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At least 1,189 records · Page 66
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NASA NTRS1992

The effect of topography on radar scattering from vegetated areas

It is shown how radar scattering from vegetated areas is affected by the topography of the surface underneath the vegetation, and that by the use of a discrete scatterer model the dominant scattering mechanism may change drastically when the ground surface is tilted relative to the horizontal. In the case of a horizontal ground surface total scattering may be dominated by scattering off the tree trunks, followed by a reflection off the ground surface. It is shown that for a relatively small tilt in the ground surface the ground-trunk interaction term may be replaced by scattering from the branches alone as the dominant scattering mechanism. The effect of the topography is more pronounced for scattering by longer wavelengths. The implications for algorithms designed to infer forest woody biomass and soil and vegetation moisture using polarimetric synthetic aperture radar (SAR) data are discussed.

Van Zyl, Jakob J.↗
NASA NTRS1990

CLIPS enhanced with objects, backward chaining and explanation facilities

In this project we extend C Language Production System (CLIPS), an existing Expert System shell, by creating three new options. Specifically, first we create a compatible with CLIPS environment that allows for defining objects and object hierarchies, second we provide means to implement backward chaining in a pure forward chaining environment, and finally we give some simple explanation facilities for the derivations the system has made. Objects and object hierarchies are extended so that facts can be automatically inferred, and placed in the fact base. Backward chaining is implemented by creating run time data structures which hold the derivation process allowing for a depth first search. The backward chaining mechanism works not only with ground facts, but also creates bindings for every query that involves variables, and returns the truth value of such a query as well as the relevant variable bindings. Finally, the WHY and HOW explanation facilities allow for a complete examination of the derivation process, the rules triggered, and the bindings created. The entire system is integrated with the original CLIPS code, and all of its routines can be invoked as CLIPS commands.

Aldrobi, M.↗
NASA NTRS1999

A Heuristic Approach to Examining Volatile Equilibrium at Titan's Surface

R. D. Lorenz, J. I. Lunine, and C. P. McKay have shown in a manuscript accepted for publication that, for a given ethane abundance and surface temperature, the nitrogen and methane abundances in Titan's atmosphere can be calculated, yielding a surface pressure that can be compared with the observed value. This is potentially a very valuable tool for examining the evolution of Titan's climatology. Its validity does depend on two important assumptions, however: 1) that the atmosphere of Titan is in global radiative equilibrium, and 2) that volatiles present are in vapor equilibrium with the surface. The former assumption has been shown to be likely, but the latter has not. Water vapor in the Earth's atmosphere, in fact, is generally not very close to equilibrium in a global sense. In the present work a heuristic approach is used to examine the likelihood that methane vapor is in equilibrium with Titan's surface. Plausible climate scenerios are examined that are consistent with methane vapor abundances derived from Voyager IRIS data. Simple precipitation and surface diffusion models are incorporated into the analysis. It is tentatively inferred that methane may be in surface equilibrium near the poles, but that equilibrium at low latitudes is more difficult to establish.

Samuelson, Robert E.↗
NASA NTRS2001

The Effect of Changes in the ASCA Calibration on the Fe-K Lines in Active Galaxies

The ASCA calibration has evolved considerably since launch and indeed, is still evolving. There have been concerns in the literature that changes in the ASCA calibration have resulted in the Fe-K lines in active galaxies (AGN) now being systematically narrower than was originally thought. If this were true, a large body of ASCA results would be impacted. In particular, it has been claimed that the broad red wing (when present) of the Fe-K line has been considerably weakened by changes in the ASCA calibration. We demonstrate explicitly that changes in the, ASCA calibration over a period of about eight years have a negligible effect on the width, strength, or shape of the Fe-K lines. The reduction in both width and equivalent width is only approximately 8% or less. We confirm this with simulations and individual sources, as well as sample average profiles. The average profile for type 1 AGN is still very broad, with the red wing extending down to approximately 4 keV. The reason for the claimed, apparently large, discrepancies is that in some sources the Fe-K line is complex, and a single-Gaussian model, being an inadequate description of the line profile, picks up different portions of the profile with different calibration. However, one cannot make inferences about calibration or astrophysics of the sources using models which do not describe the data. Better modeling of the Fe-K in such cases gives completely consistent results with both old and current calibration. Thus, inadequate modeling of the Fe-K line in these sources can seriously underestimate the line width and equivalent width, and therefore lead to incorrect deductions about the astrophysical implications.

