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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Multi-band reflector antenna with double-ring element frequency selective subreflector

Frequency selective subreflectors (FSS) are often employed in the reflector antenna system of a communication satellite or a deep space exploration vehicle for multi-frequency operations. In the past, FSS's have been designed for diplexing two frequency bands. For example, the Voyager FSS was designed to diplex S and X bands and the TDRSS FSS was designed to diplex S and Ku bands. Recently, NASA's CASSINI project requires an FSS to multiplex four frequency (S/X/Ku/Ka) bands. Theoretical analysis and experimental verifications are presented for a multi-band flat pannel FSS with double-ring elements. Both the exact formulation and the thin-ring approximation are described for analyzing and designing this multi-ring patch element FSS. It is found that the thin-ring approximation fails to predict the electrically wide ring element FSS's performance. A single screen double-ring element FSS is demonstrated for the tri-band system that reflects the X-band signal while transmitting through the S- and Ku-band signals. In addition, a double screen FSS with non-similar double-ring elements is presented for the Cassini's four-band system which reflects the X- and Ka-band signals while passing the S- and Ku-band signals. To accurately predict the FSS effects on a dual reflector antenna's radiation pattern, the FSS subreflector's transmitted/reflected field variation as functions of the polarization and incident angles with respect to the local coordinates was taken into account. An FSS transmission/reflection coefficient table is computed for TE and TM polarizations at various incident angles based on the planar FSS model. Next, the hybrid Geometric Optics (GO) and Physical Optics (PO) technique is implemented with linearly interpolating the FSS table to efficiently determine the FSS effects in a dual reflector antenna.

Wu, Te-Kao↗
NASA NTRS1987

Measuring and modeling the backscattering cross section of a leaf

Leaves are a significant feature of any vegetation canopy, and for remote sensing purposes it is important to develop an effective model for predicting the scattering from a leaf. From measurements of the X band backscattering cross section of a coleus leaf in varying stages of dryness, it is shown that a uniform resistive sheet constitutes such a model for a planar leaf. The scattering is determined by the (complex) resistivity which is, in turn, entirely specified by the gravimetric moisture content of the leaf. Using an available asymptotic expression for the scattering from a rectangular resistive plate which includes, as a special case, a metallic plate whose resistivity is zero, the computed backscattering cross sections for both principal polarizations are found to be in excellent agreement with data measured for rectangular sections of leaves with different moisture contents. If the resistivity is sufficiently large, the asymptotic expressions do not differ significantly from the physical optics ones, and for naturally shaped leaves as well as rectangular sections, the physical optics approximation in conjunction with the resistive sheet model faithfully reproduces the dominant feataures of the scattering patterns under all moisture conditions.

Senior, T. B. A.↗
NASA NTRS1987

Analysis of a Four-Reflector S/X-Band Antenna

Physical optics accounts for near field, cross polarization, and higher-order modes. Report presents physical-optics analysis of four-reflector, 64-m antennas of Deep Space Network. Analysis thorough and detailed. Report has instructional value as example for designers of large microwave dishes with subreflectors and involving reflector surfaces with hyperboloidal, paraboloidal, ellipsoidal, and more complex shapes.

Cha, Alan G.↗
NASA NTRS2005

Laboratory Investigations of the Physical and Optical Properties of the Analogs of Individual Cosmic Dust Grains

Microdsub-micron size cosmic dust grains play an important role in the physical and dynamical process in the galaxy, the interstellar medium, and the interplanetary and planetary environments. The dust grains in various astrophysical environments are generally charged by a variety of mechanisms that include collisional process with electrons and ions, and photoelectric emissions with UV radiation. The photoelectric emission process is believed to be the dominant process in many astrophysical environments with nearby UV sources, such as the interstellar medium, diffuse clouds, the outer regions of the dense molecular clouds, interplanetary medium, dust in planetary environments and rings, cometary tails, etc. Also, the processes and mechanisms involved in the rotation and alignment of interstellar dust grains are of great interest in view of the polarization of observed starlight as a probe for evaluation of the galactic magnetic field.

Abbas, M. M.↗
NASA NTRS1981

Scattering of waves from periodic surfaces

In order to study the scattering of waves from periodic surfaces, the basic grating equations are reviewed, and a general approach for scattering from impenetrable and penetrable media is formulated. Three analytical methods for scattering on a conducting sinusoidal surface for both TE and TM polarized waves are compared. In The Masel, Merrill, and Miller (MMM) method for quantum scattering of atoms (1975, 1976), the surface field expansions are expressed in terms of Fourier series. The Modified Physical Optics (MPO) method (DeSanto, 1975; Whitman and Schwering, 1977) uses surface field expansions consisting of a leading term proportional to the physical optics approximation multiplied by a Fourier series expansion. The Waterman's Plane Harmonics (WPH) approach (1975) makes use of basis functions which are downward plane harmonics evaluated on the surface. The MMM method proved to be the most efficient one in terms of rate and range of convergence. For dielectric media with periodic rough surfaces, an improved method is developed for calculating the reflected and transmitted powers, and the results are compared with experimental data obtained at optical frequencies.

