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NASA NTRS1977

Transfer of polarized radiation in self-absorbed synchrotron sources. II - Treatment of inhomogeneous media and calculation of emergent polarization

As an extension of previous treatments of polarized radiation transfer in homogeneous synchrotron sources, a transfer procedure is derived, based on the coupled-wave formulation, which allows consideration of inhomogeneous sources. It is shown that plasma characteristic wave eccentricities in a source must be fairly large unless Faraday rotation per absorption depth is very large, and that coupling between characteristic waves can be strong in inhomogeneous self-absorbed sources. Calculations are made of the polarized radiation emergent from a source with a boundary of finite thickness. It is found that neglect of propagation effects in the source boundary introduces no significant errors at optically thin frequencies, but that below the self-absorption turnover the emergent polarization may depend significantly upon such effects. In particular, the sign of the circular polarization and the linear polarization position angle depend upon the absorption depth in the boundary region.

Jones, T. W.↗

NASA NTRS1972

Experimental investigation of the visual field dependency in the erect and supine positions

The increasing utilization of simulators in many fields, in addition to aeronautics and space, requires the efficient use of these devices. It seemed that personnel highly influenced by the visual scene would make desirable subjects, particularly for those simulators without sufficient motion cues. In order to evaluate this concept, some measure of the degree of influence of the visual field on the subject in necessary. As part of this undertaking, 37 male and female subjects, including eight test pilots, were tested for their visual field dependency or independency. A version of Witkin's rod and frame apparatus was used for the tests. The results showed that nearly all the test subjects exhibited some degree of field dependency, the degree varying from very high field dependency to nearly zero field dependency in a normal distribution. The results for the test pilots were scattered throughout a range similar to the results for the bulk of male subjects. The few female subjects exhibited a higher field dependency than the male subjects. The male subjects exhibited a greater field dependency in the supine position than in the erect position, whereas the field dependency of the female subjects changed only slightly.

Lichtenstein, J. H.↗

NASA NTRS1972

On the analytic lunar and solar perturbations of a near earth satellite

The disturbing function of the moon (sun) is expanded as a sum of products of two harmonic functions, one depending on the position of the satellite and the other on the position of the moon (sun). The harmonic functions depending on the position of the perturbing body are developed into trigonometric series with the ecliptic elements l, l', F, D, and Gamma of the lunar theory which are nearly linear with respect to time. Perturbation of elements are in the form of trigonometric series with the ecliptic lunar elements and the equatorial elements omega and Omega of the satellite so that analytic integration is simple and the results accurate over a long period of time.

Estes, R. H.↗

NASA NTRS1974

On the analytic lunar and solar perturbations of a near earth satellite

The disturbing function of the moon (sun) is expanded as a sum of products of two harmonic functions, one depending on the position of the satellite and the other on the position of the moon (sun). The harmonic functions depending on the position of the perturbing body are developed into trigonometric series with the ecliptic elements l, l prime, F, D and Gamma of the lunar theory which are nearly linear with respect to time. Perturbation of elements are in the form of trigonometric series with the ecliptic lunar elements and the equatorial elements omega and Omega of the satellite, so that analytic integration is simple and the results are accurate over a long period of time.-

Estes, R. H.↗

NASA NTRS1990

The position of the magnetotail neutral sheet in the near-earth region

The position of the near-earth neutral sheet throughout the nightside region between R = 5 R(E) and R = 8.8 R(E) is examined statistically using magnetic field data from AMPTE/CCE. As has been demonstrated previously, the latitudinal position of the neutral sheet is primarily controlled by the dipole tilt angle. The position also depends on local time, radial distance, and magnetic activity. These dependencis may be represented by the empirical formula MLAT = - (0.14 Kp + 0.69) /cos(Phi)/exp 1/3 (0.065 R exp 0.8 - 0.16) DTA, where MLAT is the position in degrees of the neutral sheet relative to the magnetic equator, Kp is the 3-hourly magnetic activity index, Phi is the magnetic local time in degrees (midnight = 0 deg), R is the radius in R(E), and DTA is the dipole tilt angle in degrees.

Lopez, Ramon E.↗

NASA NTRS1981

Temperature dependence of the hydrogen atom abstraction reactions of Cl/+/ and HCl/+/ with H2

Rate constants are reported for the reactions of Cl(+) and HCl(+) with H2 over the temperature range 150-400 K. The Cl(+) reaction has a positive temperature dependence that follows the Arrhenius form k1 = A exp(-169/T). The reaction of HCl(+) has a negative temperature dependence that follows the form k2 = AT to the -0.6 power. The implication of these results for interstellar chemistry is commented on. Schematic potential surfaces for the reactions are suggested.