Yaqoob, T.↗
NASA NTRS2004

Retrieval of Marine Water Constituents Using Atmospherically Corrected AVIRIS Hyperspectral Data

This paper reports on the validation of bio-optical models in estuarine and nearshore (case 2) waters of New Jersey-New York to retrieve accurate water-leaving radiance spectra and chlorophyll concentration from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) imaging spectrometer data. MODTRAN-4 was applied to remove the effects of the atmosphere so as to infer the water-leaving radiance. The study area - Hudson/Raritan of New York and New Jersey (Figure 1) is an extremely complex estuarine system where tidal and wind-driven currents are modified by freshwater discharges from the Hudson, Raritan, Hackensack, and Passaic rivers. Over the last century, the estuarine water quality has degraded in part due to eutrophication, which has disrupted the preexisting natural balance, resulting in phytoplankton blooms of both increased frequency and intensity, increasing oxygen demand, and leading to episodes of hypoxia. As the end result, a thematic map of chlorophyll-a concentration was generated using an atmospherically corrected AVIRIS ratio image. This thematic map serves as an indication of phytoplankton concentration. Such maps are important input into the geographic information system (GIS) for use as a management tool for monitoring water resources.

Bagheri, Sima↗
NASA NTRS2005

Global, Multi-Year Analysis of Clouds and Earth's Radiant Energy System Terra Observations and Radiative Transfer Calculations

An extended record of the Terra Surface and Atmosphere Radiation Budget (SARB) computed by CERES (Clouds and Earth s Radiant Energy System) is produced in gridded form, facilitating an investigation of global scale direct aerosol forcing. The new gridded version (dubbed FSW) has a spacing of 1 at the Equator. A companion document (Rutan et al. 2005) focuses on advances to (and validation of) the ungridded, footprint scale calculations (dubbed CRS), primarily in clear-sky conditions. While mainly intended to provide observations of fluxes at the top of atmosphere (TOA), CERES (Wielicki et al. 1996) includes a program to also compute the fluxes at TOA, within the atmosphere and at the surface, and also to validate the results with independent ground based measurements (Charlock and Alberta 1996). ARM surface data has been a focus for this component of CERES. To permit the user to infer cloud forcing and direct aerosol forcing with the computed SARB, CERES includes surface and TOA fluxes that have been computed for cloud-free (clear) and aerosol free (pristine) footprints; this accounts for aerosol effects (SW scattering and absorption, and LW scattering, absorption and emission) to both clear and cloudy skies.

Charlock, T. P.↗
NASA NTRS2007

Ring-Down Spectroscopy for Characterizing a CW Raman Laser

.A relatively simple technique for characterizing an all-resonant intracavity continuous-wave (CW) solid-state Raman laser involves the use of ring-down spectroscopy. As used here, characterizing signifies determining such parameters as threshold pump power, Raman gain, conversion efficiency, and quality factors (Q values) of the pump and Stokes cavity modes. Heretofore, in order to characterize resonant-cavity-based Raman lasers, it has usually been necessary to manipulate the frequencies and power levels of pump lasers and, in each case, to take several sets of measurements. In cases involving ultra-high-Q resonators, it also has been desirable to lock pump lasers to resonator modes to ensure the quality of measurement data. Simpler techniques could be useful. In the present ring-down spectroscopic technique, one infers the parameters of interest from the decay of the laser out of its steady state. This technique does not require changing the power or frequency of the pump laser or locking the pump laser to the resonator mode. The technique is based on a theoretical analysis of what happens when the pump laser is abruptly switched off after the Raman generation reaches the steady state. The analysis starts with differential equations for the evolution of the amplitudes of the pump and Stokes electric fields, leading to solutions for the power levels of the pump and Stokes fields as functions of time and of the aforementioned parameters. Among other things, these solutions show how the ring-down time depends, to some extent, on the electromagnetic energy accumulated in the cavity. The solutions are readily converted to relatively simple equations for the parameters as functions of quantities that can be determined from measurements of the time-dependent power levels. For example, the steady-state intracavity conversion efficiency is given by G1/G2 1 and the threshold power is given by Pin(G2/G1)2, where Pin is the steady-state input pump power immediately prior to abrupt switch-off, G1 is the initial rate of decay of the pump field, and G2 is the final rate of decay of the pump field. Hence, it is possible to determine all the parameters from a single ring-down scan, provided that the measurements taken in that scan are sufficiently accurate and complete.

