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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Radar depolarization signatures of rain in cumulus clouds measured with a dual-frequency air-borne radar

Experimental data are presented on dual-wavelength scattering characteristics of subtropical rain observed during the convection and precipitation/electrification (CaPE) experiment. Linear depolarization ratios (LDRs) are calculated at both X-band and Ka-band channels. In stratiform rain, the cross-polarized return signal is generally below the noise level except within the melting layer where the maximum LDR at both frequencies is typically between -12 and -10 dB. In convective rain the range profiles of the LDR in X-band and those in Ka-band do not coincide. The X-band LDR values in convective rain typically remain smaller than -25 dB at high altitudes and take somewhat larger values of -25 to -15 dB at low altitudes. The Ka-band LDR values, in contrast, typically increase with radar range, sometimes reaching a level of -3 dB just before the cross-polarized signal falls below the noise level. Such high values of LDR in Ka-band are most likely attributable to multiple scattering effects.

Iguchi, Toshio↗
NASA NTRS1993

The backscattered Lyman-alpha glow emission as a signature of the termination shock

The location of the termination shock is a problem of great interest in astrophysics and the current availability of deep space probes with UV detectors has made it possible in principle to remotely sense the termination shock. The method depends upon a measurement of the neutral hydrogen distribution which results from the presence of the heliospheric plasma structure. The charge exchange interaction between the inflowing neutral hydrogen and the plasma structure leads to an observable modification of the inflowing hydrogen distribution. Hence, both the radial dependence of the backscattered Lyman-alpha glow and the absolute difference in the upstream and downstream glow brightness are modified. Recent Voyager 1 and 2 and Pioneer 10 spacecraft data are consistent with the presence of a termination shock. The observational evidence and theoretical models from which a termination shock is inferred will be presented.

Gangopadhyay, P.↗
NASA NTRS1993

Effect of tree structure on X-band microwave signature of conifers

Experimental studies are performed on some coniferous trees (Austrian pine, Nordmann spruce, and Norway spruce) to investigate the relation between the tree architecture and radar signal at X-band. For a single tree, the RCS is measured as a function of the scatterer location at 90 deg incidence. It is found that the main scatterers are the leafy branches and the difference between sigma(vv) and sigma(hh) is significant at the upper portion of the tree. At the lower portion of the tree, sigma(vv) and sigma(hh) have almost the same level. For a group of trees the angular trends of sigma(vv) and sigma(hh) are measured. It is found that the levels of sigma(vv) and sigma(hh) are of the same order, but their angular trends vary from one tree species to the other depending on the tree species structure. The interpretation of these experimental results is carried out with the help of a theoretical model which accounts for the structure of the tree. According to this theoretical study, the major scattering trend is due to the leaves, while the perturbation to the angular trend and the level difference between sigma(vv) and sigma(hh) are due to the branch orientation distributions (i.e., the tree architecture).

Mougin, Eric↗
NASA NTRS1993

The boron-to-beryllium ratio in halo stars - A signature of cosmic-ray nucleosynthesis in the early Galaxy

We discuss Galactic cosmic-ray (GCR) spallation production of Li, Be, and B in the early Galaxy with particular attention to the uncertainties in the predictions of this model. The observed correlation between the Be abundance and the metallicity in metal-poor Population II stars requires that Be was synthesized in the early Galaxy. We show that the observations and such Population II GCR synthesis of Be are quantitatively consistent with the big bang nucleosynthesis production of Li-7. We find that there is a nearly model independent lower bound to B/Be of about 7 for GCR synthesis. Recent measurements of B/Be about 10 in HD 140283 are in excellent agreement with the predictions of Population II GCR nucleosynthesis. Measurements of the boron abundance in additional metal-poor halo stars is a key diagnostic of the GCR spallation mechanism. We also show that Population II GCR synthesis can produce amounts of Li-6 which may be observed in the hottest halo stars.

Walker, T. P.↗
NASA NTRS1993

Extended ecosystem signatures with application to Eos synergism requirements

The primary objective is to define the advantages of synergistically combining optical and microwave remote sensing measurements for the determination of biophysical properties important in ecosystem modeling. This objective was approached in a stepwise fashion starting with ground-based observations of controlled agricultural and orchard canopies and progressing to airborne observations of more natural forest ecosystems. This observational program is complemented by a parallel effort to model the visible reflectance and microwave scattering properties of composite vegetation canopies. The goals of the modeling studies are to verify our basic understanding of the sensor-scene interaction physics and to provide the basis for development of inverse models optimized for retrieval of key biophysical properties. These retrieval algorithms can then be used to simulate the expected performance of various aspects of Eos including the need for simultaneous SAR and HIRIS observations or justification for other (non-synchronous) relative timing constraints and the frequency, polarization, and angle of incidence requirements for accurate biophysical parameter extractions. This program completed a very successful series of truck-mounted experiments, made remarkable progress in development and validation of optical reflectance and microwave scattering models for vegetation, extended the scattering models to accommodate discontinuous and periodic canopies, developed inversion approaches for surface and canopy properties, and disseminated these results widely through symposia and journal publications. In addition, the third generation of the computer code for the microwave scattering models was provided to a number of other US, Canadian, Australian, and European investigators who are currently presenting and publishing results using the MIMICS research code.