Cates, R. D.↗

NASA NTRS2012

Multi-Factor Impact Analysis of Agricultural Production in Bangladesh with Climate Change

Diverse vulnerabilities of Bangladesh's agricultural sector in 16 sub-regions are assessed using experiments designed to investigate climate impact factors in isolation and in combination. Climate information from a suite of global climate models (GCMs) is used to drive models assessing the agricultural impact of changes in temperature, precipitation, carbon dioxide concentrations, river floods, and sea level rise for the 2040-2069 period in comparison to a historical baseline. Using the multi-factor impacts analysis framework developed in Yu et al. (2010), this study provides new sub-regional vulnerability analyses and quantifies key uncertainties in climate and production. Rice (aman, boro, and aus seasons) and wheat production are simulated in each sub-region using the biophysical Crop Environment REsource Synthesis (CERES) models. These simulations are then combined with the MIKE BASIN hydrologic model for river floods in the Ganges-Brahmaputra-Meghna (GBM) Basins, and the MIKE21Two-Dimensional Estuary Model to determine coastal inundation under conditions of higher mean sea level. The impacts of each factor depend on GCM configurations, emissions pathways, sub-regions, and particular seasons and crops. Temperature increases generally reduce production across all scenarios. Precipitation changes can have either a positive or a negative impact, with a high degree of uncertainty across GCMs. Carbon dioxide impacts on crop production are positive and depend on the emissions pathway. Increasing river flood areas reduce production in affected sub-regions. Precipitation uncertainties from different GCMs and emissions scenarios are reduced when integrated across the large GBM Basins' hydrology. Agriculture in Southern Bangladesh is severely affected by sea level rise even when cyclonic surges are not fully considered, with impacts increasing under the higher emissions scenario.

Ruane, Alex C.↗

NASA NTRS2010

Inductive Non-Contact Position Sensor

Optical hardware has been developed to measure the depth of defects in the Space Shuttle Orbiter's windows. In this hardware, a mirror is translated such that its position corresponds to the defect's depth, so the depth measurement problem is transferred to a mirror-position measurement problem. This is preferable because the mirror is internal to the optical system and thus accessible. Based on requirements supplied by the window inspectors, the depth of the defects needs to be measured over a range of 200 microns with a resolution of about 100 nm and an accuracy of about 400 nm. These same requirements then apply to measuring the position of the mirror, and in addition, since this is a scanning system, a response time of about 10 ms is needed. A market search was conducted and no sensor that met these requirements that also fit into the available housing volume (less than one cubic inch) was found, so a novel sensor configuration was constructed to meet the requirements. This new sensor generates a nearly linearly varying magnetic field over a small region of space, which can easily be sampled, resulting in a voltage proportional to position. Experiments were done with a range of inductor values, drive voltages, drive frequencies, and inductor shapes. A rough mathematical model was developed for the device that, in most aspects, describes how it operates and what electrical parameters should be chosen for best performance. The final configuration met all the requirements, yielding a small rugged sensor that was easy to use and had nanometer resolution over more than the 200-micron range required. The inductive position sensor is a compact device (potentially as small as 2 cubic centimeters), which offers nanometer-position resolution over a demonstrated range of nearly 1 mm. One of its advantages is the simplicity of its electrical design. Also, the sensor resolution is nearly uniform across its operational range, which is in contrast to eddy current and capacitive sensors whose sensitivity is dependent upon position.

Youngquist, Robert↗

NASA NTRS1980

Ion chemistry in interstellar space

The temperature dependence of the bimolecular reactions of Cl(+), HCl(+), CH(+), CH2(+), N(+), NH(+), and NH2(+) reacting with H2 have been investigated. For Cl(+) and HCl(+) rate constants have been determined over the temperature range of 150 to 400 K. Preliminary data have been obtained for the other systems. The Cl(+)/H2 system shows a weak, but significant positive temperature dependence that could be important in interstellar modeling studies. The HCl(+)/H2 system shows a substantial negative temperature dependence with the rate constant approaching the collision rate at low temperatures. The association reaction CH3(+) + HCN yields CH3 HCN(+) has been theoretically modeled using statistical phase space theory. Both radiative and collisional stabilization have been included. The results are compared with experiment with good agreement obtained over wide variations in T and p. This system is potentially important in the mechanism of large molecule synthesis in interstellar space.

