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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 253 records · Page 14

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NASA NTRS1983

Physical aging in graphite/epoxy composites

Sub-Tg annealing has been found to affect the properties of graphite/epoxy composites. The network epoxy studied was based on the chemistry of tetraglycidyl 4,4'-diamino-diphenyl methane (TGDDM) crosslinked by 4,4'-diamino-diphenyl sulfone (DDS). Differential scanning calorimetry, thermal mechanical analysis, and solid-state cross-polarized magic-angle-spinning nuclear magnetic resonance spectroscopy have been utilized in order to characterize this process of recovery towards thermodynamic equilibrium. The volume and enthalpy recovery as well as the 'thermoreversibility' aspects of the physical aging are discussed. This nonequilibrium and time-dependent behavior of network epoxies are considered in view of the increasingly wide applications of TGDDM-DDS epoxies as matrix materials of structural composites in the aerospace industry.

Kong, E. S. W.↗

NASA NTRS1982

Broadcasting satellite feeder links - Characteristics and planning

The paper presents the results of recent studies by the Feeder Link Sub-Working Group of the FCC Advisory Committee for the 1983 Regional Administrative Radio Conference (RARC). These studies conclude that specification of a few key parameters will make feeder link planning relatively straightforward. Feeder links can be located anywhere within a country if satellite orbit locations are separated by 10 deg for adjacent service areas and key parameter values presented in the paper are adopted. Colocated satellites serving a common service area need special attention to attain sufficient isolation between a desired channel and its adjacent cross-polarized channels and alternate co-polarized channels. In addition to presenting planning conclusions by the Advisory Committee, the paper presents and analyzes actions of the International Radio Consultative Committee's Conference Planning Meeting (CPM) concerning feeder links. The CPM reached conclusions similar to, and compatible with, those of the Advisory Committee.

Kiebler, J. W.↗

NASA NTRS1984

Active microwave responses - An aid in improved crop classification

A study determined the feasibility of using visible, infrared, and active microwave data to classify agricultural crops such as corn, sorghum, alfalfa, wheat stubble, millet, shortgrass pasture and bare soil. Visible through microwave data were collected by instruments on board the NASA C-130 aircraft over 40 agricultural fields near Guymon, OK in 1978 and Dalhart, TX in 1980. Results from stepwise and discriminant analysis techniques indicated 4.75 GHz, 1.6 GHz, and 0.4 GHz cross-polarized microwave frequencies were the microwave frequencies most sensitive to crop type differences. Inclusion of microwave data in visible and infrared classification models improved classification accuracy from 73 percent to 92 percent. Despite the results, further studies are needed during different growth stages to validate the visible, infrared, and active microwave responses to vegetation.

Rosenthal, W. D.↗

NASA NTRS1984

Worst-month rain attenuation and XPD statistics for satellite paths at 12 GHz

The SIRIO 11.6-GHz beacon has been used to obtain worst-month attenuation and cross-polarization discrimination (XPD) data during a three-year measurement program at Blacksburg, Virginia. The ratio of worst month to annual percentage occurrence levels for the attenuation and XPD data are noted to be in close agreement with each other and with the CCIR model.

Yon, K. M.↗

NASA NTRS1984

Mobile spectrometer measures radar backscatter

The present article is concerned with a helicopter-borne spectrometer (Heloscat), which has been developed to permit high-quality scattering measurements from a mobile platform at remote sites. The term 'spectrometer' referes to a class of scatterometers. The term 'scatterometer' is employed to denote a specialized radar for measuring scattering coefficients as a function of angle. A spectrometer, on the other hand, is a scatterometer which can measure backscatter at several frequencies. The Heloscat system is discussed, taking into account two antennas, RF hardware, and an externally mounted pendulum for angle encoding. A dual-antenna configuration is used for cross-polarized measurements, while a single-antenna system is used for like-polarized measurements. Attention is also given to oscillator characteristics, efficient data handling, and aspects of calibration.

Gogineni, S.↗

NASA NTRS1984

Spaceborne distributed SAR antenna

The MSAR c-band array panel was developed and tested. Each element of the antenna system is discussed individually as well as collectively. Textual descriptions together with test data are both used to enable a clear understanding of the antenna system performance. In general, test data has confirmed or exceeded expectations. Of particular note is the excellent cross polarization achieved with a field cancelling geometry.

