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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

A statistical evaluation and comparison of VISSR Atmospheric Sounder (VAS) data

In order to account for the temporal and spatial discrepancies between the VAS and rawinsonde soundings, the rawinsonde data were adjusted to a common hour of release where the new observation time corresponded to the satellite scan time. Both the satellite and rawinsonde observations of the basic atmospheric parameters (T Td, and Z) were objectively analyzed to a uniform grid maintaining the same mesoscale structure in each data set. The performance of each retrieval algorithm in producing accurate and representative soundings was evaluated using statistical parameters such as the mean, standard deviation, and root mean square of the difference fields for each parameter and grid level. Horizontal structure was also qualitatively evaluated by examining atmospheric features on constant pressure surfaces. An analysis of the vertical structure of the atmosphere were also performed by looking at colocated and grid mean vertical profiles of both the satellite and rawinsonde data sets. Highlights of these results are presented.

Jedlovec, G. J.↗
NASA NTRS1982

Broadcasting satellite feeder links - Characteristics and planning

The paper presents the results of recent studies by the Feeder Link Sub-Working Group of the FCC Advisory Committee for the 1983 Regional Administrative Radio Conference (RARC). These studies conclude that specification of a few key parameters will make feeder link planning relatively straightforward. Feeder links can be located anywhere within a country if satellite orbit locations are separated by 10 deg for adjacent service areas and key parameter values presented in the paper are adopted. Colocated satellites serving a common service area need special attention to attain sufficient isolation between a desired channel and its adjacent cross-polarized channels and alternate co-polarized channels. In addition to presenting planning conclusions by the Advisory Committee, the paper presents and analyzes actions of the International Radio Consultative Committee's Conference Planning Meeting (CPM) concerning feeder links. The CPM reached conclusions similar to, and compatible with, those of the Advisory Committee.

Kiebler, J. W.↗
NASA NTRS1984

Cloud distributions in a Bay of Bengal monsoon depression

Much of the volume of a monsoon depression studied over the Bay of Bengal on July 7, 1979 featured thin, fragmentary layers of stratus, implying an absence of strong vertical motion. Coverage by cumulus updrafts was about 20 times less than coverage by inert cumulus remnants, and a great number density of humilis was found moistening the central area, following subsidence and drying. Detailed observations were made of a cloud line growing out of the southwesterly flow south of the center of the depression, which propagated faster than the low level winds. Aircraft altimetry showed an abrupt height drop from 6097 to 6090 m at 483 hPa, over a distance of 50 km from southeast to northwest through the line. Southwesterly momentum was lifted from 900 to 600 hPa and from southeast to northwest through the line. Other colocated singularities in convection and wind fields were found.

Warner, C.↗
NASA NTRS1984

Enhanced vibration controllability by minor structural modifications

A procedure for checking whether small changes in a structure have the potential for significant enhancements of its vibration control system is described. The first step in the procedure consists of the calculation of the sensitivity of the required strength of the control system to small changes in structural parameters. The second step consists of the optimization of the structural parameters to produce maximal reduction in required control system strength with minimal change in the structure. The procedure has been demonstrated for a flexible beam supported by four cables and controlled by a rate feedback single-colocated force-actuator velocity-sensor pair. Large changes in control strength requirement were obtained with small structural modifications. Analytical predictions of such effects have also been validated experimentally.

Haftka, R. T.↗
NASA NTRS1984

Experiments in control of flexible structures with noncolocated sensors and actuators

While the stable control of such flexible structures as limber spacecraft is easily achieved through the colocation of control actuators with sensors, noncolocation renders this extremely difficult. The most difficult case in point is that in which structural damping is low and spacecraft stiffness and inertia values are uncertain and changing. Attention is presently given to an apparatus in which each basic sensor/actuator noncolocation configuration is available, and inertias can be abruptly halved or doubled during control maneuvers. This feature can impose a sudden reversal in the plant's pole-zero sequence, which is a very difficult condition for the controller. Test results obtained to date demonstrate the inherent difficulty of achieving robustness in the case of noncolocation. It is noted that there may be very simple configurations in which there is no alternative to adaptive control.

Cannon, R. H., Jr.↗
NASA NTRS1985

Sensitivity of optimized control systems to minor structural modifications

A procedure for checking whether small changes in a structure have the potential for significant enhancements of its optimized vibration control system is described. The first step in the procedure consists of the calculation of the sensitivity of the parameters of the optimized control system to small changes in the structural parameters. Theh second step consists of the optimization of the structural parameters to produce maximal increase in the performance of the control system with minimal change in the structure. The procedure has been demonstrated for a flexible laboratory structure controlled by several rate-feedback colocated force-actuator velocity-sensor pairs. Significant improvements in the performance of the control system were obtained with small structural modifications. Analytical predictions of such effects have also been validated experimentally.