Bowers, M. T.↗

NASA NTRS1989

On the signal-to-noise ratio in IUE high-dispersion spectra

An observational and data reduction technique for fixed pattern noise (FPN) and random noise (RN) in fully extracted IUE high-dispersion spectra is described in detail, along with actual empirical values of signal-to-noise ratio (S/N) achieved. A co-addition procedure, involving SWP and LWR cameras observations of the same spectrum at different positions in the image format, provides a basis to disentangle FPN from RN, allowing each average amplitude, within a given wavelength interval, to be estimated as a function of average flux number. Empirical curves, derived with the noise algorithm, make it possible to estimate the S/N in individual spectra at the wavelengths investigated. The average S/N at the continuum level in well-exposed stellar spectra varies from 10 to 20, for the orders analyzed, depending on position in the spectral format. The co-addition procedure yields an improvement in S/N by factors ranging from 2.3 to 2.9. Direct measurements of S/N in narrow, line-free wavelength intervals of individual and co-added spectra for weak-lined stars yield comparable, or in some cases somewhat higher, S/N values and improvement factors.

Leckrone, David S.↗

NASA NTRS1993

The reaction of atomic hydrogen with germane - Temperature dependence of the rate constant and implications for germane photochemistry in the atmospheres of Jupiter and Saturn

Studies of the formation and loss processes for GeH4 are required in order to provide data to help determine the major chemical form in which germanium exists in the atmospheres of Jupiter and Saturn. The reaction of hydrogen atoms with germane is one of the most important of these reactions. The absolute rate constant for this reaction as a function of temperature and pressure is studied. Flash photolysis of dilute mixtures of GeH4 in argon, combined with time-resolved detection of H atoms via Lyman alpha resonance fluorescence, is employed to measure the reaction rate. The reaction is shown to be moderately rapid, independent of total pressure, but possessing a positive temperature dependence.

Nava, David F.↗

NASA NTRS2008

Using Transponders on the Moon to Increase Accuracy of GPS

It has been proposed to place laser or radio transponders at suitably chosen locations on the Moon to increase the accuracy achievable using the Global Positioning System (GPS) or other satellite-based positioning system. The accuracy of GPS position measurements depends on the accuracy of determination of the ephemerides of the GPS satellites. These ephemerides are determined by means of ranging to and from Earth-based stations and consistency checks among the satellites. Unfortunately, ranging to and from Earth is subject to errors caused by atmospheric effects, notably including unpredictable variations in refraction. The proposal is based on exploitation of the fact that ranging between a GPS satellite and another object outside the atmosphere is not subject to error-inducing atmospheric effects. The Moon is such an object and is a convenient place for a ranging station. The ephemeris of the Moon is well known and, unlike a GPS satellite, the Moon is massive enough that its orbit is not measurably affected by the solar wind and solar radiation. According to the proposal, each GPS satellite would repeatedly send a short laser or radio pulse toward the Moon and the transponder(s) would respond by sending back a pulse and delay information. The GPS satellite could then compute its distance from the known position(s) of the transponder(s) on the Moon. Because the same hemisphere of the Moon faces the Earth continuously, any transponders placed there would remain continuously or nearly continuously accessible to GPS satellites, and so only a relatively small number of transponders would be needed to provide continuous coverage. Assuming that the transponders would depend on solar power, it would be desirable to use at least two transponders, placed at diametrically opposite points on the edges of the Moon disk as seen from Earth, so that all or most of the time, at least one of them would be in sunlight.

Penanen, Konstantin↗

NASA NTRS2008

Measuring Positions of Objects using Two or More Cameras

An improved method of computing positions of objects from digitized images acquired by two or more cameras (see figure) has been developed for use in tracking debris shed by a spacecraft during and shortly after launch. The method is also readily adaptable to such applications as (1) tracking moving and possibly interacting objects in other settings in order to determine causes of accidents and (2) measuring positions of stationary objects, as in surveying. Images acquired by cameras fixed to the ground and/or cameras mounted on tracking telescopes can be used in this method. In this method, processing of image data starts with creation of detailed computer- aided design (CAD) models of the objects to be tracked. By rotating, translating, resizing, and overlaying the models with digitized camera images, parameters that characterize the position and orientation of the camera can be determined. The final position error depends on how well the centroids of the objects in the images are measured; how accurately the centroids are interpolated for synchronization of cameras; and how effectively matches are made to determine rotation, scaling, and translation parameters. The method involves use of the perspective camera model (also denoted the point camera model), which is one of several mathematical models developed over the years to represent the relationships between external coordinates of objects and the coordinates of the objects as they appear on the image plane in a camera. The method also involves extensive use of the affine camera model, in which the distance from the camera to an object (or to a small feature on an object) is assumed to be much greater than the size of the object (or feature), resulting in a truly two-dimensional image. The affine camera model does not require advance knowledge of the positions and orientations of the cameras. This is because ultimately, positions and orientations of the cameras and of all objects are computed in a coordinate system attached to one object as defined in its CAD model.