Ulaby, Fawwaz T.↗
NASA NTRS1993

Geophysical signature of the Pretoria saltpan impact structure and a possible satellite crater

The Pretoria Saltpan Crater is located in the southern portion of the Bushveld Igneous Complex, some 40 km NNW of Pretoria, South Africa, at 25 deg 24 min 30 sec S/28 deg 4 min 59 sec E. An origin by impact for this crater structure was recently confirmed. The results of the only gravity reconnaissance carried out over the crater to date failed to support an impact origin. With the aid of recent results obtained from a central drill-core, it was necessary to carry out more geophysical work which would include a gravity profile of higher resolution. A second, smaller, circular depression (about 400 m in diameter) to the SW of the crater is suggestive of a twin crater. This site had never been investigated, and thus various geophysical surveys were conducted.

Brandt, D.↗
NASA NTRS1993

Work at Graz on satellite signatures

The size and shape of the satellites retroreflector arrays have a major impact on the distribution and scatter of the return signal; this can be seen clearly when reaching sub-cm ranging accuracies and when using Single-Photon Detectors with single- or multi-photon returns; for other receiver systems (using MCP's), it should be checked also. As a consequence, the necessary center-of-mass correction for some satellites will differ, depending on the receiver systems. While this effect is not yet visible on small satellites or small retro-reflector arrays (like STARLETTE, ERS1), it can be in the order of centimeters on AJISAI or ETALON.

Kirchner, Georg↗
NASA NTRS1993

Nitrogen isotopic signatures in agglutinates from breccia 79035

Agglutinates in the size range 125-175 microns from regolith breccia 79035 are substantially depleted in N compared with bulk 79035. Isotopically, agglutinate N closely resembles that found previously in ilmenite separates. The minimum (delta)N-15 value found during stepwise pyrolysis of agglutinates is significantly heavier than that observed for bulk 79035. The major host phase for trapped N in 79035, and the host phase of the lightest isotopic component(s), remain unidentified.

Kerridge, John F.↗
NASA NTRS1993

Possible flexural signatures around Olympus and Ascraeus Montes, Mars

The effective elastic thickness (h) of the lithosphere provides a measure of the thermal and mechanical state of a planet's shallow interior. An estimate of h in the vicinity of a feature that constitutes a load on a planetary surface can be determined from the flexural response of the lithosphere to the load. This approach has been applied to Mars by calculating radial stresses associated with lithospheric flexure associated with surface loads, and comparing the results to the positions of circumferential graben surrounding the major Martian shield volcanoes and mascon basins. However, many prominent surface loads on Mars, most notably the Olympus Mons volcano, do not exhibit flexural graben. In these instances application of the above method can provide only a lower limit of effective elastic thickness. An alternative method of determining h is to calculate the vertical displacements associated with the flexural loading and to compare the amplitude and shape of the flexural profile to observed topography. This method has not been applied to flexural problems on Mars because of the poor resolution of Martian topographic data. However, previous analyses have shown that the lithosphere around major volcanic shields should exhibit vertical deflections of order 1 km over horizontal baselines of order 100 km. We were thus motivated to search for the presence of flexural troughs in the existing Mars topography data.

Zuber, M. T.↗
NASA NTRS1993

Orbital Signature Analyzer (OSA): A spacecraft health/safety monitoring and analysis tool

Fixed or static limit sensing is employed in control centers to ensure that spacecraft parameters remain within a nominal range. However, many critical parameters, such as power system telemetry, are time-varying and, as such, their 'nominal' range is necessarily time-varying as well. Predicted data, manual limits checking, and widened limit-checking ranges are often employed in an attempt to monitor these parameters without generating excessive limits violations. Generating predicted data and manual limits checking are both resource intensive, while broadening limit ranges for time-varying parameters is clearly inadequate to detect all but catastrophic problems. OSA provides a low-cost solution by using analytically selected data as a reference upon which to base its limits. These limits are always defined relative to the time-varying reference data, rather than as fixed upper and lower limits. In effect, OSA provides individual limits tailored to each value throughout all the data. A side benefit of using relative limits is that they automatically adjust to new reference data. In addition, OSA provides a wealth of analytical by-products in its execution.

Weaver, Steven↗
NASA NTRS1994

Implications for high speed research: The relationship between sonic boom signature distortion and atmospheric turbulence

In this study there were two primary tasks. The first was to develop an algorithm for quantifying the distortion in a sonic boom. Such an algorithm should be somewhat automatic, with minimal human intervention. Once the algorithm was developed, it was used to test the hypothesis that the cause of a sonic boom distortion was due to atmospheric turbulence. This hypothesis testing was the second task. Using readily available sonic boom data, we statistically tested whether there was a correlation between the sonic boom distortion and the distance a boom traveled through atmospheric turbulence.

