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At least 127 records · Page 7
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NASA NTRS1974

Millimeter wavelength propagation studies

The investigations conducted for the Millimeter Wavelength Propagation Studies during the period December, 1966, to June 1974 are reported. These efforts included the preparation for the ATS-5 Millimeter Wavelength Propagation Experiment and the subsequent data acquisition and data analysis. The emphasis of the OSU participation in this experiment was placed on the determination of reliability improvement resulting from the use of space diversity on a millimeter wavelength earth-space communication link. Related measurements included the determination of the correlation between radiometric temperature and attenuation along the earth-space propagation path. Along with this experimental effort a theoretical model was developed for the prediction of attenuation statistics on single and spatially separated earth space propagation paths. A High Resolution Radar/Radiometer System and Low Resolution Radar System were developed and implemented for the study of intense rain cells in preparation for the ATS-6 Millimeter Wavelength Propagation Experiment.

Hodge, D. B.↗
NASA NTRS1973

Lunar polarization studies at 3.1 mm wavelength

Observations of the distribution of linearly polarized lunar thermal emission were made at a wavelength of 3.1 mm with The University of Texas 4.88 m parabolic reflector (0.042 deg HPBW). A shadow corrected, rough surface, thermal emission model for a homogeneous moon was least-squares-fitted to the polarization data. Results indicate an effective lunar dielectric constant of 1.34 plus or minus 0.04 with surface roughness characterized by a standard deviation of 17 (plus or minus 5) deg for surface slopes with a normal probability density, independent of lunar phase. A comparison of these results with published values at other wavelengths suggests that the effective lunar dielectric constant, as obtained by lunar emission measurements, decreases with decreasing wavelength of observation. This wavelength dependence may be interpreted in terms of an inhomogeneous surface and/or a surface that possesses intermediate scale surface roughness.

White, T. L.↗
NASA NTRS1976

Wavelength dependence in radio-wave scattering and specular-point theory

Radio-wave scattering from natural surfaces contains a strong quasispecular component that at fixed wavelengths is consistent with specular-point theory, but often has a strong wavelength dependence that is not predicted by physical optics calculations under the usual limitations of specular-point models. Wavelength dependence can be introduced by a physical approximation that preserves the specular-point assumptions with respect to the radii of curvature of a fictitious, effective scattering surface obtained by smoothing the actual surface. A uniform low-pass filter model of the scattering process yields explicit results for the effective surface roughness versus wavelength. Interpretation of experimental results from planetary surfaces indicates that the asymptotic surface height spectral densities fall at least as fast as an inverse cube of spatial frequency. Asymptotic spectral densities for Mars and portions of the lunar surface evidently decrease more rapidly.

Tyler, G. L.↗
NASA NTRS1976

Subresolution imaging - Use of wavelength and angular diversity techniques

The paper demonstrates the possibility of using an illumination system tunable in angle or wavelength to determine the amplitude and height structure of an object within the resolution cell defined by the resolving power of some imaging system. It is noted that since the value of the electric field at a point on the image of an object varies with the wavelength or angle of illumination on the object, the Fourier transform of this variation provides directly the amplitude and phase structure of the object within the resolution cell corresponding to the image point of interest. This Fourier transform is calculated, and it is shown that the transform of the electric-field intensity with respect to the inverse wavelength dimension is simply the autocorrelation of the electric-field variation with wavelength. Practical techniques that can be employed to measure the subresolution detail of an object are briefly discussed.

Jain, A.↗
NASA NTRS1976

Doppler wavelength shifts of transition zone lines measured in Skylab solar spectra

Wavelengths of lines of the transition-zone ions Si IV, C IV, O IV, N V, and O V are observed to be redshifted relative to the wavelengths of chromospheric lines in XUV spectra obtained from the normal-incidence spectrograph on Skylab. The spectra cover the wavelength range from 1200 to 1565 A and were obtained with the slit positioned over chromospheric network and cell regions, on coronal holes, and above the limb. The network-area and coronal-hole spectra were obtained near the disk center. Only some of the spectra show redshifted transition-zone lines. The observed shifts are between 0.03 and 0.08 A, implying velocities of 15 km/s or less. The amount of wavelength shift does not always appear to be the same for lines of different ions. The shifts imply that descending plasma in the solar atmosphere produces more emission than ascending plasma at temperatures between approximately 70,000 and 200,000 K.