Haftka, R. T.↗
NASA NTRS1984

Thunderstorm cloud top dynamics as inferred from satellite observations and models

Satellite observations of convective towers penetrating the tropopause are summarized. Three classes of thunderstorm cloud tops including storm tops that have a monotonic temperature-height pattern with the highest point being the coldest point (class 1), tops that have a colocated cold and high point with the warm point on the downward slope to the rear of the highest point (class 2), and tops in which the cold point is offset upwind from the high point, with a cold-warm couplet straddling the convective cloud top (class 3), are described. A one-dimensional cloud parcel model applied in the overshooting region is used to explore the causes of the storm top's unique temperature/height structures.

Adler, R. F.↗
NASA NTRS1985

Direct model reference adaptive control of a flexible robotic manipulator

Quick, precise control of a flexible manipulator in a space environment is essential for future Space Station repair and satellite servicing. Numerous control algorithms have proven successful in controlling rigid manipulators wih colocated sensors and actuators; however, few have been tested on a flexible manipulator with noncolocated sensors and actuators. In this thesis, a model reference adaptive control (MRAC) scheme based on command generator tracker theory is designed for a flexible manipulator. Quicker, more precise tracking results are expected over nonadaptive control laws for this MRAC approach. Equations of motion in modal coordinates are derived for a single-link, flexible manipulator with an actuator at the pinned-end and a sensor at the free end. An MRAC is designed with the objective of controlling the torquing actuator so that the tip position follows a trajectory that is prescribed by the reference model. An appealing feature of this direct MRAC law is that it allows the reference model to have fewer states than the plant itself. Direct adaptive control also adjusts the controller parameters directly with knowledge of only the plant output and input signals.

Meldrum, D. R.↗
NASA NTRS1985

On the design of large flexible space structures (LFSS)

For a general finite-element model of an LFSS, a strictly passive compensator results in an exponentially stable feedback system when actuators and sensors are calculated. In the general case (no colocation), necessary and sufficient conditions are stated on the parameter Q for stabilizing a certain number of modes. Conditions for robust stability are given, and it is shown that feedback does not destabilize the unmodeled modes under certain conditions.

Bhaya, A.↗
NASA NTRS1986

Microprocessor controlled force actuator

The mechanical and electrical design of a prototype force actuator for vibration control of large space structures (LSS) is described. The force actuator is an electromagnetic system that produces a force by reacting against a proof-mass. The actuator has two colocated sensors, a digital microcontroller, and a power amplifier. The total weight of actuator is .998 kg. The actuator has a steady state force output of approximately 2.75 N from approximately 2 Hz to well beyond 1000 Hz.

Zimmerman, D. C.↗
NASA NTRS1986

Evolution of lightning flash density and reflectivity structure in a multicell thunderstorm

The radar reflectivity structure and the distribution of lightning in a storm cell was investigated using S-band and UHF-band radar data for six storm cells over Wallops Island. The S-band scans were vertical and continuous, while the UHF data were taken in steps of 2.5 deg elevation. The peak in lightning activity during the study corresponded to a merging of two storm cells. A minimum height of 7 km was found necessary for the appearance of a 40 dBZ core with lightning, which first appears in a multicell thunderstorm at the leading edge of the 50 dBZ core of the cell and between a cell and its decaying neighbor. The lightning moves further into the cell during cell decay and decreases in density. Finally, the lightning is offset horizontally from the precipitation core during cell growth but colocates with the precipitation core as the cell dissipates.

Mazur, V.↗
NASA NTRS1986

The poleward edge of the mid-latitude trough - Its formation, orientation and dynamics

Data from the Advanced Ionospheric Sounder (AIS) deployed at Halley, Antarctica (76-deg S, 27-deg W; L = 4.2) and the Dynamics Explorer-2 spacecraft (DE-2) are used to investigate several aspects of the formation processes and dynamics of the poleward edge of the midlatitude electron density trough. These include a study of the flux and energy of charged particles precipitating into the F-region as a function of Magnetic Local Time. It is found that local energetic electron precipitation is a major source of ionization of the poleward edge in the evening sector, but only after magnetic midnight transport processes become more important. Occasionally a significant increase in the flux of conjugate photoelectrons is colocated with the poleward edge of the trough in the morning sector. The combination of AIS and DE-2 data has allowed identification of significant longitudinal structure on the poleward edge of the trough that may be the result of substorm activity. It is found that the orientation of the poleward edge of the trough and the locus of the plasmapause predicted from the 'tear-drop' model vary in rather a similar manner with local time, though no close physical link between the two features is inferred from this coincidence.

Rodger, A. S.↗
NASA NTRS1986

The coordinate frame of the lunar laser ranging network

The geocentric coordinates for four instruments, which were derived using lunar laser ranging, are compared with the 84L02 coordinates determined from the Lageos satellite. The determination of the geocentric coordinates for the 2.7 m and McDonald Observatory laser ranging system telescopes at McDonald Observatory, the Haleakala site, and the CERGA site near Grasse, France is described. Consideration is given to the McDonald Observatory colocation and station motion due to continential drift. A rms difference of 18 cm is determined for the two sets of geocentric coordinates; however, removing a data anomaly reduces the rms difference to 13 cm.

