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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Signatures of solar wind latitudinal structure in interplanetary Lyman-alpha emissions - Mariner 10 observations

A detailed analysis is conducted which shows that signatures in the interplanetary Lyman-alpha emissions observed in three different data sets from Mariner 10 (corresponding to different locations of the spacecraft) provide firm evidence that the intensity departures are correlated with a decrease in solar wind flux with increasing latitude. It is suggested that observations of the interplanetary emission can be used to monitor average solar wind activity at high latitudes. The asymmetry in the solar radiation field as a source of observed departures in L-alpha data is considered and attention is given to the interstellar hydrogen and helium density.

Kumar, S.↗
NASA NTRS1979

Signature extension methods in crop area estimation

The Procedure B multispectral processing system is both multisegment and multistratum. It uses data from several LACIE-sized segments together and makes a proportion estimate for the entire group of segments as well as for the individual segments. In the clustering of data features, Procedure B produces multiple classes or strata rather than just two strata (as in Procedure 1), and performs stratified sampling on each of these mutliple strata in order to make a proportion estimate. Tests results for the components and for the overall performance of Procedure B are presented, and conclusions that can be drawn from these tests are discussed. The rationale for signature extension for crop area estimation is summarized.

Kauth, R. J.↗
NASA NTRS1980

Microwave signatures of the natural snow cover at Weissfluhjoch

The natural snow cover on a high altitude alphine test site was monitored with a multi-frequency radiometer for more than three years. Some measurements were also made with a 10.5 GHz scatterometer. The microwave observations are supported by a large set of ground truth data. From year to year a wide variation in the development of the snowpack above and below average was observed. Typical microwave data are presented for the different snow conditions in view of the applicability as signatures for remote sensing.

Hofer, R.↗
NASA NTRS1981

The signature of auroral kilometric radiation on Isis 1 ionograms

Auroral kilometric radiation (AKR) appears on the Isis 1 topside sounder ionograms as intense noise bands between the electron cyclotron frequency and 700 kHz. A variable gap occurs between the cyclotron frequency and the lowest AKR frequency. As Isis 1 traverses the source region, the gap narrows, and the AKR signals at higher frequencies weaken. This signature suggests that the AKR waves are generated directly in the extraordinary mode at frequencies just above the local cutoff frequency and that the radiation is initially perpendicular to the magnetic field.

Calvert, W.↗
NASA NTRS1980

Infrared and radar signatures of lunar craters - Implications about crater evolution

Geological models accounting for the strongly crater size-dependent IR and radar signatures of lunar crater floors are examined. The simplest model involves the formation and subsequent 'gardening' of an impact melt layer on the crater floor, but while adequate in accounting for the gradual fading of IR temperatures and echo strengths in craters larger than 30 km in diameter, it is inadequate for smaller ones. It is concluded that quantitative models of the evolution of rock populations in regoliths and of the interaction of microwaves with regoliths are needed in order to understand crater evolutionary processes.

Thompson, T. W.↗
NASA NTRS1981

A magnetospheric signature of some F layer positive storms

Calculations of electron density distributions in the global thermosphere-ionosphere system perturbed by high-latitude thermospheric heating are presented which indicate a link between the heating and magnetospheric plasma disturbances near the equator. The calculations were made using a self-consistent model of the global sunlit thermosphere-ionosphere system describing the evolution of equatorial plasma disturbances. The heat input is found to cause electron density enhancements that propagate along magnetic field lines from the F2 maximum over mid-latitudes to the equator in the magnetosphere and which correspond to the positive phase of an F layer storm. The positive phase is shown to be generated by the induction of equatorward winds that raise the mid-latitude F layer through momentum transfer from neutral atoms to ionospheric ions, which ions pull electrons with them. Model results are used to identify plasma signatures of equatorward winds and an intensified magnetospheric electric field in Explorer 45 and Arial 4 measurements taken during the positive phase of an F layer storm.

Miller, N. J.↗
NASA NTRS1981

Determination of wind tunnel constraint effects by a unified pressure signature method. Part 1: Applications to winged configurations

A new, fast, non-iterative version of the "Wall Pressure Signature Method" is described and used to determine blockage and angle-of-attack wind tunnel corrections for highly-powered jet-flap models. The correction method is complemented by the application of tangential blowing at the tunnel floor to suppress flow breakdown there, using feedback from measured floor pressures. This tangential blowing technique was substantiated by subsequent flow investigations using an LV. The basic tests on an unswept, knee-blown, jet flapped wing were supplemented to include the effects of slat-removal, sweep and the addition of unflapped tips. C sub mu values were varied from 0 to 10 free-air C sub l's in excess of 18 were measured in some cases. Application of the new methods yielded corrected data which agreed with corresponding large tunnel "free air" resuls to within the limits of experimental accuracy in almost all cases. A program listing is provided, with sample cases.