Mckenna, D. B.↗

NASA NTRS1985

A study of backscattering and emission from closely packed inhomogeneous media

The effects of close spacing between small scattering spheres were examined by keeping the distance-dependent terms in the expressions for the transverse scattered fields. The phase matrix was then derived from these fields and was used in the radiative transfer formulation to model scattering and emission from a densely populated, inhomogeneous layer. Computed results were compared with those obtained when the phase matrix was specialized to the far-field condition. It was found that the use of the far-zone condition tended to underestimate both the level of the copolarized backscattering and the cross-polarized backscattering. In emission computations, the use of the far-zone condition overestimated the level of the brightness temperature. These effects decreased with a decrease in the volume fraction or an increase in the exploring frequency, as expected. An improvement on the snow parameter (density and crystal size) estimation was shown to be possible when this new phase matrix was used.

Fung, A. K.↗

NASA NTRS1986

Mutual coupling effects in antenna arrays, volume 1

Mutual coupling between rectangular apertures in a finite antenna array, in an infinite ground plane, is analyzed using the vector potential approach. The method of moments is used to solve the equations that result from setting the tangential magnetic fields across each aperture equal. The approximation uses a set of vector potential model functions to solve for equivalent magnetic currents. A computer program was written to carry out this analysis and the resulting currents were used to determine the co- and cross-polarized far zone radiation patterns. Numerical results for various arrays using several modes in the approximation are presented. Results for one and two aperture arrays are compared against published data to check on the agreement of this model with previous work. Computer derived results are also compared against experimental results to test the accuracy of the model. These tests of the accuracy of the program showed that it yields valid data.

Collin, R. E.↗

NASA NTRS1986

Physical optics analysis of a four-reflector antenna, part 1

Concern has been raised for the 64-m to 70-m antenna upgrade project that the 70-m system may experience greater S-band beam-pointing perturbations than the 64-m system. The S-band perturbations are due to minor (higher order) mode generation, causing subtle cross-polarization fields affecting beam pointing direction, as described herein. For the antennas in their present configuration (64 m), a slight S-band gain degradation of about 0.05 dB can be attributed to these effects. Therefore, a full physical optics analysis was performed for the present-day 64-m system, as described herein. The results were compared with past analyses and experimental observations in order to verify the algebra and computer code with the intent of deriving a valid analysis method for accurately analyzing the 70-m shaped dual reflector Cassegrainian antenna. The results of the new analysis appear to be in excellent agreement with previous analyses and experimental data.

Cha, A. G.↗

NASA NTRS1986

Interstellar grain mantles

Interstellar molecular grain mantles are an important component of the interstellar dust inside dense molecular clouds as evidenced by the detection of absorption bands at 2.97, 3.08, 4.61, 6.0 and 6.8 microns. Mantles may also be the precursors of more complex grain mantles in the diffuse interstellar medium. The molecular composition of these icy grain mantles were calculated employing gas phase as well as grain surface reactions. The calculated mixtures consist mainly of the molecules H2O, H2CO, N2, CO, O2, H2O2, NH2, and their deuterated counterparts in varying ratios. The exact compositions depend strongly on the physical conditions in the gas phase. The absorption spectra of H2O with other molecules was studied in the laboratory. Optical constants were determined for a few selected mixtures. Extinction and polarization cross sections across the 3 micron ice band were calculated. A comparison with the observations towards BN shows that the low frequency wing observed on this feature is due to absorption by a mixture of H2O and other molecules rather than scattering by large, pure H2O ice grains.

Bregman, J.↗

NASA NTRS1985

Active microwave remote sensing of an anisotropic random medium layer

A two-layer anisotropic random medium model has been developed to study the active remote sensing of the earth. The dyadic Green's function for a two-layer anisotropic medium is developed and used in conjunction with the first-order Born approximation to calculate the backscattering coefficients. It is shown that strong cross-polarization occurs in the single scattering process and is indispensable in the interpretation of radar measurements of sea ice at different frequencies, polarizations, and viewing angles. The effects of anisotropy on the angular responses of backscattering coefficients are also illustrated.