Sparrow, Victor W.↗
NASA NTRS1994

Variability of measured sonic boom signatures

The topics discussed include the following: atmospheric turbulence; BOOMFILE Database description; BOOMFILE flight conditions; XB-70 Database descriptions; analysis progression; extended database; prediction method; overpressure variability dependence on flight conditions; loudness variability on flight conditions; sonic boom variability in repeat flights; and statistical distributions.

Elmer, K. R.↗
NASA NTRS1992

The effect of turbulence and molecular relaxation on sonic boom signatures

Our objectives are to assess the effect of turbulence and molecular absorption (which is now known to be a key factor in sonic boom shock structure) on shaped sonic booms. Today I will discuss the combination of physical mechanisms for idealized turbulence. In parallel, we are reviewing models for mixed layer turbulence, and these physical effects will eventually be generalized.

Plotkin, Kenneth J.↗
NASA NTRS1994

Vibration Signature Analysis of a Faulted Gear Transmission System

A comprehensive procedure in predicting faults in gear transmission systems under normal operating conditions is presented. Experimental data was obtained from a spiral bevel gear fatigue test rig at NASA Lewis Research Center. Time synchronous averaged vibration data was recorded throughout the test as the fault progressed from a small single pit to severe pitting over several teeth, and finally tooth fracture. A numerical procedure based on the Winger-Ville distribution was used to examine the time averaged vibration data. Results from the Wigner-Ville procedure are compared to results from a variety of signal analysis techniques which include time domain analysis methods and frequency analysis methods. Using photographs of the gear tooth at various stages of damage, the limitations and accuracy of the various techniques are compared and discussed. Conclusions are drawn from the comparison of the different approaches as well as the applicability of the Wigner-Ville method in predicting gear faults.

Choy, F. K.↗
NASA NTRS1994

Control theory and splines, applied to signature storage

In this report the problem we are going to study is the interpolation of a set of points in the plane with the use of control theory. We will discover how different systems generate different kinds of splines, cubic and exponential, and investigate the effect that the different systems have on the tracking problems. Actually we will see that the important parameters will be the two eigenvalues of the control matrix.

Enqvist, Per↗
NASA NTRS1994

Signatures of flux erosion from the dayside magnetosphere

The rate of merging and the strength of the region 1 Birkeland currents increase during periods of southward interplanetary magnetic field (IMF). Fringe fields of the Birkeland currents depress dayside magnetospheric magnetic field strengths and remove magnetic flux from the dayside magnetophere, thereby allowing the dayside magnetopause to move inward and the cusp equatorward. We use previously derived fits to the magnetospause location as a function of IMF B(sub z), the condition of pressure balance at the magnetopause, and an idealized model of region 1 Birkeland currents to estimate that strong southward IMF turnings will produce approximately 13- to 26-nT depressions in the geosynchronous magnetic field strength over periods of 30-60 min. We then present three case studies of geosynchronous magnetic field strength variations during periods of nearly constant solar wind dynamic pressure and southward IMF. The dayside magnetospheric magnetic field strength was depressed approximately 10 nT during a period of strongly southward IMF (B(sub z) = -6 nT), but only approximately 5 nT during two more typical periods of slightly southward IMF (B(sub z) = -2 to -3 nT). The depressions correspond to periods of enhanced AL index, which we interpret as evidence for directly driven solar wind-magnetosphere interaction rather than the unloading of energy stored within the magnetotail. Dayside geosynchronous magnetic field strengths are weakly correlated with IMF B(sub z).

Sibeck, D. G.↗
NASA NTRS1994

Optical signatures of high-redshift galaxy clusters

We combine an N-body and gasdynamic simulation of structure formation with an updated population synthesis code to explore the expected optical characteristics of a high-redshift cluster of galaxies. We examine a poor (2 keV) cluster formed in a biased, cold dark matter cosmology and employ simple, but plausible, threshold criteria to convert gas into stars. At z = 2, the forming cluster appears as a linear chain of very blue (g-r approximately equals 0) galaxies, with 15 objects brighter than r = 25 within a 1 square arcmin field of view. After 2 Gyr of evolution, the cluster viewed at z = 1 displays both freshly infalling blue galaxies and red galaxies robbed of recent accretion by interaction with the hot intracluster medium. The range in G-R colors is approximately 3 mag at z = 1, with the reddest objects lying at sites of highest galaxy density. We suggest that red, high-redshift galaxies lie in the cores of forming clusters and that their existence indicates the presence of a hot intracluster medium at redshifts z approximately equals 2. The simulated cluster viewed at z = 2 has several characteristics similar to the collection of faint, blue objects identified by Dressler et al. in a deep Hubble Space Telescope observation. The similarities provide some support for the interpretation of this collection as a high-redshift cluster of galaxies.

Evrard, August E.↗