SEARCH · Engineering Papers

Results for “Microwave”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,387 records · Page 77

All sources NASA NTRS DOE OSTI

NASA NTRS1990

Impulsive microwave burst and solar noise storm emission resolved with the VLA

The evolution of a microwave burst at 20.7 cm wavelength and a type I noise storm at 91.6 cm wavelength are examined using VLA. The magnetic loops in the two spectral regions are studied. The sizes and brightness temperatures of the 20.7 cm burst sources are compared with those predicted by multithermal and nonthermal models of microwave burst emission of Dulk and Dennis (1982). The data reveal that: (1) the precursor, impulsive, and post-impulsive phases of the 20.7 cm burst are located in spatially separated sources; (2) the gradual enhancement of a 91.6 cm noise storm continuum source suggests a feed-back mechanism exists between activity in higher and lower lying loops; and (3) the derived magnetic field strengths at 20.7 cm for the nonthermal models are 35-160 G with core field strengths of 125-220 G.

Willson, Robert F.↗

NASA NTRS1990

Bragg scattering of electromagnetic waves by microwave-produced plasma layers

A set of parallel plasma layers is generated by two intersecting microwave pulses in a chamber containing dry air at a pressure comparable to the upper atmosphere. The dependencies of breakdown conditions on the pressure and pulse length are examined. The results are shown to be consistent with the appearance of tail erosion of the microwave pulse caused by air breakdown. A Bragg scattering experiment, using the plasma layers as a Bragg reflector, is then performed. Both time domain and frequency domain measurements of wave scattering are conducted. The experimental results are found to agree very well with the theory.

Kuo, S. P.↗

NASA NTRS1989

Passive microwave observations of asteroids

The advances made since 1979 in the quantity and quality of the radio observations of asteroids and in the understanding of the physics of asteroidal microwave emission are reviewed. Radio continuum spectra analyses are now available for the four largest asteroids (Ceres, Vesta, Pallas, and Hygiea) at several wavelengths and several smaller asteroids (including Interamnia and Eunomia) at one wavelength. The spectra show that most asteroids are covered by a layer of material with physical properties of finely divided dust. This surface material is in layers of variable depth and has dielectric properties which vary from asteroid to asteroid. The effect of instrumentation on the interpretation of microwave observations is examined.

Webster, William J., Jr.↗

NASA NTRS1990

Topology of microwave background fluctuations - Theory

Topological measures are used to characterize the microwave background temperature fluctuations produced by 'standard' scenarios (Gaussian) and by cosmic strings (non-Gaussian). Three topological quantities: total area of the excursion regions, total length, and total curvature (genus) of the isotemperature contours, are studied for simulated Gaussian microwave background anisotropy maps and then compared with those of the non-Gaussian anisotropy pattern produced by cosmic strings. In general, the temperature gradient field shows the non-Gaussian behavior of the string map more distinctively than the temperature field for all topology measures. The total contour length and the genus are found to be more sensitive to the existence of a stringy pattern than the usual temperature histogram. Situations when instrumental noise is superposed on the map, are considered to find the critical signal-to-noise ratio for which strings can be detected.

Gott, J. Richard, III↗

NASA NTRS1989

Empirical studies of the microwave radiometric response to rainfall in the tropics and midlatitudes

Results are presented from quantitative comparisons between satellite microwave radiometer observations and digital radar observations of equatorial convective cloud clusters and midlatitude frontal precipitation. Simultaneous data from the Winter Monsoon Experiment digital radar and the SMMR for December 1978 are analyzed. It is found that the most important differences between the microwave response to rainfall in the equatorial tropics and to stratiform rain in oceanic midlatitude fronts is caused by the different spatial characteristics of stratiform and convective rainfall and by the different background brightness temperature fields associated with tropical and midlatitude levels of atmospheric water vapor.

Petty, Grant W.↗

NASA NTRS1989

Passive microwave retrievals of rainfall and rain system structure

Consideration is given to the development of algorithms for the Tropical Rainfall Measuring Mission to retrieve rain and rain vertical structure from passive microwave radiometry. The self-consistency approach for surface rain rate retrieval of Kummerow et al. (1989) is applied to microwave brightness temperature data from the Cooperative Huntsville Meteorological Experiment (Dodge et al., 1986). Because the algorithm simultaneously retrieves rainfall rate and rain system structure, it is suggested that the algorithm can be used to overcome the problem of the dependence of brightness temperatures on the vertical profile of hydrometeors.

