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At least 235 records · Page 13

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NASA NTRS1972

Skylab S193 and the analysis of the wind field over the ocean

The present status of the program to provide proof of concept for the idea that simultaneously observed radar scattering cross section measurements and passive microwave measurements can be used to determine the winds in the planetary boundary layer over the ocean, is given. The role of S193 in Skylab is providing the final clinching proof that an operational instrument will obtain data of great value to both meteorology and oceanography is described.

Pierson, W. J., Jr.↗

NASA NTRS1972

Virginia precipitation scatter experiment: Experiment description

A year's statistical data were acquired for the purposed of refining existing coordination procedures for the sharing of common frequency allocations between terrestrial and space services. The measurement program was conducted between October 3, 1970 and October 2, 1971, to empirically determine the cumulative distributions of scattering cross section per unit volume of hydrometeors at heights up to 9 km above the surface and of rain rate at the surface. The measurements were made in southeastern Virginia using a bistatic, continuous-wave, vertically polarized radar system operating at S- and X-band frequencies and tipping-bucket rain gages. The bistatic radar system was configured to represent an interference situation between a radio-relay system and a space communication system Earth terminal; the results may be interpreted as measured distributions of transmission loss for interference due to rain. A description of the experiment and equipment is included.

Levine, E.↗

NASA NTRS1973

Curve crossing for low-energy elastic scattering of He/+/ by Ne.

The perturbation seen in the experimental differential elastic-scattering cross section for the 40-eV He/+/ + Ne system has been attributed to a single crossing of two intermolecular potential-energy curves. A new theoretical treatment of the curve-crossing problem, namely, that of Delos and Thorson, is employed to obtain the crossing probabilities and phases associated with the crossing. These are determined by utilizing ab initio potentials involved in the crossing and are further used in a partial-wave calculation of the cross section, which is compared with our experiment. The origin of the oscillatory structure observed in the differential cross section is discussed in semiclassical terms by defining the problem in terms of two pseudo-deflection-functions. A rainbow effect is shown to be related to a particular feature (a maximum rather than a minimum) of these deflection functions.

Bobbio, S. M.↗

NASA NTRS1975

Comparison between reflectivity statistics at heights of 3 and 6 km and rain rate statistics at ground level

An experiment was conducted to study the relations between the empirical distribution functions of reflectivity at specified locations above the surface and the corresponding functions at the surface. A bistatic radar system was used to measure continuously the scattering cross section per unit volume at heights of 3 and 6 km. A frequency of 3.7 GHz was used in the tests. It was found that the distribution functions for reflectivity may significantly change with height at heights below the level of the melting layer.

Crane, R. K.↗

NASA NTRS1977

Low-temperature rotational relaxation in gaseous H2 and D2

Various potentials have been used for generating scattering cross sections for rotational transitions in hydrogen isotope systems. Low-temperature rates were calculated and the relaxation times compared with the results of sound absorption experiments. It is seen that the existing potentials could still be improved since they do not exactly reproduce the experimental results.

Ramaswamy, R.↗

NASA NTRS1976

Recent measurements concerning uranium hexafluoride-electron collision processes

Scattering of electrons by UF6 molecules was studied at impact energies ranging from 5 to 100 eV and momentum transfer, elastic and inelastic scattering cross sections were determined. The measurements also yielded spectroscopic information which made possible to extend the optical absorption cross sections from 2000 angstroms to 435 angstroms. It was found that UF6 is a very strong absorber in the vacuum UV region. No transitions were found to lie below the onset of the optically detected 3.0 eV feature.

Trajmar, S.↗

NASA NTRS1979

Implications of cyclotron features in the X-ray spectrum of Her X-1

The X-ray spectra from binary pulsars is investigated through the modification of physical processes due to an intense magnetic field. An effective scattering cross section for line photons and the Coulomb cross section for electron-ion collisions, treating the electrons relativistically, are calculated. The quantization of electron orbits in strong fields are discussed. The absorption cross sections and emission rates for cyclotron photons are calculated. An application of the results to the data from Hercules X-1 is examined.

Bussard, R.↗

NASA NTRS1980

The dependence of zenith angle of the strength of 3-meter equatorial electrojet irregularities

Radar measurements in Peru were used to deduce the zenith angle dependence of the scattering cross section of plasma irregularities generated by instabilities in the equatorial electrojet. The irregularities probed by the 50 MHz Jicamarca radar had a wavelength of 3m. The cross section for the type 2 irregularities was isotropic in the plane perpendicular to the magnetic field, while the cross section for the stronger type 1 irregularities varied with zenith angle at a rate of approximately 0.3 dB/degree; the horizontally traveling waves were more than 100 times stronger than those traveling vertically.