Doschek, G. A.↗
NASA NTRS1976

Wavelengths and levels of the Na I isoelectronic sequence from K IX through Mn XV

The seven spectra K IX through Mn XV have been analyzed and the series limits determined. We present the adopted wavelengths and the derived energy levels and term splittings. The highly ionized atoms were produced in a spark source and recorded mainly on a 3 m grazing incidence spectrograph. Many new lines have been identified and many previously given wavelengths have been revised. All wavelengths have been examined for consistency by least-squares-fitting procedures and other tests. These procedures indicated misidentified lines and incorrect wavelengths which we corrected when possible. A discussion of the errors is given.

Cohen, L.↗
NASA NTRS1977

Evaluation of spectral channels and wavelength regions for separability of agricultural cover types

The spectral channels were evaluated as well as wavelength regions; visible, near infrared, middle infrared and thermal infrared were evaluated with respect to their estimated probability of correct classification (P sub c) in discriminating agricultural cover types. Multispectral scanner data in twelve spectral channels in the wavelength range of 0.4 to 11.7 micrometers acquired in the middle of July for three flightlines were analyzed by applying automatic pattern recognition techniques. The same analysis was performed for the data acquired in the middle of August, over the same three flightlines, to investigate the effect of time on the results. The effect of deletion of each spectral channel as well as each wavelength region on P sub c was given. Values of P sub c for all possible combinations of wavelength regions in the subsets of one to twelve spectral channels were also given. The overall values of P sub c were found to be greater for the data of the middle of August than the data of the middle of July.

Kumar, R.↗
NASA NTRS1977

Evaluation of spectral channels and wavelength regions for separability of agricultural cover types

The purpose of this study was to evaluate the spectral channel as well as wavelength regions - visible, near infrared, middle infrared and thermal infrared - with respect to their estimated probability of correct classification (P-c) in discriminating agricultural cover types. Multispectral scanner data in twelve spectral channels in the wavelength range of 0.4 to 11.7 micron acquired in the middle of July for three flightlines were analysed by applying automatic pattern recognition techniques. The same analysis was performed for the data acquired in the middle of August, over the same three flightlines, to investigate the effect of time on the results. The effect of deletion of each spectral channel as well as each wavelength region on P-c is given. Values of P-c for all possible combinations of wavelength regions in the subsets of one to twelve spectral channels are also given. The overall values of P-c were found to be greater for the data of the middle of August than the data of the middle of July.

Kumar, R.↗
NASA NTRS1978

Radar studies of the Martian surface at centimeter wavelengths - The 1975 opposition

The Goldstone radar system was operated at wavelengths of 3.5 and 12.6 cm to probe the Martian surface during the 1975 opposition. Regions studied in detail by range-Doppler techniques are Syrtis Major, Sinus Meridiani, and the crater Schiaparelli. Average rms slopes of 1.6 and 1.1 deg were measured in Syrtis Major at 3.5 and 12.6 cm, respectively, while the average reflectivity was about 0.064 at both wavelengths. No wavelength dependence of surface roughness was seen in Sinus Meridiani, where rms surface slopes averaged 1.8 deg and the reflectivity was about 0.08. The regions around Schiaparelli were probed at a 12.6-cm wavelength. The echo from the bottom of the crater was undetectable. Operating at 3.5 cm during May and June of 1976, 149 CW echo spectra were obtained near latitude 18 deg, sampling most longitudes including the early Viking landing sites. The average total radar cross section is 4.8% of the geometrical cross section. The diffuse component was estimated to be 1.9%, leaving 2.9% to the average quasi-specular component. The average rms slope is 4.1 deg.