Williams, J. G.↗
NASA NTRS1987

A comparison of SAGE 1, SBUV, and Umkehr ozone profiles including a search for Umkehr aerosol effects

Using a spatially weighted average for the stratospheric aerosol and gas experiment 1 (SAGE 1) events derived from an autocorrelation analysis, 337 colocated SAGE 1 and Umkehr ozone profiles are found. The total column ozone in layers two through nine measured by SAGE 1 is found to be 4.6 + or - 1.3 percent higher at the 95 percent confidence level than the approximate total column ozone measured by Umkehr. Average layer ozone differences indicate that most of this discrepancy resides in the lower layers. Intercomparison of SAGE 1, Nimbus 7 solar backscattered ultraviolet (SBUV), and Umkehr ozone at stations north of 30 deg indicates that, in layer six, Umkehr values are consistently higher than both SAGE 1 and SBUV by about 10 percent. In layer eight, SBUV ozone is higher than both SAGE 1 and SBUV by about 10 percent. In the upper stratosphere, the SAGE 1-Umkehr ozone differences are small for low stratospheric aerosol optical depth cases, but vary from -3 percent in layer six to -8 percent in layer nine for high optical depth cases.

Newchurch, M. J.↗
NASA NTRS1987

Optimized retrievals of precipitable water from the VAS 'split window'

Precipitable water fields have been retrieved from the VISSR Atmospheric Sounder (VAS) using a radiation transfer model for the differential water vapor absorption between the 11- and 12-micron 'split window' channels. Previous moisture retrievals using only the split window channels provided very good space-time continuity but poor absolute accuracy. This note describes how retrieval errors can be significantly reduced from plus or minus 0.9 to plus or minus 0.6 gm/sq cm by empirically optimizing the effective air temperature and absorption coefficients used in the two-channel model. The differential absorption between the VAS 11- and 12-micron channels, empirically estimated from 135 colocated VAS-RAOB observations, is found to be approximately 50 percent smaller than the theoretical estimates. Similar discrepancies have been noted previously between theoretical and empirical absorption coefficients applied to the retrieval of sea surface temperatures using radiances observed by VAS and polar-orbiting satellites. These discrepancies indicate that radiation transfer models for the 11-micron window appear to be less accurate than the satellite observations.

Chesters, Dennis↗
NASA NTRS1987

Comparison of Southern Hemisphere radiosonde and LIMS temperatures at 100 mb

Radiosonde (RS)and satellite-derived (Nimbus-7 LIMS) 100-mb temperatures over New Zealand at 12 GMT are compared for the 1978-79 summer. The colocated LIMS temperature information consists of synoptically mapped values (for 12 GMT), as well as the primary nighttime orbital retrievals valid at about 1030 GMT. The RS time series of temperature is dominated by temporal fluctuations associated mainly with the eastward passage of waves which have characteristic periods of 4-5 and 11-12 days and peak-to-peak amplitudes of 10-15 K. The LIMS temperatures and the corresponding temperature time series are also found to exhibit quite close agreement (in terms of temporal phase for the latter) with the RS data. However, the LIMS-mapped temperature fluctuations suffer from a noticeable attenuation in amplitude (approaching 50 percent for higher-frequency fluctuations), which will affect the accuracy of LIMS-derived estimates of dynamical quantities such as wind velocity and relative vorticity in the lower stratosphere.

Miles, T.↗
NASA NTRS1987

A parameter robust LQG design synthesis with applications to control of flexible structures

An asymptotic LQG design synthesis technique is described which explicitly includes a class of structured plant uncertainties by viewing plant parameter variations as an internal feedback loop. A direct structural relationship between this class of parameter uncertainties and the weighting matrices in the design of the LQG compensator is exploited by an asymptotic procedure in which either the regulator or the filter become insensitive to parameter variations. This asymptotic approach represents a generalization of the LQG/LTR technique. Controllers designed by this new LQG method and LQG/LTR are compared by application to a mass-spring-damper system which approximates the dynamics of flexible structures. For both colocated and noncolocated sensors and actuators configurations, the LQG/LTR design is extremely sensitive to parameter variations whereas the new LQG design allows considerably improved parameter robustness.

Tahk, Minjea↗
NASA NTRS1987

Robustness issues in boundary feedback of flexible structures

Transfer function models for basic structural elements with boundary control reveal certain inherent properties relevant to questions of control system realization, design, and analysis of robustness. The transfer functions involved are not strictly proper, may be nonminimum phase (except for the special case of colocated actuation and sensing), and often have large number of poles on the imaginary axis. In this paper these basic questions are considered in terms of modeling and control system design for robustness. The application of algebraic methods for computing stabilizing control, based on certain exact, irrational transfer functions is investigated. Boundary control of the wave equation is considered and extension of the method to more general problems is suggested. Special attention is given to implementation issues associated with a class of infinite dimensional control laws.

Bennett, W. H.↗