Matsko, Andrey↗
NASA NTRS2010

Evaluation of Satellite-Based Upper Troposphere Cloud Top Height Retrievals in Multilayer Cloud Conditions During TC4

Upper troposphere cloud top heights (CTHs), restricted to cloud top pressures (CTPs) less than 500 hPa, inferred using four satellite retrieval methods applied to Twelfth Geostationary Operational Environmental Satellite (GOES-12) data are evaluated using measurements during the July August 2007 Tropical Composition, Cloud and Climate Coupling Experiment (TC4). The four methods are the single-layer CO2-absorption technique (SCO2AT), a modified CO2-absorption technique (MCO2AT) developed for improving both single-layered and multilayered cloud retrievals, a standard version of the Visible Infrared Solar-infrared Split-window Technique (old VISST), and a new version of VISST (new VISST) recently developed to improve cloud property retrievals. They are evaluated by comparing with ER-2 aircraft-based Cloud Physics Lidar (CPL) data taken during 9 days having extensive upper troposphere cirrus, anvil, and convective clouds. Compared to the 89% coverage by upper tropospheric clouds detected by the CPL, the SCO2AT, MCO2AT, old VISST, and new VISST retrieved CTPs less than 500 hPa in 76, 76, 69, and 74% of the matched pixels, respectively. Most of the differences are due to subvisible and optically thin cirrus clouds occurring near the tropopause that were detected only by the CPL. The mean upper tropospheric CTHs for the 9 days are 14.2 (+/- 2.1) km from the CPL and 10.7 (+/- 2.1), 12.1 (+/- 1.6), 9.7 (+/- 2.9), and 11.4 (+/- 2.8) km from the SCO2AT, MCO2AT, old VISST, and new VISST, respectively. Compared to the CPL, the MCO2AT CTHs had the smallest mean biases for semitransparent high clouds in both single-layered and multilayered situations whereas the new VISST CTHs had the smallest mean biases when upper clouds were opaque and optically thick. The biases for all techniques increased with increasing numbers of cloud layers. The transparency of the upper layer clouds tends to increase with the numbers of cloud layers.

Chang, Fu-Lung↗
NASA NTRS2005

Magnetic Variations Associated With Solar Flares

A report summarizes an investigation of helioseismic waves and magnetic variations associated with solar flares, involving analysis of data acquired by the Michelson Doppler Imager (MDI) aboard the Solar and Heliocentric Observatory (SOHO) spacecraft, the Yohkoh spacecraft, and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. Reconstruction of x-ray flare images from RHESSI data and comparison of them with MDI magnetic maps were performed in an attempt to infer the changes in the geometry of the magnetic field. It was established that in most flares observed with MDI, downward propagating shocks were much weaker than was one observed in the July 9, 1996 flare, which caused a strong helioseismic response. It was concluded that most of the observed impulsive variations result from direct impact of high-energy particles. Computer codes were developed for further study of these phenomena.

Petrosian, Vahe↗
NASA NTRS1961

Morphology of Magnetic Storms

This publication is a product of the continuing study of the properties of charged particles and fields in space being conducted by The RAND Corporation under contract No. NAS5-276 for the National Aeronautics and Space Administration. Magnetic storms, revealed by world-wide changes in the intensity of the earth's magnetic field, and emphasized by disturbances in electromagnetic communication channels, form detectable patterns on the surface of the earth and above it. The author draws together data from various times, places, and altitudes and, coupling these with what is known or inferred about the aurora, the ionosphere, and the relationship between them and the earth's radiation belts, creates a picture of what is believed to occur during a magnetic storm.

Vestine, E. H.↗
NASA NTRSDate not supplied

Effects of Porous Gap Fillers on Leading-Edge Slat Noise of 30P30N. Part II: PIV Measurements

The leading edge slat of a high-lift system is one of the main contributors to the airframe noise during approach conditions. This paper, the second of two parts, continues our previous studies on the slat gap filler as a passive noise control device on the two-dimensional, 30P30N multielement airfoil. Whereas the earlier study was focused on the effects of an impermeable gap filler that completely blocks the flow through the gap, this follow-on assessment is devoted to permeable slat gap fillers that allow limited amounts of flow to pass through the gap. Part I of this two-part abstract described the aerodynamic and acoustic effects of the permeable gap fillers, as inferred from both the measurements of static and unsteady surface pressures and the microphone array data for the radiated noise. To understand the physical mechanism responsible for the noise reduction documented in Part I, as well as for the accompanying aerodynamic penalty due to the porous gap fillers, Particle Image Velocimetry (PIV) is used in this paper to measure the flow details in the slat-cove region of the 30P30N model. A single angle of attack (𝛼 = 5.5◦) and a chord based Reynolds number of 1.71𝑒6 are selected as the test conditions. The PIV results show that the slat flow features are significantly altered with the presence of the porous gap filler, resulting in a more stable slat-cove shear layer and, thus, reduced velocity fluctuations with a successive decrease in the permeability. The porous gap filler with the lowest permeability acts similar to the solid gap filler. However, flow separation is observed on the upper side of the porous interface, which leads to an aerodynamic performance penalty via a reduction in lift on the main wing.