Downs, G. S.↗
NASA NTRS1977

Separability of agricultural cover types in spectral channels and wavelength regions

Spectral channels and wavelength regions (visible, near infrared, middle infrared and thermal infrared) were evaluated with respect to their estimated probability of correct classification (Pc) in discriminating agricultural cover types. Multispectral scanner data in twelve spectral channels in the wavelength range of 0.4 to 11.7 microns acquired in the middle of July for three flightlines, were analyzed by applying automatic pattern recognition techniques. The same analysis was performed for the data acquired in the middle of August, 1971, over the same three flightlines, to investigate the effect of time on the results. The effect of deletion of each spectral channel as well as each wavelength region on Pc is given. Values of Pc for all possible combinations of wavelength regions in the subsets of one to twelve spectral channels are also given. The overall values of Pc were found to be greater for the data of the middle of August than the data of the middle of July.

Kumar, R.↗
NASA NTRS1979

On the wavelength dependence of the effects of turbulence on average refraction angles in occultations by planetary atmospheres

The dependence of the effects of planetary atmospheric turbulence on radio or optical wavelength in occultation experiments is discussed, and the analysis of Hubbard and Jokipii (1977) is criticized. It is argued that in deriving a necessary condition for the applicability of their method, Hubbard and Jokipii neglect a factor proportional to the square of the ratio of atmospheric or local Fresnel zone radius and the inner scale of turbulence, and fail to establish sufficient conditions, thereby omitting the square of the ratio of atmospheric scale height and the local Fresnel zone radius. The total discrepancy is said to mean that the results correspond to geometrical optics instead of wave optics, as claimed, thus being inapplicable in a dicussion of wavelength dependence. Calculations based on geometrical optics show that the bias in the average bending angle depends on the wavelength in the same way as does the bias in phase path caused by turbulence in a homogeneous atmosphere. Hubbard and Jokipii comment that the criterion of Haugstad and Eshleman is incorrect and show that there is a large wave optical domain where the results are independent of wavelength.

Haugstad, B. S.↗
NASA NTRS1979

Solar radio spectrum extended to 10 km wavelength

Solar radio spectra for quiet times and radio bursts in the wavelength range from 1 mm to 10 km are presented. Solar nonthermal emission in the range of 100 m to 10 km was monitored by means of long wire antennas connected to multi-channel radiometers in the frequency range 25 kHz to 10 MHz on board the IMP-6 and RAE-1 satellites. Flux densities for very intense type III bursts with intensities ranging from 10 to the -18th to 10 to the -14th W/sq m per Hz with maxima at 330 to 1500 m are shown, and a power-law wavelength distribution function of maximum burst intensities is derived. Intensities of a large type III noise storm are shown to increase with increasing wavelength to a peak near 500 m. The flux densities of both types of burst emission are observed to greatly exceed quiet sun intensities at long wavelengths.

Weber, R. R.↗
NASA NTRS1980

Separability of agricultural cover types in spectral channels and wavelength regions

This study was a continuation of a more complete evaluation of the spectral channels as well as wavelength regions - visible, near infrared, middle infrared, and thermal infrared - with respect to their estimated probability of correct classification Pc in discriminating agricultural cover types reported previously by Kumar and Silva (1977). Multispectral scanner data in twelve spectral channels in the wavelength range of 0.4-11.7 microns acquired in the middle of July for three flightlines were analyzed by applying automatic pattern recognition techniques. The same analysis was performed for the data acquired in the middle of August 1971, over the same three flightlines, to investigate the effect of time on the results. The effect of deletion of each spectral channel as well as each wavelength region on Pc is given. Values of Pc for all possible combinations of wavelength regions in the subsets of one to twelve spectral channels are also given. The overall values of Pc were found to be greater for the data of the middle of August than the data of the middle of July.