Hackett, J. E.↗
NASA NTRS1981

Determination of wind tunnel constraint effects by a unified pressure signature method. Part 2: Application to jet-in-crossflow

The development of an improved jet-in-crossflow model for estimating wind tunnel blockage and angle-of-attack interference is described. Experiments showed that the simpler existing models fall seriously short of representing far-field flows properly. A new, vortex-source-doublet (VSD) model was therefore developed which employs curved trajectories and experimentally-based singularity strengths. The new model is consistent with existing and new experimental data and it predicts tunnel wall (i.e. far-field) pressures properly. It is implemented as a preprocessor to the wall-pressure-signature-based tunnel interference predictor. The supporting experiments and theoretical studies revealed some new results. Comparative flow field measurements with 1-inch "free-air" and 3-inch impinging jets showed that vortex penetration into the flow, in diameters, was almost unaltered until 'hard' impingement occurred. In modeling impinging cases, a 'plume redirection' term was introduced which is apparently absent in previous models. The effects of this term were found to be very significant.

Hackett, J. E.↗
NASA NTRS1981

Substorm signatures at synchronous altitude

The signatures of magnetospheric substorms near 6.6 earth radii are statistically examined using data obtained on board ATS 6 by magnetic field and energetic particle measurements. It is confirmed that the configuration change toward a tail-like field in the dusk-to-midnight sector typically begins about one hour before the onset of the expansion phase of a substorm. Configuration changes before the onsets of moderate substorms are characterized by a directional change of magnetic field at magnetic latitude of approximately 10 deg, rather than by a change in the field magnitude. It is found during storm periods that the magnetic field orientation occasionally becomes almost parallel to the magnetic equator before expansion phase onsets. Here, the field magnitude usually increases above background levels. These facts are seen as strongly suggesting that a tail-like configuration is caused by an intensification and earthward motion of the cross-tail current system, often close to 6.6 earth radii, rather than by development of diamagnetic ring current.

Kokubun, S.↗
NASA NTRS1982

SASS measurements of the Ku-band radar signature of the ocean

SeaSat-A Satellite Scatterometer (SASS) measurements of normalized radar cross section (NRCS) have been merged with high quality surface-wind fields based on in situ, to create a large data base of NRCS-wind signature data. These data are compared to the existing NRCS-wind model used by the SASS to infer winds. Falso-color maps of SASS NRCS and ocean winds from multiple orbits show important synoptic trends.

Schroeder, L. C.↗
NASA NTRS1982

Correlated Birkeland current signatures from the TRIAD and Magsat magnetic field data

Transverse magnetic disturbances associated with Birkeland currents have been measured by the Magsat and TRIAD spacecraft, whose orbits were nearly coplanar from November to mid-December, 1979. A comparison of data received when the satellites were over the TRIAD/Chatanika receiving station shows that 75% of magnetic disturbance measurements are similar in shape and magnitude, with TRIAD peak disturbances being on average 7% larger than those of Magsat. This discrepancy is lower than measurement uncertainties due to baselines and calibration. During periods of higher Kp, despite time separations of as much as 45 min, better agreement is found in the large-scale and well-defined field-aligned current signatures.

Zanetti, L. J.↗
NASA NTRS1982

Electron acceleration and radiation signatures in loop coronal transients

It is proposed that in loop coronal transients an erupting loop moves away from the solar surface, with a velocity exceeding the local Alfven speed, pushing against the overlying magnetic fields and driving a shock in the front of the moving part of the loop. Lower hybrid waves are excited at the shock front and propagate radially toward the center of the loop with phase velocity along the magnetic field that exceeds the thermal velocity. The lower hybrid waves stochastically accelerate the tail of the electron distribution inside the loop. The manner in which the accelerated electrons are trapped in the moving loop are discussed, and their radiation signature is estimated. It is suggested that plasma radiation can explain the power observed in stationary and moving type IV bursts.

Vlahos, L.↗
NASA NTRS1982

The optical and radiation field signatures produced by lightning return strokes

Typical examples of the signals that are produced by first and subsequent return strokes in cloud-to-ground lightning on a microsecond time scale are presented. Statistics on the structure of the waveforms and the radiance of the channels are given. The relationship between the light signals and the associated electric field signatures is discussed. It is shown that the initial light signal from a return stroke tends to be linear for about 15 microsec and then rises more slowly to a peak that is delayed by approximately 60 microsec from the electric field peak. It is thought that the transition between the fast linear portion and the slower rise may be due to the return stroke entering the cloud base. A small percentage of the records suggest that two different branches of the same stepped leader can initiate separate return strokes. The light pulses from cloud discharges tend to be smaller and to vary more slowly than those from return strokes.

