure modes and causes) and (2) determine whether performance parameters could be analyzed and monitored to provide an indication of component or system degradation prior to failure. The nonconformance analysis of the APU did identify repeating component failures, which possibly could be reduced if key performance parameters were monitored and analyzed. The performance-trending analysis verified that the characteristics of hardware parameters can be effective in detecting degradation of hardware performance prior to failure.

Maloy, Joseph E.↗

NASA NTRS1991

A case study in multi-component software development

McDonnell Douglas Helicopter Company has been actively involved in the development of the U.S. Army sponsored Second Generation Comprehensive Helicopter Analysis System (2GCHAS) over the last five years. This paper describes the multidisciplinary nature of the software, the multicontractor and multi-site environment within which the development took place, the development standards used and evolution of the final product. Valuable lessons were learned during this task that can be used in future developments.

Sangha, Karan↗

NASA NTRS1991

A recent case study in system identification

Results of a recent study of a ten-bay truss structure at the NASA Langley Research Center are reported. First, the conditioning of complex eigenvectors derived by the ERA method is discussed. Results of parameter estimation using the SSID (Structural System Identification) code are then presented. Based on the results of the study, it is concluded that (1) parameter estimation based on modal data should include eigenvectors as well as eigenvalues; (2) the eigenvectors should be orthogonalized when orthogonality is poor due to closely spaced modes; and (3) the parameters used in the estimation should enable the model to match the data.

Hasselman, T. K.↗

NASA NTRS1990

Earth Observing System instrument performance - Moderate Resolution Imaging Spectrometer case study

The requirements for characterization and calibration of instruments for the Earth Observing System space segment are defined in terms of tracking the Level 1 data for a sensor. The data must be traceable over the 15 year program, and must provide corrections for radiometric, spectral and geometric coefficients which capture the combined design and performance characteristics of the sensor. These requirements are then developed for MODIS-N to identify a set of techniques which can be used in the laboratory, spacecraft integrator's facility and in-orbit. The in-orbit of the so-called reflectance and radiance methods are reviewed, and the cross-calibrations available between HIRIS and MODIS through these techniques are listed. A scheme for the use of all available tools for instrument performance is provided as a Figure showing an estimated time interval over which each tool is expected to be useful. The final calibration strategy and plan must be developed through the understanding of a useful strategy for each of these candidate mechanisms.

Guenther, Bruce W.↗

NASA NTRS1991

Modeling and measuring multiprogramming and system overheads on a shared-memory multiprocessor - Case study

The present discussion of methods for quantifying multiprogramming (MP) overhead on a computer system illustrates two such techniques, respectively for quantifying MP overheads' lower bound and determining the MP overload of real workloads, in light of the percentage of parallel processing time that is consumed by MP overhead on Alliant multiprocessors. Kernel lock spinning is found to be a major factor in MP overhead, which accounts for more than half of total system overhead. It is noted that parallel environments' MP overhead is not statistically dependent on the number of parallel jobs undergoing multiprogramming.

Dimpsey, Robert T.↗

NASA NTRS1991

A case study of the cusp electrodynamics by the Aureol-3 satellite - Evidence for FTE signatures?

Particle and field data from a pass of the Aureol-3 satellite through the polar cusp, several minutes after the southward turning of the IMF, are analyzed in detail. Superposed on the classical cusp, characterized by the typical ion and electron precipitations, several very narrow arcs are detected where large fluxes of electrons and ions, accelerated to 2-4 keV, precipitate simultaneously. These localized arcs correspond to the upward current sheets of a succession in latitude of narrow, alternatively upward and downward field-aligned current sheets. The data suggest that the satellite has crossed the ionospheric footprints of 2 adjacent flux transfer events separated by 100-150 km in latitude. Electric spikes and electromagnetic turbulence are typically associated with the region of downward currents.

Bosqued, Jean M.↗

NASA NTRS1991

Development of a computer algorithm for the analysis of variable-frequency AC drives: Case studies included

The development of computer software for performance prediction and analysis of voltage-fed, variable-frequency AC drives for space power applications is discussed. The AC drives discussed include the pulse width modulated inverter (PWMI), a six-step inverter and the pulse density modulated inverter (PDMI), each individually connected to a wound-rotor induction motor. Various d-q transformation models of the induction motor are incorporated for user-selection of the most applicable model for the intended purpose. Simulation results of selected AC drives correlate satisfactorily with published results. Future additions to the algorithm are indicated. These improvements should enhance the applicability of the computer program to the design and analysis of space power systems.