Chuang, S.-L.↗
NASA NTRS1972

A numerical study of electromagnetic scattering from ocean like surfaces

The integral equations describing electromagnetic scattering from one dimensional conducting surfaces are formulated and numerical results are presented. The results are compared with those obtained using approximate methods such as physical optics, geometrical optics, and perturbation theory. The integral equation solutions show that the surface radius of curvature must be greater than 2.5 wavelengths for either the physical optics or geometric optics to give satisfactory results. It has also been shown that perturbation theory agrees with the exact fields as long as the root mean square surface roughness is less than one-tenth of a wavelength.

Lentz, R. R.↗
NASA NTRS1974

A numerical study of electromagnetic scattering from ocean-like surfaces

The integral equations describing electromagnetic scattering from cylindrical, perfectly conducting surfaces are formulated and numerical results are presented. The results are compared with those obtained using approximate methods such as physical optics, geometrical optics, and perturbation theory. The integral equation solutions show that the surface radius of curvature must be greater than 2.5 wavelengths for either the physical optics or geometrical optics to give satisfactory results. It has also been shown that perturbation theory agrees with the exact fields as long as the root-mean-square surface roughness is less than one tenth of a wavelength.

Lentz, R. R.↗
DOE OSTI2023

Influence of oxygen on the optical and electrical properties of magnetron-sputtered indium tin oxide thin films at ambient temperature

Indium Tin Oxide (ITO) thin films are commonly used as transparent conductive layers in the production of solar cells. ITO films are typically heat-treated at temperatures exceeding 200 °C after magnetron sputtering to optimize the physical, optical, and electrical characteristics for photovoltaic devices. Here, this high temperature heating procedure greatly limits the use of potential substrates, particularly those comprised of low-temperature-rated polymers. Herein, we examine the physical, optical, and electrical transport effects of oxygen:argon plasmas during radio frequency magnetron sputtering of ITO on glass and flexible polymer surfaces without subsequent heat treatment. ITO thin films were characterized via sheet resistance testing, UV-Vis spectroscopy, X-ray diffraction, and scanning electron microscopy imaging. Sheet resistance measurements revealed an increase in surface resistance with increasing oxygen concentration during sputtering. UV-Vis optical transmittance measurements demonstrated an improvement in optical transmittance with an increase in oxygen concentration reaching a maximum transmittance at an oxygen concentration of 1-2 %. Most importantly, it was discovered that producing bilayers of ITO with pure argon sputtered plasmas followed by oxygen:argon plasmas produced ITO thin films with suitable resistance and transmittance without the need for high temperature post annealing. These results highlight a methodology for the deposition of ITO thin films on temperature sensitive substrates for flexible photovoltaics and low-cost module production.

36 MATERIALS SCIENCE↗
NASA NTRS1979

A regression technique for evaluation and quantification for water quality parameters from remote sensing data

The paper attempts to define optical physics and/or environmental conditions under which the linear multiple-regression should be applicable. It is reported that investigation of the signal response shows that the exact solution for a number of optical physics conditions is of the same form as a linearized multiple-regression equation, even if nonlinear contributions from surface reflections, atmospheric constituents, or other water pollutants are included. Limitations on achieving this type of solution are defined. Laboratory data are used to demonstrate that the technique is applicable to water mixtures which contain constituents with both linear and nonlinear radiance gradients. Finally, it is concluded that instrument noise, ground-truth placement, and time lapse between remote sensor overpass and water sample operations are serious barriers to successful use of the technique.

Whitlock, C. H.↗
NASA NTRS1991

High frequency scattering from corrugated stratified cylinders

Interest in applying radar remote sensing for the study of forested areas led to the development of a model for scattering from corrugated stratified dielectric cylinders. The model is used to investigate the effect of bark and its roughness on scattering from tree trunks and branches. The outer layer of the cylinder (bark) is assumed to be a low-loss dielectric material and to have a regular (periodic) corrugation pattern. The inner layers are treated as lossy dielectrics with smooth boundaries. A hybrid solution based on the moment method and the physical optics approximation is obtained. In the solution, the corrugations are replaced with polarization currents that are identical to those of the local tangential periodic corrugated surface, and the stratified cylinder is replaced with equivalent surface currents. New expressions for the equivalent physical-optics currents are used which are more convenient than the standard ones. It is shown that the bark layer and its roughness both reduce the radar cross-section. It is also demonstrated that the corrugations can be replaced by an equivalent anisotropic layer.

