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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Utilization of vegetation indices to improve microwave soil moisture estimates over agricultural lands

A technique is presented by means of which visible/near-IR data are used to develop corrections in remotely sensed microwave soil moisture signals, in order to account for vegetation effects. Visible/IR data collected with the NASA NS001 Thematic Mapper Simulator were used to calculate the Perpendicular Vegetation Index (PVI), which was then related to the change of sensitivity of the microwave measurement to surface soil moisture. Effective estimation of soil moisture in the presence of vegetation can be made with L-band microwave radiometers and visible/IR sensors when the PVI is lower than 4.3. This technique offers a means for the estimation of moisture from a space platform over many agricultural areas, without expensive ground data collection.

Theis, S. W.↗
NASA NTRS1984

Measurements of energy distribution in microwave plasmas of N2 and He and comparisons with results for H2

An electrothermal propulsion concept utilizing a microwave plasma system as the mechanism to convert electromagnetic energy into translational energy of the flowing gas is being investigated. Specifically, this study compares the energy transfer characteristics of three different gases, H2, N2, and He, to gain some insight as to the dominant energy transfer processes present in a microwave plasma. A calorimetric experimental system has been designed and built enclosing the microwave plasma system to accurately determine the net energy transferred to the flowing gas. Results are obtained for N2 and He discharges and compared with previously reported experimental results for H2.

Chapman, R.↗
NASA NTRS1984

Recent work on a microwave ion source

The performance of a microwave (2.45 GHz) plasma-disk ion source using a cylindrical microwave cavity is described. The operating characteristics in argon and xenon gases with 50-200 W of input power and gas flow rates from 10-80 sccm are presented. In particular, extracted beam current versus accelerating voltage, and specific energy vs. mass utilization efficiency and extracted beam current are presented. Double Langmuir probe measurements indicate that electron densities in excess of 10 to the 12th per cu cm can be readily achieved in the microwave generated plasma.

Asmussen, J.↗
NASA NTRS1983

Atmospheric water distribution in cyclones as seen with Scanning Multichannel Microwave Radiometers (SMMR)

Passive microwave measurements are used to study the distribution of atmospheric water in midlatitude cyclones. The integrated water vapor, integrated liquid water, and rainfall rate are deduced from the brightness temperatures at microwave frequencies measured by the Scanning Multichannel Microwave Radiometer (SMRR) flown on both the Seasat and Nimbus 7 satellites. The practical application of locating fronts by the cyclone moisture pattern over oceans is shown, and the relationship between the quantity of coastal rainfall and atmospheric water content is explored.

Katsaros, K. B.↗
NASA NTRS1985

Microwave and X-ray observations of delayed brightenings at sites remote from the primary flare locations

Five examples of solar flares observed with the 17-GHz interferometer at Nobeyama in which a secondary microwave burst occurred at a distance of 100,000 km to 1,000,000 km from the primary flare site are presented. The secondary microwave burst in all five cases had a similar time profile to the primary burst with a delay of 2 to 25 s. The velocity of a triggering agent inferred from this delay and spatial separation is 10,000 km to 100,000 km/s. The intensity of the secondary burst was a factor of 3 to 25 smaller than that of the primary burst in all events except for one case in which it was a factor of 2 larger. The polarization degree of the secondary burst at 17 GHz was 35 percent, significantly higher than the average value for typical impulsive bursts. Two of the events were accompanied by meterwave type III/V bursts located high in the corona between the primary and secondary sites. For two of the other events, X-ray images of the secondary source were obtained with the hard-X-ray imaging spectrometer on the Solar Maximum Mission. These observations strongly suggest that the distant microwave bursts were produced by electrons with energies of 10 keV to 100 keV which were channeled along a huge loop from the main flare site to the remote location.

Nakajima, H.↗
NASA NTRS1984

Comments on 'Inference of cloud temperature and thickness by microwave radiometry from space'

The method proposed by Pandey et al. (1983) for estimating the temperature differential and thickness of clouds from microwave data obtained with the scanning multichannel microwave radiometers of the Seasat and Nimbus-7 satellites is examined critically. It is pointed out that both the thicknesses and the temperature differentials derived from them may not be meaningful unless accurate measurements of cloud-top height (from IR radiometry) and reliable data on liquid-water content are available. It is suggested that the good fits obtained in generating regression coefficients for the proposed method may be artifacts of the fixed or limited-range liquid-water densities of the cloud models used. With respect to cloud-top height, the need to quantify and account for differences in the fields of view and spatial resolutions of the IR and microwave radiometers, as undertaken for the case of precipitating clouds by Yeh and Liou (1983), is stressed.