Lee, J. K.↗

NASA NTRS1986

Results of a three-year 11.6 GHz low-angle propagation experiment using the SIRIO satellite

The results of three years of attenuation and depolarization measurements at 11.6 GHz on a 10.7 deg elevation angle downlink from the SIRIO spacecraft are summarized. Cumulative distributions and worst-month statistics for rain rate, attenuation, and cross-polarization discrimination (XPD) are presented. Beacon attenuation and radiometric attenuation are compared, and the statistics of fade durations and of rapid XPD changes are discussed.

Bostian, C. W.↗

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 NTRS1986

The SIR-C ground data system: Digital processor, data products, information flow

The SIR-C (Shuttle Imaging Radar) instrument will collect both C-Band and L-Band data with each frequency band consisting of direct (HH or VV) and cross-polarized (HV or VH) data. Considering all possible combinations as many as eight different data channels will be available for any given target area. This data will be digitized and formatted on-board for direct downlink via the Tracking and Data Relay Satellite System (TDRSS), or it will be buffered through on-board high density digital recorders for storage or transmission when TDRSS is available. The data is received by the TDRSS ground station at White Sands and is nominally relayed via DOMSAT to the high data rate recording facility at GSFC. The tapes are then shipped to JPL for processing into imagery and eventual distribution to the SIR-C investigators.

Curlander, John C.↗

NASA NTRS1986

Physical optics analysis of a four-reflector antenna

Cha, A. G.↗

NASA NTRS1986

20-GHz phased-array-fed antennas utilizing distributed MMIC modules

The feasibility of phased-array-fed dual-reflector systems with distributed power and phase control, and utilizing monolithic microwave integrated circuit modules, is demonstrated. Secondary radiation patterns for various antenna configurations, calculated using a method in which the phased array for each scanning direction is simulated by a fictitious point source, are computed to determine the achievable EIRP levels, sidelobe isolation, and cross-polarization isolation. The focal-region-fed Cassegrain reflector was found to be best suited for fixed multiple beam applications, while the phased-array-fed dual-reflector configuration was selected for multiple scanning beams. Key elements of the phased-array design including a radiating square horn and a square orthomode transducer were fabricated and tested.

Sorbello, R. M.↗

NASA NTRS1987

Forest biomass, canopy structure, and species composition relationships with multipolarization L-band synthetic aperture radar data

The effect of forest biomass, canopy structure, and species composition on L-band synthetic aperature radar data at 44 southern Mississippi bottomland hardwood and pine-hardwood forest sites was investigated. Cross-polarization mean digital values for pine forests were significantly correlated with green weight biomass and stand structure. Multiple linear regression with five forest structure variables provided a better integrated measure of canopy roughness and produced highly significant correlation coefficients for hardwood forests using HV/VV ratio only. Differences in biomass levels and canopy structure, including branching patterns and vertical canopy stratification, were important sources of volume scatter affecting multipolarization radar data. Standardized correction techniques and calibration of aircraft data, in addition to development of canopy models, are recommended for future investigations of forest biomass and structure using synthetic aperture radar.

Sader, Steven A.↗

NASA NTRS1987

Nonuniform sampling techniques for antenna applications

A two-dimensional sampling technique, which can employ irregularly spaced samples (amplitude and phase) in order to generate the complete far-field patterns is presented. The technique implements a matrix inversion algorithm, which depends only on the nonuniform sampled data point locations and with no dependence on the actual field values at these points. A powerful simulation algorithm is presented to allow a real-life simulation of many reflector/feed configurations and to determine the usefulness of the nonuniform sampling technique for the copolar and cross-polar patterns. Additionally, an overlapped window concept and a generalized error simulation model are discussed to identify the stability of the technique for recovering the field data among the nonuniform sampled data. Numerical results are tailored for the pattern reconstruction of a 20-m offset reflector antenna operating at L-band. This reflector is planned to be used in a proposed measurement concept of large antenna aboard the Space Shuttle, whereby it would be almost impractical to accurately control the movement of the Shuttle with respect to the RF source in prescribed directions in order to generate uniform sampled points. Also, application of the nonuniform sampling technique to patterns obtained using near-field measured data is demonstrated. Finally, results of an actual far-field measurement are presented for the construction of patterns of a reflector antenna from a set of nonuniformly distributed measured amplitude and phase data.

Rahmat-Samii, Yahya↗