Kumar, R.↗
NASA NTRS1981

Improvements to the accuracy of the IUE wavelength scales in high dispersion

The data base of Pt-Ne emission lines used to calibrate the IUE high dispersion wavelength scales was scrutinized to improve the internal consistency of the adopted laboratory wavelength values and provide a homogeneous, documented line list, which IUE Guest Observers may use to evaluate quantitatively those Pt-Ne spectra taken to calibrate their data. After deletion of incorrect or inappropriate data in the old data base (lines with incorrect wavelength assignments; lines which are too faint, too bright, or blended; lines which fall near reseau marks, etc.) and the addition of several new entries, a total of 172 Pt-Ne lines for the SWP camera and 164 Pt-Ne lines for the LWR camera are now used for routine wavelength calibration in the high dispersion mode.

Turnrose, B. E.↗
NASA NTRS1980

Upper limits on the power in solar oscillations at 1.2 mm, 9 mm, 3.7 cm, and 11.1 cm wavelengths

A search for solar oscillations has been made using the NRAO 36-ft telescope at 1.2 mm, the NRL 85-ft telescope at 9 mm, and the NRAO four-element interferometer at 3.7 and 11.1 cm wavelengths. After corrections for the small coherence length of the optically observed oscillations, for their known spectral bandwidth, and for the visibility function of the interferometer, upper limits have been placed on the fluctuating power at oscillation frequencies near 3 mHz. The interferometric observations at 3.7 cm and the single-dish observations at 1.2 and 9 mm imply that less than 0.3, 0.04, and 0.1%, respectively, of the bremsstrahlung photons emitted from the chromosphere show periodic fluctuations. This is 1 to 3 orders of magnitude less than the fraction (approximately 1 to 2%) reported at ultraviolet wavelengths. Simple arguments in terms of weak shock theory suggest that the observable oscillations at centimeter and millimeter wavelengths should be intrinsically weaker in the thermal bremmstrahlung radiation than the optical, but the 3.7 cm wavelength results are explained only by the absence of periodic fluctuations at the level of formation.

Kundu, M. R.↗
NASA NTRS1981

Effect of short wavelength illumination on the characteristic bulk diffusion length in ribbon silicon solar cells

The influence of short wavelength light on the characteristic bulk minority carrier diffusion length of the ribbon silicon photovoltaic cell has been investigated. We have measured the intensity and wavelength dependence of the diffusion length in an EFG ribbon cell, and compared it with a standard Czochralski grown silicon cell. While the various short wavelength illuminations have shown no influence on the diffusion length in the CZ cell, the diffusion lengths in the ribbon cell exhibit a strong dependence on the volume generation rate as well as on the wavelength of the superimposed lights. We have concluded that the trap-filling phenomenon at various depths in the bulk neutral region of the cell is consistent with the experimental observation.

Ho, C. T.↗
NASA NTRS1982

Stress induced long wavelength photoconductivity in doped silicon infrared detectors

The long wavelength cutoff of a Si:P detector was extended to 34 microns by the application of a uniaxial stress. An unstressed Si:P photoconductive detector responds to photons of up to 28 microns wavelength. By applying a uniaxial stress to a detector along the /100/ crystal axis, the response was extended to approximately 34 microns. The /100/ axis was chosen as the stress direction because theoretical calculations predicted that such a stress extends the wavelength response more than one along the /110/ axis. These theoretical calculations were based upon fits to experimental data obtained at stresses of up to approximately kbar, and indicated that the extension in wavelength response continues to increase at much larger stresses.

Houck, J. R.↗
NASA NTRS1981

Long wavelength gravity and topography anomalies

It is shown that gravity and topography anomalies on the earth's surface may provide new information about deep processes occurring in the earth, such as those associated with mantle convection. Two main reasons are cited for this. The first is the steady improvement that has occurred in the resolution of the long wavelength gravity field, particularly in the wavelength range of a few hundred to a few thousand km, mainly due to increased coverage of terrestrial gravity measurements and the development of radar altimeters in orbiting satellites. The second reason is the large number of numerical and laboratory experiments of convection in the earth, including some with deformable upper and lower boundaries and temperature-dependent viscosity. The oceans are thought to hold the most promise for determining long wavelength gravity and topography anomalies, since their evolution has been relatively simple in comparison with that of the continents. It is also shown that good correlation between long wavelength gravity and topography anomalies exists over some portions of the ocean floor

Watts, A. B.↗