slat noise↗
NASA NTRS2022

Multidecadal Trends in Ozone Chemistry in the Baltimore-Washington Region

Over the past four decades, policy-led reductions in anthropogenic emissions have improved air quality over the Baltimore-Washington region (BWR). Most of the improvements in meeting the ozone air quality metrics (NAAQS) did not occur until the early 2000s despite large reductions in ozone precursors (NOx, CO, and volatile organic compounds (VOCs)) in the prior decades. We use observations of ozone and ozone precursors from satellites, ground-based sites, and the 2011 DISCOVER-AQ aircraft campaign in Maryland to illustrate how ozone chemistry in the BWR evolved between 1972 and 2019. Analysis of weekday vs weekend probability of ozone exceedance indicates the BWR transitioned to the NOx-limited regime by 2000–2003. A data-constrained box model agrees with this transition period and illustrates the key roles of reduced emissions of formaldehyde (HCHO), aromatics, and other VOCs since 1996, which reduced the peak of ozone production at the time of the transition and likely prevented the BWR from experiencing worsening surface air quality as the region transitioned to NOx-limited chemistry. Analysis of satellite observations of tropospheric column HCHO to NO2 analyzed using a new approach for evaluation of chemical regimes derived from DISCOVER-AQ data also provide a consistent depiction of the timing of the transition period that we infer from ground-based observations and the box model. Finally, despite significant improvements in air quality over the past two decades, the BWR still has not met the EPA standard for surface ozone. With predominantly NOx-limited ozone chemistry over the BWR, continued decreases in emission of NOx will slow the rate of ozone production and help improve air quality. We highlight emissions of NO2 from the diesel truck fleet as a worthwhile focus for future policy because emissions from this source appear to influence day-of-week variations in observed NO2, with an accompanying effect on ozone.

Ozone↗
NASA NTRS1993

The upper clouds of Venus: Determination of the scale height from NIMS-Galileo infrared data

The 3-5 micrometer thermal emission of the nightside of Venus, recorded by the Near-Infrared Mapping Spectrometer (NIMS) instrument at the time of the Galileo flyby of Venus, is analysed to infer the properties of the upper cloud boundary. From the global maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By using the nominal Pioneer Venus thermal profile, these data give access to two parameters: the cloud deck temperature and the cloud scale height. It is verified independently, from the NIMS spectra, that this thermal profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (25 deg S, 30 deg N). Within this range, the cloud scale height is found to be constant with latitude, and is H = 5.2 km, with an accuracy of about 15%, taking into account the various sources of theoretical and observational uncertainties. At higher latitudes, the temperature profile becomes more isothermal and the presented method to retrieve H is no longer valid.

Roos, M.↗
NASA NTRS1989

Carrier phase ambiguity resolution for the Global Positioning System applied to geodetic baselines up to 2000 km

A technique for resolving the ambiguities in the GPS carrier phase data (which are biased by an integer number of cycles) is described which can be applied to geodetic baselines up to 2000 km in length and can be used with dual-frequency P code receivers. The results of such application demonstrated that a factor of 3 improvement in baseline accuracy could be obtained, giving centimeter-level agreement with coordinates inferred by very-long-baseline interferometry in the western United States. It was found that a method using pseudorange data is more reliable than one using ionospheric constraints for baselines longer than 200 km. It is recommended that future GPS networks have a wide spectrum of baseline lengths (ranging from baselines shorter than 100 km to those longer than 1000 km) and that GPS receivers be used which can acquire dual-frequency P code data.

Blewitt, Geoffrey↗
NASA NTRS2013

Smarter Earth Science Data System

The explosive growth in Earth observational data in the recent decade demands a better method of interoperability across heterogeneous systems. The Earth science data system community has mastered the art in storing large volume of observational data, but it is still unclear how this traditional method scale over time as we are entering the age of Big Data. Indexed search solutions such as Apache Solr (Smiley and Pugh, 2011) provides fast, scalable search via keyword or phases without any reasoning or inference. The modern search solutions such as Googles Knowledge Graph (Singhal, 2012) and Microsoft Bing, all utilize semantic reasoning to improve its accuracy in searches. The Earth science user community is demanding for an intelligent solution to help them finding the right data for their researches. The Ontological System for Context Artifacts and Resources (OSCAR) (Huang et al., 2012), was created in response to the DARPA Adaptive Vehicle Make (AVM) programs need for an intelligent context models management system to empower its terrain simulation subsystem. The core component of OSCAR is the Environmental Context Ontology (ECO) is built using the Semantic Web for Earth and Environmental Terminology (SWEET) (Raskin and Pan, 2005). This paper presents the current data archival methodology within a NASA Earth science data centers and discuss using semantic web to improve the way we capture and serve data to our users.