Kummerow, Christian↗

NASA NTRS1989

Cloud model-based simulations of satellite microwave data and their application to an SSM/I rain algorithm

Satellite microwave observations of convective rain systems are simulated using a three-dimensional cloud model and a microwave radiative-transfer model. The simulated data sets are then used as a basis to derive rain estimation algorithms which are tested on actual satellite data. A tropical simulation is discussed, and the algorithms are applied to SSM/I data.

Adler, Robert F.↗

NASA NTRS1990

Precipitation observed over the South China Sea by the Nimbus-7 Scanning Multichannel Microwave Radiometer during Winter Monex

Mesoscale cloud clusters near the northwestern coast of Borneo were observed by the Scanning Multichannel Microwave Radiometer (SMMR) on three occasions during the Winter Monsoon Experiment in December 1978. A nondimensional form of the SMMR 37 GHz polarization difference is introduced and used to identify regions of precipitation, and these are compared with visible and infrared imagery from the GMS-1 geostationary satellite. For two of the three cloud cluster cases, quantitative comparisons are made between nearly simultaneous SMMR observations and reflectivity observations made by the MIT WR-73 digital weather radar at Bintulu. Though limited in scope, these represent the first known direct comparisons between digital radar-derived rain parameters and satellite passive microwave observations of near-equatorial precipitation. SMMR 37 GHz observations are found to be much better indicators of fractional coverage of each SMMR footprint by rain than of average rain rate within the footprint. Total area coverage by precipitation is estimated for all three clusters using this result.

Petty, Grant W.↗

NASA NTRS1990

The secondary spectral component of solar microwave bursts

The multicomponent characteristics of bursts are discussed, with emphasis on the relationship between their primary and secondary components. It is shown that microwave bursts with complex spectra, consisting of two or more spectral components, can be divided into two classes. One class may result from bursts in which the microwave radiation is generated in two different gyrosynchrotron sources that evolve differently in time. The second class is characterized by a common temporal evolution of the spectral components, the similarity of the circular polarization of both components, and the commonality in the ratio of primary to secondary peak frequencies from event to event. These properties suggest that the two components originate in a common source or from individual sources that are strongly coupled. An additionally observed event of this class also suggests that the primary and secondary components have a similar location, but that the surface area of the secondary component is larger.

Stahli, M.↗

NASA NTRS1990

Microwave characterization of high-temperature superconducting thin films using stripline resonators

A method was derived for the extraction of the surface resistivity (Rs) of high-Tc superconducting (HTS) thin films at microwave frequencies, using a stripline resonator circuit. Surface resistivity values were calculated by measuring the difference in Q-values of a stripline resonator configuration when an HTS thin film was substituted for one of the metallic ground planes. Cryogenic tests were completed for several stripline resonator structures, yielding accurate predictions of the HTS thin-film performance at frequencies between 3 and 26 GHz (in harmonics of approximately 3 GHz). These results were then used to illustrate the dependence of HTS surface resistivity on frequency and temperature in the microwave frequency band.

Macdonald, Alan D.↗

NASA NTRS1990

A semiempirical model for interpreting microwave emission from semiarid land surfaces as seen from space

A radiative-transfer model for simulating microwave brightness temperatures over land surfaces is described. The model takes into account sensor viewing conditions (spacecraft altitude, viewing angle, frequency, and polarization) and atmospheric parameters over a soil surface characterized by its moisture, roughness, and temperature and covered with a layer of vegetation characterized by its temperature, water content, single scattering albedo, structure, and percent coverage. In order to reduce the influence of atmospheric and surface temperature effects, the brightness temperatures are expressed as polarization ratios that depend primarily on the soil moisture and roughness, canopy water content, and percentage of cover. The sensitivity of the polarization ratio to these parameters is investigated. Simulation of the temporal evolution of the microwave signal over semiarid areas in the African Sahel is presented and compared to actual satellite data from the SMMR instrument on Nimbus-7.

Kerr, Yann H.↗

NASA NTRS1990

Microwave radiative transfer studies using combined multiparameter radar and radiometer measurements during Cohmex

Theoretical calculations of the upwelling microwave radiances from clouds containing layers of rain, ice, and a melting region were performed at frequencies of 18, 37, and 92 GHz. These frequencies coincide with high-resolution microwave radiometer measurements taken aboard the NASA ER-2 high-altitude aircraft during the summer 1986 Cohmex (Cooperative Huntsville Meteorological Experiment) in Alabama. For purposes of brightness temperature computations, the storms were modeled with rain, melting phase, and ice layers. The melting phase region was composed of water-coated ice spheres defined by a 'melt index' in terms of the volume fraction of water. Single scatter albedo, scattering, and extinction coefficients were computed at the above frequencies as a function of the rain rate and melt index. In addition, multiparameter radar observations of the storm were mapped into a Cartesian space and averaged over regions comparable to the radiometer footprint.

