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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 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Ice depolarization of the Comstar beacon at 28.56 GHz during low fades and correlation with radar backscatter

Radar correlation with significant ice depolarization events accompanied by low copolarization fades of the 28.56-GHz Comstar beacon signal are described for an experimental program at Wallops Island, VA. Using a Faraday switch at the front end of the receiver, the copolarization and cross-polarization levels of the 28.56-GHz beacon signal are sequentially monitored. A nearby high resolution S-band radar pointing along the earth-satellite path monitors the simultaneous ice and rain reflectivity. For a first case considered, excellent correlation is noted between the cross-polarization events and relatively large and extended ice reflectivities along a segment of the earth-satellite path at altitudes near and above the 0 C isotherm. In a second case, the radar and receiver data strongly suggest the cross-polarization mechanism is due to a hailshaft which intersects the path at altitudes well below the 0 deg isotherm.

Goldhirsh, J.↗
NASA NTRS1983

C-band measurements of radar backscatter from ice project summary report

The ability to measure the radar scattering coefficient of ice with a helicopter or surface spectrometer was extended into the 4-8 GHz spectral region. The scattering coefficient was measured at Mould Bay, N.W.T., over a frequency range from 4 to 18 GHz for both summer and fall conditions. Scatter from fresh water ice in the St. Lawrence River and from numerous seasonal sea-ice types along the coast of Newfoundland were also measured. The C-band (near 5 GHz) scattering cross section for different types of ice shows poorer contrast than the scattering coefficient at higher frequencies, but better contrast than the negligible value found at L-band (1.5 GHz). At frequencies above 4 GHz the contrast in scattering coefficient between the different ice types is much less in summer than in other seasons; at most times of year the scattering is much stronger from multiyear than from other ice types, but in early summer it is actually slightly weaker than that from first year ice.

Onstott, R. G.↗
NASA NTRS1983

Pulse stuttering as a remedy for aliased ground backscatter

An algorithm that aides in the removal of ground scatter from low frequency Mesosphere, Stratosphere, Troposphere (MST) radar signals is examined. The unwanted ground scatter is shown as a sequence of velocity plots which are almost typical at the various altitudes. The interpulse period is changed in a cyclic way, thereby destroying the coherence of the unwanted signal. The interpulse period must be changed by an amount at least equal to the transmitted pulse width, and optimum performance is obtained when the number of different interpulse period occupies a time span greater than the coherence time of the unwanted signal. Since a 20-msec pulse width is used, it was found convenient to cycle through 50 pulses, the interpulse period changing from 2 msec to 3 msec during the 1/8-second time. This particular pattern of interpulse periods was provided by a software radar controller. With application of this algorithm, the unwanted scatter signal becomes incoherent from one pulse to the next, and therefore is perceived as noise by the coherent integrator and correlator.

Bowhill, S. A.↗
NASA NTRS1984

Optimum backscatter cross section of the ocean as measured by synthetic aperture radars

The interaction of the radar signals from Synthetic Aperture Radar (SAR) and Side Looking Airborne Radar (SLAR) is particularly important for the ocean surface where the radar modulation can yield information about the long ocean wave field. Radar modulation measurements from fixed platforms are made in wavetanks and the open oceans. The surfaces are described in terms of two scale models. The radar modulation is considered to be principally due to: (1) geometrical tilt due to the slope of the long ocean waves, and (2) the straining of the short waves (by hydrodynamic interaction). For application to moving platforms, this modulation needs to be described in terms of a general geometry for both like and cross polarization since the long ocean waves, in general, travel in arbitrary directions. The finite resolution of the radar is considered for tilt modulation with hydrodynamic effects neglected.

Bahar, E.↗
NASA NTRS1984

Development of a global model for atmospheric backscatter at CO2 wavelengths

The effect of gravitational sedimentation on the atmospheric aerosol size distribution has been simulated using a one layer numerical model. A nonlinear curve fitting program is being developed to study aerosol size parameters. The Sage/Sam 2 data establishing the geographical and temporal variations of aerosol concentration in the troposphere are being studied. The basic version of the Sam 2 program and the alternative version of the Sage 2 program are operational. Above 6 km, latitudinal differences are more important in determining aerosol concentration than surface conditions.

Source record↗
NASA NTRS1984

Development of a global model for atmospheric backscatter at CO2 wavelengths

Computer graphics software was developed to examine in detail the simulated variations of aerosol size distribution due to gravitational sedimentation effects. This examination contributes to understanding the effect of aerosol microphysical processes occurring in an aerosol plume which undergoes transport in the atmosphere. Results are presented in graphical form. Future research directions are outlined.

Source record↗
NASA NTRS1984

Development of a global model for atmospheric backscatter at CO2 wavelengths

Work on the following tasks is reported briefly: (1) to investigate the effect of aerosol microphysical processes, occurring in an aerosol plume which undergoes transport in the atmosphere, on its beta sub CO2 value and (2) to examine experimentally measured size distributions for the free troposphere, fit bimodal log-normal models with different retractive indices for the two modes, and calculate beta sub CO2.