data center↗
NASA NTRS1989

Stratospheric effects of solar ultraviolet variations on the solar rotation time scale

A summary is presented of some current work on measurement and interpretation of stratospheric ozone and temperature responses to observed short term solar ultraviolet variations. Although some studies have yielded provisional evidence for a nearly in-phase ozone-solar cycle relationship, they extend at most over only one or two 11 year cycles so the statistical significance of the correlations is not large. Similarly, the relatively short lengths of individual satellite data sets combined with the problem of estimating the effect of changes in instrument sensitivity (drift) during the observing period have complicated attempts to infer long term or solar cycle ozone trends. The solar rotation and active region development time scale provides an alternate time scale for which detailed studies of middle atmospheric ozone and temperature responses to solar ultraviolet variability are currently possible using available satellite data sets. At tropical latitudes where planetary wave amplitudes are relatively small, clear correlative evidence for the existence of middle atmospheric ozone and temperature responses to short term solar ultraviolet variations has been obtained in recent years. These measurements will ultimately allow improved empirical and theoretical calculations of longer term solar induced ozone and temperature variations at low and middle latitudes.

Hood, L. L.↗
NASA NTRS2019

New Estimates of Ice and Oxygen Fluxes Across the Entire Lid of Lake Vostok From Observations of Englacial Radio Wave Attenuation

Over deep Antarctic subglacial lakes, spatially varying ice thickness and the pressure‐dependent melting point of ice result in distinct areas of melting and freeze‐on at the ice‐water interface, that is, at the lake lid. These ice mass fluxes drive lake circulation and, because basal Antarctic ice contains air clathrate, affect the input of oxygen to the lake, with implications for subglacial life. Inferences of rates of melting and freeze‐on, that is, accretion rates, from radar layer tracking and geodesy are limited in spatial coverage and resolution. Here we develop a new method to estimate accretion rate, and the resulting oxygen input at a lake lid, using airborne radar data over Lake Vostok together with ice‐temperature and chemistry data from the Vostok ice core. Because the lake lid is a coherent reflector of known reflectivity (at our radar frequency), we can infer depth‐averaged radio wave attenuation in the ice, with a spatial resolution of ~1 km along flight lines. Spatial variation in attenuation depends mostly on variation in ice temperature near the lid, which in turn varies strongly with ice mass flux at the lid. We model ice temperature versus depth with ice mass flux as a parameter, thus linking that flux to observed depth‐averaged attenuation. The resulting map of melt and accretion rates independently reproduces features known from earlier studies but now covers the entire lid. We find that freeze‐on is dominant when integrated over the lid, with an ice imbalance of 0.05 to 0.07 cu.km/year, which is robust against uncertainties.

Dale P. Winebrenner↗
NASA NTRS1995

ULF waves in the foreshock

We review our current knowledge of ULF waves in planetary foreshocks. Most of this knowledge comes from observations taken within a few Earth radii of the terrestrial bow shock. Terrestrial foreshock ULF waves can be divided into three types, large amplitude low frequency waves (approximately 30-s period), upstream propagating whistlers (1-Hz waves), and 3-s waves. The 30-s waves are apparently generated by back-streaming ion beams, while the 1-Hz waves are generated at the bow shock. The source of the 3-s waves has yet to be determined. In addition to issues concerning the source of ULF waves in the foreshock, the waves present a number of challenges, both in terms of data acquisition, and comparison with theory. The various waves have different coherence scales, from approximately 100 km to approximately 1 Earth radius. Thus multi-spacecraft separation strategies must be tailored to the phenomenon of interest. From a theoretical point of view, the ULF waves are observed in a plasma in which the thermal pressure is comparable to the magnetic pressure, and the rest-frame wave frequency can be moderate fraction of the proton gyro-frequency. This requires the use of kinetic plasma wave dispersion relations, rather than multi-fluid MHD. Lastly, and perhaps most significantly, ULF waves are used to probe the ambient plasma, with inferences being drawn concerning the types of energetic ion distributions within the foreshock. However, since most of the data were acquired close to the bow shock, the properties of the more distant foreshock have to be deduced mainly through extrapolation of the near-shock results. A general understanding of the wave and plasma populations within the foreshock, their interrelation, and evolution, requires additional data from the more distant foreshock.

Greenstadt, E. W.↗