Kankam, M. David↗

NASA NTRS1992

STS-48 case study, 17-18 September 1991

Weather conditions are documented prior to and during the STS-48 attempted landing at the Shuttle Landing Facility at KSC on 18 Sep. 1991. Trends in meteorological data during 17 and 18 Sep. are examined along with their relationship to the overall weather pattern observed over the KSC region. The primary weather problems during the landing were the formation of showers within 10 nautical miles of the SLF and any ceiling less than 10,000 ft. The controlling factor of the weather was a high pressure ridge that was gradually weakening and moving off the northeast. As this occurred, the low level flow was switching from a easterly to a southeasterly direction. This change in wind direction was reflected by shower movement on the McGill radar and by trends in rawinsondes launched from the Cape. These rawinsondes also indicated that the boundary layers was becoming slightly more unstable several hours prior to the attempted landing which may have aided in the development of clouds and small isolated showers. Also, analyses of Doppler wind profiler and rawinsondes indicated a possible midlevel disturbance in the easterly flow pattern near 700 mb. This weak disturbance may have made the atmosphere a little more unstable early on 18 Sep. Finally, embedded within the southeasterly flow were several bands of low clouds. These clouds were rather difficult to see in unenhanced IR satellite imagery available to forecasters in real time. However, post analyses using several different enhancement curves, adapted from NESDIS, clearly reveals the presence of these clouds.

Atchison, Michael K.↗

NASA NTRS1992

Taguchi methods in electronics: A case study

Total Quality Management (TQM) is becoming more important as a way to improve productivity. One of the technical aspects of TQM is a system called the Taguchi method. This is an optimization method that, with a few precautions, can reduce test effort by an order of magnitude over conventional techniques. The Taguchi method is specifically designed to minimize a product's sensitivity to uncontrollable system disturbances such as aging, temperature, voltage variations, etc., by simultaneously varying both design and disturbance parameters. The analysis produces an optimum set of design parameters. A 3-day class on the Taguchi method was held at the Marshall Space Flight Center (MSFC) in May 1991. A project was needed as a follow-up after the class was over, and the motor controller was selected at that time. Exactly how to proceed was the subject of discussion for some months. It was not clear exactly what to measure, and design kept getting mixed with optimization. There was even some discussion about why the Taguchi method should be used at all.

Kissel, R.↗

NASA NTRS1990

Radiative heating rates in the tropical monsoon environment - A case study

A study designed to investigate the radiative impact of high clouds through a combination of modeling and data analysis is presented. A series of high-altitude survey flights was conducted utilizing a NASA Ames ER-2 aircraft carrying a nadir-viewing narrow field-of-view radiometer, with channels at 6.5 and 10.5 microns. The two intensities obtained by this radiometer can be employed to deduce optical depth as described by Liou et. al. (1990).

Heffernan, C. M.↗

NASA NTRS1992

A case study of lightning, whistlers, and associated ionospheric effects during a substorm particle injection event

The relationships among cloud-to-ground (CG) lightning, sferics, whistlers, VLF amplitude perturbations, and other ionospheric phenomena occurring during substorm events were investigated using data from simultaneous ground-based observations of narrow-band and broad-band VLF radio waves and of CG lightning made during the 1987 Wave-Induced Particle Precipitation campaign conducted from Wallops Island (Virginia). Results suggest that the data collected on ionospheric phenomena during this event may represent new evidence of direct coupling of lightning energy to the lower ionosphere, either in conjunction with or in the absence of gyroresonant interactions between whistler mode waves and electrons in the magnetosphere.

Rodriguez, J. V.↗

NASA NTRS1990

Moisture and heat budgets of a cirrus cloud from aircraft measurements during fire - October 31 case study

Increasing knowledge of cirrus cloud properties can contribute to general circulation model development and ultimately to a better understanding of climate. The objective of this study is to gain better understanding of cirrus cloud characteristics. Observations from different sensors during the FIRE experiment which took place in Wisconsin over Oshkosh (Starr, 1987), together with pertinent calculations, will be used to understand the dynamic, microphysical, and radiative characteristics of these clouds.