Source record↗
NASA NTRS1984

NIMBUS-7 SBUV (Solar Backscatter Ultraviolet) observations of solar UV spectral irradiance variations caused by solar rotation and active-region evolution for the period November 7, 1978 - November 1, 1980

Observations of temporal variations of the solar UV spectral irradiance over several days to a few weeks in the 160-400 nm wavelength range are presented. Larger 28-day variations and a second episode of 13-day variations occurred during the second year of measurements. The thirteen day periodicity is not a harmonic of the 28-day periodicity. The 13-day periodicity dominates certain episodes of solar activity while others are dominated by 28-day periods accompanied by a week 14-day harmonic. Techniques for removing noise and long-term trends are described. Time series analysis results are presented for the Si II lines near 182 nm, the Al I continuum in the 190 nm to 205 nm range, the Mg I continuum in the 210 nm to 250 nm range, the MgII H & K lines at 280 nm, the Mg I line at 285 nm, and the Ca II K & H lines at 393 and 397 nm.

Heath, D. F.↗
NASA NTRS1984

A reexamination of soil textural effects on microwave emission and backscattering

Microwave frequency measurements of moist soil dielectric properties are noted to challenge the validity of percent-of-field-capacity as a moisture indicator that is independent of soil texture in terms of microwave sensitivity. In arriving at this view, gravimetric, volumetric, and percent-of-field-capacity were tested for their ability to reduce dielectric behavior divergence between soil textures at 1.4 and 5.0 GHz. The most congruent dielectric behavior between soil textures is found to occur when soil moisture is expressed on a volumetric basis that is proportional to the number of water dipoles/unit volume. An inadequate characterization of soil bulk density in the field, combined with the dependency of bulk density on water retention at field capacity, offers the most plausible explanation for the earlier conclusions.

Dobson, M. C.↗
NASA NTRS1985

The effect of vegetation type, microrelief, and incidence angle on radar backscatter

The NASA/JPL Synthetic Aperture Radar (SAR) was flown over a 20 x 110 km test site in the Texas High Plains regions north of Lubbock during February/March 1984. The effect of incidence angle was investigated by comparing the pixel values of the calibrated and uncalibrated images. Ten-pixel-wide transects along the entire azimuth were averaged in each of the two scenes, and plotted against the calculated incidence angle of the center of each range increment. It is evident from the graphs that both the magnitudes and patterns exhibited by the corresponding transect means of the two images are highly dissimilar. For each of the cross-poles, the uncalibrated image displayed very distinct and systematic positive trends through the entire range of incidence angles. The two like-poles, however, exhibited relatively constant returns. In the calibrated image, the cross-poles exhibited a constant return, while the like-poles demonstrated a strong negative trend across the range of look-angles, as might be expected.

Owe, M.↗
NASA NTRS1986

The effect of turbulence on lidar backscattered signal fluctuations

CO2 lidar measurements being subjected to uncertainties caused by strong fluctuations were explained to appear primarily due to speckle effects whereas turbulence was to be negligible. Spectral cross correlation measurements performed in the near IR and in the visible show that the main contribution to the fluctuation for the above lidar wavelengths originates from the atmospheric turbulence. The standard deviation of the fluctuation signal was measured for the two wavelengths simultaneously. A typical curve representing mid-summer day changes in the value of sigma as a function of time is shown. The results, indicating the strong dependence on the turbulence, is analyzed and discussed.

Fastig, S.↗
NASA NTRS1987

Backscatter from a periodic rough surface at near grazing incidence

The effect of periodic surface roughness on the radar cross section (RCS) was studied. The surface roughness was formed by a small sinusoidal variation in a planar surface. RCS measurements were obtained for two different sinusoidal variations near grazing incidence for both principle polarizations. Significant grating lobes were observed in the measurements which directly correspond to the roughness characteristics. A physical optics solution was generated and compared to the measurements with reasonable agreement.

Dominek, A. K.↗
NASA NTRS1989

Improvement and extension of a radar forest backscattering model

Radar modeling of mangal forest stands, in the Sundarbans area of Southern Bangladesh, was developed. The modeling employs radar system parameters with forest data on tree height, spacing, biomass, species combinations, and water (including slightly conductive water), content both in leaves and trunks of the mangal. For Sundri and Gewa tropical mangal forests, six model components are proposed, which are required to explain the contributions of various forest species combinations in the attenuation and scattering of mangal vegetated nonflooded or flooded surfaces. Statistical data of simulated images were compared with those of SIR-B images both to refine the modeling procedures and to appropriately characterize the model output. The possibility of delineation of flooded or nonflooded boundaries is discussed.

Simonett, David S.↗
NASA NTRS1989

Aerosol speckle effects on atmospheric pulsed lidar backscattered signals

For coherently pulsed lidar systems, the effects of refractive turbulence along the path on the aerosol speckle field propagation and on the decorrelation time are investigated. For a coherence bandwidth of 4.1 x 10 to the 11th rad/s and a time delay of 1.65 x 10 to the -14th s, the effect of pulse broadening is found to be negligible. It is suggested that pulse broadening effects need to be taken into account when the correlation time due to aerosol dephasing approaches the nanosecond range.

Murty, S. R.↗