Yeh, H.-Y. M.↗
NASA NTRS1985

Temporal variations of the microwave signatures of sea ice during the late spring and early summer near Mould Bay, NWT

It has been shown that passive microwave imagery obtained from satellite-borne sensors provides an important basis for the study of the polar regions. Because of the optical thinness of high-latitude clouds at microwave frequencies, radiometry can provide all-weather all-time observing capability. However, in order to clarify observational uncertainties and investigate the information content of passive microwave imagery, detailed ground-based observations are needed. Multifrequency data are also required to utilize the strong spectral dependence of both the dielectric properties of liquid water and volume scattering. The present investigation has the aim to provide information of the considered type for the calibration and interpretation of satellite observations of the Arctic during the summer season. Attention is given to instruments and calibration, the field program and the state of the ice cover, and the results.

Grenfell, T. C.↗
NASA NTRS1985

Measuring the global distribution of intense convection over land with passive microwave radiometry

The global distribution of intense convective activity over land is shown to be measurable with satellite passive-microwave methods through a comparison of an empirical rain rate algorithm with a climatology of thunderstorm days for the months of June-August. With the 18 and 37 GHz channels of the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR), the strong volume scattering effects of precipitation can be measured. Even though a single frequency (37 GHz) is responsive to the scattering signature, two frequencies are needed to remove most of the effect that variations in thermometric temperatures and soil moisture have on the brightness temperatures. Because snow cover is also a volume scatterer of microwave energy at these microwavelengths, a discrimination procedure involving four of the SMMR channels is employed to separate the rain and snow classes, based upon their differences in average thermometric temperature.

Spencer, R. W.↗
NASA NTRS1985

Active Microwave Properties of Vegetation Canopies

Potential users of radar imagery need a better fundamental understanding of the capabilities of radar systems for vegetation studies than past studies provide. One approach is the use of theoretical models to predict observable active microwave properties of vegetation. This in turn requires accurate observations of backscattering coefficients and other active microwave properties in field research studies. The background document for the SRAEC program emphasizes the need to relate electromagnetic parameters to classical biophysical descriptors and to understand the role of polarization, especially cross-polarization. The broad goal of this study is to increase the understanding of the effects of canopy structure on the active microwave properties of vegetation canopies, with particular attention to polarization.

Paris, J. F.↗
NASA NTRS1985

Microwave electric propulsion for orbit transfer applications

A Microwave Electric Propulsion (MEP) concept is developed for an unmanned Orbit Transfer Vehicle (OTV). The concept is based on the use of beamed microwave power and on an extrapolation of ion thruster technology. Beamed microwave power transmission is discussed in terms of its relationship to spacecraft propulsion. Characteristics of an MEP OTV are determined and performance is evaluated by comparison to a variety of alternative propulsion systems for the completion of a ten-year mission model.

Sercel, J. C.↗
NASA NTRS1986

High-spectral resolution solar microwave observations

The application of high-spectral resolution microwave observations to the study of solar activity is discussed with particular emphasis on the frequency dependence of microwave emission from solar active regions. A shell model of gyroresonance emission from active regions is described which suggest that high-spectral resolution, spatially-resolved observations can provide quantitative information about the magnetic field distribution at the base of the corona. Corresponding observations of a single sunspot with the Owens Valley frequency-agile interferometer at 56 frequencies between 1.2 and 14 Ghs are presented. The overall form of the observed size and brightness temperature spectra was consistent with expectations based on the shell model, although there were differences of potential physical significance. The merits and weaknesses of microwave spectroscopy as a technique for measuring magnetic fields in the solar corona are briefly discussed.

Hurford, G. J.↗
NASA NTRS1985

Emission, absorption and group delay of microwaves in the atmosphere in relation to water vapour content over the Indian subcontinent

The advent of satellite communication for global coverage has apparently indicated a renewed interest in the studies of radio wave propagation through the atmosphere, in the VHF, UHF and microwave bands. The extensive measurements of atmosphere constituents, dynamics and radio meterological parameters during the Middle Atmosphere Program (MAP) have opened up further the possibilities of studying tropospheric radio wave propagation parameters, relevant to Earth/space link design. The three basic parameters of significance to radio propagation are thermal emission, absorption and group delay of the atmosphere, all of which are controlled largely by the water vapor content in the atmosphere, particular at microwave bands. As good emitters are also good absorbers, the atmospheric emission as well as the absorption attains a maximum at the frequency of 22.235 GHz, which is the peak of the water vapor line. The group delay is practically independent of frequency in the VHF, UHF and microwave bands. However, all three parameters exhibit a similar seasonal dependence originating presumably from the seasonal dependence of the water vapor content. Some of the interesting results obtained from analyses of radiosonde data over the Indian subcontinent collected by the India Meteorological Department is presented.