Vivekanandan, J.↗

NASA NTRS1990

Arctic multiyear ice classification and summer ice cover using passive microwave satellite data

Passive microwave data collected by Nimbus 7 were used to classify and monitor the Arctic multilayer sea ice cover. Sea ice concentration maps during several summer minima are analyzed to obtain estimates of ice floes that survived summer, and the results are compared with multiyear-ice concentrations derived from these data by using an algorithm that assumes a certain emissivity for multiyear ice. The multiyear ice cover inferred from the winter data was found to be about 25 to 40 percent less than the summer ice-cover minimum, indicating that the multiyear ice cover in winter is inadequately represented by the passive microwave winter data and that a significant fraction of the Arctic multiyear ice floes exhibits a first-year ice signature.

Comiso, J. C.↗

NASA NTRS1990

COBE Differential Microwave Radiometers - Instrument design and implementation

Differential Microwave Radiometers (DMRs) at frequencies of 31.5, 53, and 90 GHz have been designed and built to map the large angular scale variations in the brightness temperature of the cosmic microwave background radiation. The instrument is being flown aboard NASA's Cosmic Background Explorer (COBE) satellite, launched on November 18, 1989. Each receiver input is switched between two antennas pointing 60 deg apart on the sky. The satellite is in near-polar orbit with the orbital plane precessing at 1 deg per day, causing the beams to scan the entire sky in 6 months. In 1 year of observation, the instruments are capable of mapping the sky to an rms sensitivity of 0.1 mK per 7 deg field of view. The mission and the instrument have been carefully designed to minimize the need for systematic corrections to the data.

Smoot, G.↗

NASA NTRS1991

Coating Solar Cells By Microwave Plasma Deposition

Antireflection films deposited on silicon solar cells at high production rates with microwave-enhanced plasma deposition. Microwave energy at frequency of 2.45 GHz generates plasma in mixture of gases, from which thin film of silicon nitride deposits on silicon substrates. Reaction temperature relatively low (only 250 degrees C), and film deposition rate more than 500 Angstrom/minute - 2 to 5 times faster. Quality of antireflection film similar to that produced by chemical-vapor deposition. Uses less power and consumes smaller quantities of gas. Species formed in plasma longer lived and dissociate reactants in region of chamber well away from plasma-generation region.

Minaee, Behrooz↗

NASA NTRS1991

Microwave Oscillator Would Have Reduced Phase Noise

Microwave oscillators of proposed new type incorporate suppressed-carrier/negative-feedback feature to reduce phase noise near their carrier frequencies. Concept results in phase noise less than achievable by cryogenically stabilized microwave components or by room-temperature oscillators stabilized by quartz crystals. Implemented in three different versions.

Dick, G. John↗

NASA NTRS1991

Spaceborne Microwave Imagers

Monograph presents comprehensive overview of science and technology of spaceborne microwave-imaging systems. Microwave images used as versatile orbiting, remote-sensing systems to investigate atmospheres and surfaces of planets. Detect surface objects through canopies of clouds, measure distributions of raindrops in clouds that their views penetrate, find meandering rivers in rain forests and underground water in arid regions, and provide information on ocean currents, wakes, ice/water boundaries, aircraft, ships, buoys, and bridges.

Stacey, J. M.↗

NASA NTRS1990

Radiometer system requirements for microwave remote sensing from satellites

An area of increasing interest is the establishment of a significant research program in microwave remote sensing from satellites, particularly geosynchronous satellites. Due to the relatively small resolution cell sizes, a severe requirement is placed on beam efficiency specifications for the radiometer antenna. Geostationary satellite microwave radiometers could continuously monitor several important geophysical parameters over the world's oceans. These parameters include the columnar content of atmospheric liquid water (both cloud and rain) and water vapor, air temperature profiles, and possibly sea surface temperature. Two principle features of performance are of concern. The first is the ability of the radiometer system to resolve absolute temperatures with a very small absolute error, a capability that depends on radiometer system stability, on frequency bandwidth, and on footprint dwell time. The second is the ability of the radiometer to resolve changes in temperature from one resolution cell to the next when these temperatures are subject to wide variation over the overall field-of-view of the instrument. Both of these features are involved in the use of the radiometer data to construct high-resolution temperature maps with high absolute accuracy.

Juang, Jeng-Nan↗