Klinko, Steve↗

NASA NTRS1971

Optimal analytic multiburn trajectories

Fuel optimal analytic multiburn transfer trajectories and shuttle rendezvous, assuming gravity vector linear dependence on position vector for burn arcs

Jezewski, D. J.↗

NASA NTRS1975

Altitudinal, diurnal and latitudinal dependences for ionic concentrations formed by corpuscular fluxes in the stratosphere and mesosphere

On the basis of over 15 rocket measurements of ionic concentrations below 80 km, the height, daily and latitudinal dependences between positive ionic concentrations and cosmic ray intensity below 60 km as well as between ionic concentrations and corpuscular streams within the 60-80 km altitude range are compared. It is shown that ionic concentration and cosmic ray intensity below 60 km at night are likely to be interrelated, in conformity with Chapman's theory of the simple layer. In daytime, all phenomena are aggravated by photodetachment and ion exchange reactions with the participation of ozone. Between 60-80 km, besides ordinary cosmic rays, there must exist an additional corpuscular stream.

Bragin, Y. A.↗

NASA NTRS1975

Aeroelastic stability of trimmed helicopter blades in forward flight

Equations for moderately large amplitude coupled flap-lag motion of a torsionally rigid hingeless elastic helicopter blade in forward flight are derived. Quasi-steady aerodynamic loads are considered and the effects of reversed flow are included. By using Galerkin's method the spatial dependence of the problem is eliminated and the equations are linearized about a time dependent equilibrium position determined from the trimmed equilibrium position of the rotor in forward flight. In the first trim procedure the rotor is maintained at a fixed value of thrust coefficient with forward flight and horizontal and vertical force equilibrium is satisfied in addition to maintaining zero pitch and roll moments. The second trim procedure maintains only zero pitch and roll moment simulating conditions under which a rotor would be tested in the wind tunnel.

Friedmann, P.↗

NASA NTRS1977

Aeroelastic stability of coupled flap-lag-torsional motion of helicopter rotor blades in forward flight

A set of coupled periodic nonlinear differential flap-lag-torsional equations of blade motion is described which can simulate a blade having a precone, twist, distributed torsion, root torsion, torque offset, blade-root offsets, and offsets among the elastic axis, the aerodynamic center, and the blade cross-sectional center of mass. It is noted that the aerodynamic loads derived for these equations are applicable to cases of both hover and forward flight. The aeroelastic stability of a hingeless helicopter blade is investigated by linearizing the nonlinear differential equations of motion about a time-dependent equilibrium position of the helicopter in forward flight, using propulsion and moment trim procedures. A comparison of the results with those obtained previously for coupled flap-lag-torsional boundaries in hover indicates that aeroelastic-stability margins are degraded due to forward flight.

Friedmann, P.↗

NASA NTRS1978

Aeroelastic stability and response of horizontal axis wind turbine blades

The coupled flap-lag-torsion equations of motion of an isolated horizontal axis wind turbine blade are formulated. Quasi-steady blade-element strip theory was applied to derive the aerodynamic operator which includes boundary layer type gradient winds. The final equations which have periodic coefficients were solved in order to obtain the aeroelastic response and stability of large horizontal axis wind turbine blade. A new method of generating an appropriate time-dependent equilibrium position (required for the stability analysis) has been implemented. Representative steady-state responses and stability boundaries, applicable mainly to an existing blade design (NASA/-ERDA MOD-0), are presented. The results indicate that the MOD-0 configuration is a basically stable design and that blade stability is not sensitive to offsets between blade elastic axis and aerodynamic center. Blade stability appears to be sensitive to precone. The tower shadow (or wake) has a considerable effect on the flap response but leaves blade stability unchanged. Finally, it was found that non linear terms in the equations of motion can significantly affect the linearized stability boundaries, however, these terms have a negligible effect on blade response at operating conditions.

Kottapalli, S. B. R.↗