Sen, A. K.↗
NASA NTRS1985

Earth to space dc to dc power transmission system utilizing a microwave beam as source of energy for electric propelled interorbital vehicles

The paper contributes to the credibility of an electric propelled interorbital transportation system by introducing a new low-mass source of continuous dc power for electric propulsion and illustrating how the source can be economically tied to an electric utility on earth by an electronically steered microwave beam. The new thin-film rectenna, which functions as the receiving end of an earth-to-space microwave power transmission system is described. It is easily fabricated, is over 80 percent efficient, has a specific mass of no more than 2 kilograms per kilowatt of continuous dc power output, and is well adapted for deployment in space. The paper then describes a complete system consisting of the interorbital vehicle and the microwave power transmission system that supplies it with power. A design scenario is used to obtain performance data from the system in terms of vehicle transfer times, payload fractions, and costs. Electric energy costs are found to be less than $1000 per kilogram of payload delivered to geosynchronous orbit from low-earth orbit.

Brown, W. C.↗
NASA NTRS1985

Microwave imaging of a solar limb flare - Comparison of spectra and spatial geometry with hard X-rays

A solar limb flare was mapped using the Very Large Array (VLA) together with hard X-ray (HXR) spectral and spatial observations of the Solar Maximum Mission satellite. Microwave flux records from 2.8 to 19.6 GHz were instrumental in determining the burst spectrum, which has a maximum at 10 GHz. The flux spectrum and area of the burst sources were used to determine the number of electrons producing gyrosynchrotron emission, magnetic field strength, and the energy distribution of gyrosynchrotron-emitting electrons. Applying the thick target model to the HXR spectrum, the number of high energy electrons responsible for the X-ray bursts was found to be 10 to the 36th, and the electron energy distribution was approximately E exp -5, significantly different from the parameters derived from the microwave observations. The HXR imaging observations exhibit some similiarities in size and structure o the first two burst sources mapped with the VLA. However, during the initial burst, the HXR source was single and lower in the corona than the double 6 cm source. The observations are explained in terms of a single loop with an isotropic high-energy electron distribution which produced the microwaves, and a larger beamed component which produced the HXR at the feet of the loop.

Schmahl, E. J.↗
NASA NTRS1986

The SIR-B observations of microwave backscatter dependence on soil moisture, surface roughness, and vegetation covers

An experiment was conducted from an L-band SAR aboard Space Shuttle Challenger in October 1984 to study the microwave backscatter dependence on soil moisture, surface roughness, and vegetation cover. The results based on the analyses of an image obtained at 21-deg incidence angle show a positive correlatlion between scattering coefficient and soil moisture content, with a sensitivity comparable to that derived from the ground radar measurements reported by Ulaby et al. (1978). The surface roughness strongly affects the microwave backscatter. A factor of two change in the standard deviation of surface roughness height gives a corresponding change of about 8 dB in the scattering coefficient. The microwave backscatter also depends on the vegetation types. Under the dry soil conditions, the scattering coefficient is observed to change from about -24 dB for an alfalfa or lettuce field to about -17 dB for a mature corn field. These results suggest that observations with a SAR system of multiple frequencies and polarizations are required to unravel the effects of soil moisture, surface roughness, and vegetation cover.

Wang, J. R.↗
NASA NTRS1986

Interpretation of temporal features in an unusual X-ray and microwave bursts

Observations are briefly discussed of an event in which microwave and hard X-ray emissions were not correlated in the accepted way. Two impulsive peaks of roughly equal intensity were observed at three different microwave frequencies. The hard X-ray peaks accompanying these, however, differ in intensity by almost two orders of magnitude. Various possible interpretations of this burst are discussed, in the context of familiar models of these emissions. The most likely explanation is that the electron spectrum in the first burst has a break at about 350 keV. General implications for interpretation of X-rays and microwaves are discussed.

Mackinnon, A. L.↗
NASA NTRS1986

High-spatial-resolution microwave and related observations as diagnostics of coronal loops

High spatial resolution microwave observations of coronal loops, together with theoretical models for the loop emission, can provide detailed information about the temperature, density, and magnetic field within the loop, as well as the environment around the loop. The capability for studying magnetic fields is particularly important, since there is no comparable method for obtaining direct information about coronal magnetic fields. Knowledge of the magnetic field strength and structure in coronal loops is important for understanding both coronal heating and flares. With arc-second-resolution microwave observations from the Very Large Array (VLA), supplemental high-spectral-resolution microwave data from a facility such as the Owens Valley frequency-agile interferometer, and the ability to obtain second-of-arc resolution EUV aor soft X ray images, the capability already exists for obtaining much more detailed information about coronal plasma and magnetic structures than is presently available. This capability is discussed.

Holman, Gordon D.↗
NASA NTRS1986

Theoretical models of free-free microwave emission from solar magnetic loops

The free-free microwave emission is calculated from a series of model magnetic loops. The loops are surrounded by a cooler external plasma, as required by recent simultaneous X ray and microwave observations, and a narrow transition zone separating the loops from the external plasma. To be consistent with the observational results, upper limits on the density and temperature scale lengths in the transition zone are found to be 360 km and 250 km, respectively. The models which best produce agreement with X ray and microwave observations also yielded emission measure curves which agree well with observational emission measure curves for solar active regions.

Brosius, Jeffrey W.↗