Sarabandi, Kamal↗
NASA NTRS2002

The Opposition Effect: A Very Unusual Case

The reflection of electromagnetic radiation from a planetary regolith involves a combination of geometric and physical optics processes which contribute to the signal returned to the remote observer. The geometric optics effects are the product of singly and multiply scattered radiation from the surfaces of the regolith particles, combined with radiation which has undergone various combinations of transmission through one or more regolith grains followed by one or more scatterings from other particles. The physical optics effects include diffraction of radiation around the edges of large irregular particles and cooperative coherent scattering between particles which are small when compared to the wavelength of the incident radiation. These effects produce measurable changes in the intensity and polarization of reflected light as a function of illumination and viewing geometry. In particular, as phase angle becomes small, the reflectance of a particulate material will increase non-linearly and exhibit the 'opposition effect'. In the planetary science context, the phase curve, and in particular the size of the opposition surge and the width of the phase curve near zero degrees, have been attributed to two processes commonly called 'shadow hiding' (SHOE) and 'coherent backscattering' (CBOE).

Nelson, R. M.↗
NASA NTRS1990

Analysis of blended rolled edge reflectors using numerical UTD

The uniform geometrical theory of diffraction (UTD) concept is used to predict the scattered fields in the target zones of compact-range blended rolled-edge reflectors. Since the necessary diffraction coefficients are not known in a closed form, a numerical method to calculate the diffraction coefficients is described. In the numerical method, the problem is reduced to two dimensions, and physical optics line integration is used to compute the diffraction coefficients. Thus, the method is computationally efficient. The method is used to analyze two compact-range reflectors. The results obtained using the numerical UTD show good agreement with the scattered fields obtained using a corrected physical optics surface integration.

Ellingson, Steven W.↗
NASA NTRS1979

A regression technique for evaluation and quantification for water quality parameters from remote sensing data

The objective of this paper is to define optical physics and/or environmental conditions under which the linear multiple-regression should be applicable. An investigation of the signal-response equations is conducted and the concept is tested by application to actual remote sensing data from a laboratory experiment performed under controlled conditions. Investigation of the signal-response equations shows that the exact solution for a number of optical physics conditions is of the same form as a linearized multiple-regression equation, even if nonlinear contributions from surface reflections, atmospheric constituents, or other water pollutants are included. Limitations on achieving this type of solution are defined.

Whitlock, C. H.↗
NASA NTRS2023

The Extreme Forest Fires in California/Oregon in 2020: Aerosol Optical and Physical Properties and Comparisons of Aged Versus Fresh Smoke

Wildfire activity in the western United States during August to October 2020 was exceptional in terms of the fire severity and area burned. Extremely dry biomass fuels from near historic multi-year drought conditions were further exacerbated with very hot and dry conditions in 2020. These conditions when coupled with strong offshore flow allowed many ignitions to grow into extremely large and severe wildfires. Long-term monitoring at a few AERONET sites in California showed that the number of days with high Aerosol Optical Depth at 440 nm (AOD 440 >1) in 2020 was greater than any other year going back to the beginning of the data records in 2002. A wide range of fine mode particle volume median radii were retrieved from AERONET data over the course of these fires suggesting significant variability in combustion conditions and aging processes. Additionally, the fine mode radii in some of these smoke plumes in 2020 were very large especially at high AOD (∼0.22–0.32 μm volume median radius), likely due to both coagulation and condensation occurring during aging at very high particulate concentrations. The largest fine mode particle radii combined with narrow distributions resulted in some very rare AOD spectra showing peak AOD at 500 nm and decreasing to lower AOD at both shorter and longer wavelengths. The most extreme retrieved size distributions and associated measured AOD spectra were principally observed in long-distance transported smoke plumes from these western United States fires at sites in Colorado, Maryland and Virginia, possibly due to further aging during transport. Additionally, strong absorption was sometimes observed at short wavelengths with much lower single scattering albedo at 440 nm compared to 675 nm in some plumes consistent with significant brown carbon (BrC) and/or coated black carbon (BC) absorption in biomass burning particles. This strong spectral absorption signature observed at some California sites and dates remained similarly strong in some smoke plumes observed at some east coast sites in Maryland and Virginia, thereby suggesting that the lifetime of these particular BrC and/or coated BC absorbing species was greater than 5 days.