Gultepe, I.↗

NASA NTRS1992

A GCM case study on the maintenance of short-term subtropical wind maxima in the summer hemisphere during SOP-1, FGGE

In order to substantiate earlier findings of pronounced short-term deviations in Southern Hemisphere tropical convection during the first Special Observing Period (SOP-1) of FGGE, the hypothesis that the observed summertime subtropical wind maxima were strengthened by episodes of enhanced tropical divergence is numerically tested using a fourth-order GCM. FGGE observational data for January 6-20, 1979 suggested that strong convection over the South Pacific helped to maintain a nearby subtropical jet. The dominant term in the zonal momentum budget was the Coriolis force applied to the diabatically driven meridional circulation. The present GCM study simulates these processes to a reasonable degree of accuracy. Experimental reduction of the latent energy supplied to the GCM is used to substantiate that summertime subtropical westerly maxima are largely forced by divergent outflow from the tropics.

Hurrell, James W.↗

NASA NTRS1991

Space Shuttle processing - A case study in artificial intelligence

A scheduling system incorporating AI is described and applied to the automated processing of the Space Shuttle. The unique problem of addressing the temporal, resource, and orbiter-configuration requirements of shuttle processing is described with comparisons to traditional project management for manufacturing processes. The present scheduling system is developed to handle the late inputs and complex programs that characterize shuttle processing by incorporating fixed preemptive scheduling, constraint-based simulated annealing, and the characteristics of an 'anytime' algorithm. The Space-Shuttle processing environment is modeled with 500 activities broken down into 4000 subtasks and with 1600 temporal constraints, 8000 resource constraints, and 3900 state requirements. The algorithm is shown to scale to very large problems and maintain anytime characteristics suggesting that an automated scheduling process is achievable and potentially cost-effective.

Mollikarimi, Cindy↗

NASA NTRS1992

Staircase ion signature in the polar cusp - A case study

On 15 October, 1981 Dynamics Explorer 2 crossed the polar cusp at 1015 MLT and observed three distinct ion populations as it was moving poleward. These three populations had peak-flux energy around 2.7 keV, 850 eV, and 360 eV. At the time of observation, the IMF was southward. The first step coincided with a rotation of the flow from westward to poleward and then eastward. The second and third steps showed a flow directed principally poleward. Furthermore, the magnetic and electric perturbations in the first step are well fitted by an elongated flux tube footprint model. These results suggest that three consecutive Flux Transfer Events (FTEs) have injected solar wind plasma into the ionosphere forming the polar cusp. The individual FTE signatures can only be identified by the jumps in the precipitation pattern. The newest reconnected FTE footprint was crossed near the edge of the event while the two oldest ones were crossed around the event center. The small latitudinal size of these FTE footprints (40 km) and their short recurrence rate (3, 6 min) is consistent with an intermittent reconnection taking place at the subsolar point on a short time scale.

Escoubet, C. P.↗

NASA NTRS1992

HL-20 personnel launch system - A concept definition case study

For several years, the NASA Langley Research Center has been involved in defining options for future space transportation systems. As part of this effort, for the past 2 years, an analysis of a candidate Personnel Launch System to deliver and return people and small payloads to and from Space Station Freedom has been conducted. This effort has involved a government/industry/university team in conducting an indepth analysis of the HL-20 lifting body concept to provide a technically viable, affordable Personnel Launch System. This paper will use the HL-20 PLS definition activity to illustrate the process that is used by Langley to mature vehicle concepts to identify technical/development risks and costs for future transportation systems.

Freeman, Delma C., Jr.↗

NASA NTRS1992

SeaWiFS technical report series. Volume 4: An analysis of GAC sampling algorithms. A case study

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument will sample at approximately a 1 km resolution at nadir which will be broadcast for reception by realtime ground stations. However, the global data set will be comprised of coarser four kilometer data which will be recorded and broadcast to the SeaWiFS Project for processing. Several algorithms for degrading the one kilometer data to four kilometer data are examined using imagery from the Coastal Zone Color Scanner (CZCS) in an effort to determine which algorithm would best preserve the statistical characteristics of the derived products generated from the one kilometer data. Of the algorithms tested, subsampling based on a fixed pixel within a 4 x 4 pixel array is judged to yield the most consistent results when compared to the one kilometer data products.

Yeh, Eueng-Nan↗