Guo, C.↗
NASA NTRS1982

Comparison of environmental conditions in the Bering Sea and Davis Strait and the effects on microwave signature returns; March and April, 1979

Aircraft data collected in the Bering Sea in March, 1979 using a 6.6 GH sub z (C Band) microwave radiometer and a 13.9 GH sub z (Ku Band) scatterometer, reinforce the difficulties in interpreting first year ice types found near the ice edge in a marginal ice zone. An ice interpretation scheme using data taken with a 13.3 GH sub z (Ku Band) scatterometer and a 19.4 GH sub z (K Band) radiometer in Davis Strait also shows ambiguity in the first year ice signal and indicates that ice interpretation becomes more difficult near the ice edge and under warmer conditions. This report also compares X Band SAR data taken in Davis Strait with similar imagery collected in the Bering Sea. Ice core samples from the Bering test area offer a basis for speculation on changes in ice morphology which affect the signature return at the ice edge, and help explain the difficulty of the sensors in discerning the two different ice types found on the photography and in the core samples.

Mcnutt, S. L.↗
NASA NTRS1982

Comparison of LANDSAT-2 and field spectrometer reflectance signatures of south Texas rangeland plant communities

The accuracy was assessed for an atmospheric correction method that depends on clear water bodies to infer solar and atmospheric parameters for radiative transfer equations by measuring the reflectance signature of four prominent south Texas rangeland plants with the LANDSAT satellite multispectral scanner (MSS) and a ground based spectroradiometer. The rangeland plant reflectances produced by the two sensors were correlated with no significant deviation of the slope from unity or of the intercept from zero. These results indicated that the atmospheric correction produced LANDSAT MSS estimates of rangeland plant reflectances that are as accurate as the ground based spectroradiometer.

Richardson, A. J.↗
NASA NTRS1982

The effects of seasonal differences in climatic conditions on Landsat spectral signatures and associated land cover classification

Unsupervised classification algorithms are used to analyze Landsat computer-compatible tape data for an area of approximately 840 sq km in central Oklahoma, over the period from June 12 to August 4, 1979. The results obtained show that changes in remotely sensed spectral signatures and land cover classes are associated with a period of transition from moisture availability in late spring to moisture deficit in midsummer, with the latter being marked by greater visible spectrum reflectance and greater near-IR absorption, although each surface cover type has responded differently to the seasonal change in water availability. Consideration of these results has led to the identification of important factors in the use of multidate satellite data in environmental change monitoring. Naturally induced trends in surface albedo introduce noise into studies aimed at identifying anthropogenic land cover change. Specific problems associated with prairie-forest ecotonal areas in the southern Great Plains involve the seasonally induced differences in separability of forest, bush, and grassland cover types.

Harrington, J. A., Jr.↗
NASA NTRS1984

Flight measurement and analysis of AAFE RADSCAT wind speed signature of the ocean

The advanced aerospace flight experiment radiometer scatterometer (AAFE RADSCAT) which was developed as a research tool to evaluate the use of microwave frequency remote sensors to provide wind speed information at the ocean surface is discussed. The AAFE RADSCAT helped establish the feasibility of the satellite scatterometer for measuring both wind speed and direction. The most important function of the AAFE RADSCAT was to provide a data base of ocean normalized radar cross section (NRCS) measurements as a function of surface wind vector at 13.9 GHz. The NRCS measurements over a wide parametric range of incidence angles, azimuth angles, and winds were obtained in a series of RADSCAT aircraft missions. The obtained data base was used to model the relationship between k sub u band radar signature and ocean surface wind vector. The models developed therefrom are compared with those used for inversion of the SEASAT-A satellite scatterometer (SASS) radar measurements to wind speeds.

Schroeder, L. C.↗
NASA NTRS1984

Modeling of SAR signatures of shallow water ocean topography

A hydrodynamic/electromagnetic model was developed to explain and quantify the relationship between the SEASAT synthetic aperture radar (SAR) observed signatures and the bottom topography of the ocean in the English Channel region of the North Sea. The model uses environmental data and radar system parameters as inputs and predicts SAR-observed backscatter changes over topographic changes in the ocean floor. The model results compare favorably with the actual SEASAT SAR observed backscatter values. The developed model is valid for only relatively shallow water areas (i.e., less than 50 meters in depth) and suggests that for bottom features to be visible on SAR imagery, a moderate to high velocity current and a moderate wind must be present.

Shuchman, R. A.↗