Aerosol↗
NASA NTRS1994

High-frequency techniques for RCS prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors, parts 1 and 2

Formulations for scattering from the coated plate and the coated dihedral corner reflector are included. A coated plate model based upon the Uniform Theory of Diffraction (UTD) for impedance wedges was presented in the last report. In order to resolve inaccuracies and discontinuities in the predicted patterns using the UTD-based model, an improved model that uses more accurate diffraction coefficients is presented. A Physical Optics (PO) model for the coated dihedral corner reflector is presented as an intermediary step in developing a high-frequency model for this structure. The PO model is based upon the reflection coefficients for a metal-backed lossy material. Preliminary PO results for the dihedral corner reflector suggest that, in addition to being much faster computationally, this model may be more accurate than existing moment method (MM) models. An improved Physical Optics (PO)/Equivalent Currents model for modeling the Radar Cross Section (RCS) of both square and triangular, perfectly conducting, trihedral corner reflectors is presented. The new model uses the PO approximation at each reflection for the first- and second-order reflection terms. For the third-order reflection terms, a Geometrical Optics (GO) approximation is used for the first reflection; and PO approximations are used for the remaining reflections. The previously reported model used GO for all reflections except the terminating reflection. Using PO for most of the reflections results in a computationally slower model because many integrations must be performed numerically, but the advantage is that the predicted RCS using the new model is much more accurate. Comparisons between the two PO models, Finite-Difference Time-Domain (FDTD) and experimental data are presented for validation of the new model.

Balanis, Constantine A.↗
DOE OSTI2024

Efficient end-to-end simulation of time-dependent coherent X-ray scattering experiments

Physical optics simulations for beamlines and experiments allow users to test experiment feasibility and optimize beamline settings ahead of beam time in order to optimize valuable beam time at synchrotron light sources like NSLS-II. Further, such simulations also help to develop and test experimental data processing methods and software in advance. The Synchrotron Radiation Workshop ( SRW ) software package supports such complex simulations. We demonstrate how recent developments in SRW significantly improve the efficiency of physical optics simulations, such as end-to-end simulations of time-dependent X-ray photon correlation spectroscopy experiments with partially coherent undulator radiation (UR). The molecular dynamics simulation code LAMMPS was chosen to model the sample: a solution of silica nanoparticles in water at room temperature. Real-space distributions of nanoparticles produced by LAMMPS were imported into SRW and used to simulate scattering patterns of partially coherent hard X-ray UR from such a sample at the detector. The partially coherent UR illuminating the sample can be represented by a set of orthogonal coherent modes obtained by simulation of emission and propagation of this radiation through the coherent hard X-ray (CHX) scattering beamline followed by a coherent-mode decomposition. GPU acceleration is added for several key functions of SRW used in propagation from sample to detector, further improving the speed of the calculations. The accuracy of this simulation is benchmarked by comparison with experimental data.

36 MATERIALS SCIENCE↗
NASA NTRS1980

Vector analogues of the Maggi-Rubinowicz theory of edge diffraction

The Maggi-Rubinowicz technique for scalar and electromagnetic fields is interpreted as a transformation of an integral over an open surface to a line integral around its rim. Maggi-Rubinowicz analogues are found for several vector physical optics representations. For diffraction from a circular aperture, a numerical comparison between these formulations shows the two methods are in agreement. To circumvent certain convergence difficulties in the Maggi-Rubinowicz integrals that occur as the observer approaches the shadow boundary, a variable mesh integration is used. For the examples considered, where the ratio of the aperture diameter to wavelength is about ten, the Maggi-Rubinowicz formulation yields an 8 to 10 fold decrease in computation time relative to the physical optics formulation.

Meneghini, R.↗
NASA NTRS2016

Optical and Physical Methods for Mapping Flooding with Satellite Imagery

Flood and surface water mapping is becoming increasingly necessary, as extreme flooding events worldwide can damage crop yields and contribute to billions of dollars economic damages as well as social effects including fatalities and destroyed communities (Xaio et al. 2004; Kwak et al. 2015; Mueller et al. 2016).Utilizing earth observing satellite data to map standing water from space is indispensable to flood mapping for disaster response, mitigation, prevention, and warning (McFeeters 1996; Brakenridge and Anderson 2006). Since the early 1970s(Landsat, USGS 2013), researchers have been able to remotely sense surface processes such as extreme flood events to help offset some of these problems. Researchers have demonstrated countless methods and modifications of those methods to help increase knowledge of areas at risk and areas that are flooded using remote sensing data from optical and radar systems, as well as free publically available and costly commercial datasets.

satellite imagery↗