Ierkic, H. M.↗

NASA NTRS1981

The wind-speed measurement capability of spaceborne radar altimeters

This study represents an attempt to quantitatively assess the capability of a spaceborne radar altimeter to infer ocean surface wind speeds from a measurement of the backscattered power at vertical incidence. The study uses data acquired during 184 near overflights of NOAA data buoys with the GEOS-3 satellite radar altimeter and encompasses a wind-speed range from less than 1 to 18 m/s. An algorithm is derived from the data comparison for converting measurements of the normalized scattering cross section of the ocean surface at 13.9 GHz into estimates of the surface wind speed at the standard anemometer height of 10 m. The algorithm is straightforward and potentially useful for on-board processing of raw altimeter data for the purpose of providing real-time estimates of surface wind speed. For winds in the range of 1 to 18 m/s, the mean difference between the altimeter-inferred winds and the buoy measurements is negligible while the standard deviation of the difference is 1.74 m/s.

Brown, G. S.↗

NASA NTRS1981

AgRISTARS. Supporting research: MARS x-band scatterometer

The design, construction, and data collection procedures of the mobile agricultural radar sensor (MARS) x band scatterometer are described. This system is an inexpensive, highly mobile, truck mounted FM-CW radar operating at a center frequency of 10.2 GHz. The antennas, which allow for VV and VH polarizations, are configured in a side looking mode that allows for drive by data collection. This configuration shortens fieldwork time considerably while increasing statistical confidence in the data. Both internal calibration, via a delay line, and external calibration with a Luneberg lens are used to calibrate the instrument in terms of sigma(o). The radar scattering cross section per unit area, sigma(o), is found using the radar equation.

Ulaby, F. T.↗

NASA NTRS1981

Measurement of the oscillator strengths and autoionization widths of the neutral-aluminum multiplet 3s2 3p2 P0 - 3s 3p2 2P

The hook method is used in conjunction with absorption equivalent width measurements to determine the oscillator strengths and line widths of the Al I multiplet 3s2 3p2 P0 - 3s 3p2 2P at 176 nm. Autoionization is found to be the dominant decay channel for the two levels of the 3p2 2P term, with autoionizing widths of 4.0 and 0.87 x 10 to the 10th/sec for J of 3/2 and 1/2, respectively. The van der Waals scattering cross section for the Al I transition at 176.91 nm by He is determined to be (1.3 + or - 0.3) x 10 to the -14th sq cm at 2400 K, and a solar spectrum calculation centered at 176 nm shows that the Al I features are possible diagnostic probes of (1) solar non-LTE processes, and (2) the temperature minimum and structure of the lower chromosphere.

Lombardi, G. G.↗

NASA NTRS1981

Optimization of radar imaging system parameters for geological analysis

The use of radar image simulation to model terrain variation and determine optimum sensor parameters for geological analysis is described. Optimum incidence angle is determined by the simulation, which evaluates separately the discrimination of surface features possible due to terrain geometry and that due to terrain scattering. Depending on the relative relief, slope, and scattering cross section, optimum incidence angle may vary from 20 to 80 degrees. Large incident angle imagery (more than 60 deg) is best for the widest range of geological applications, but in many cases these large angles cannot be achieved by satellite systems. Low relief regions require low incidence angles (less than 30 deg), so a satellite system serving a broad range of applications should have at least two selectable angles of incidence.

Waite, W. P.↗

NASA NTRS1983

Angular distribution for electron excitation of the 4(2)S yields 4(2)P transition in Zn II - Comparison of experiment and theory

Differential electron-scattering cross sections for inelastic excitation of an ion have been measured for the first time. Experiments were carried out in a cross electron-ion beam geometry for the 4(2)S yields 4(2)P transition in Zn II at 75 eV. In addition, differential cross sections were calculated at energies between 15 and 100 eV in a five-state close-coupling approximation in which 4s, 4p, 3d(9)4s(2), 5s, and 4 d states were included. Agreement in shape between theory and experiment at 75 eV is excellent.

Chutjian, A.↗

NASA NTRS1983

High spectral resolution lidar to measure optical scattering properties of atmospheric aerosols. II - Calibration and data analysis

The high spectral resolution lidar (HSRL) measures optical properties of atmospheric aerosols by interferometically separating the elastic aerosol backscatter from the Doppler broadened molecular contribution. Calibration and data analysis procedures developed for the HSRL are described. Data obtained during flight evaluation testing of the HSRL system are presented with estimates of uncertainties due to instrument calibration. HSRL measurements of the aerosol scattering cross section are compared with in situ integrating nephelometer measurements.

Sroga, J. T.↗

NASA NTRS1984

Comparison of mesospheric VHF radar echoes and rocket probe electron concentration measurements

Refractive index irregularities in the equatorial mesosphere have been investigated using both the Jicamarca VHF radar and a rocket-borne Langmuir probe launched from Punta Lobos, Peru. On February 27, 1983, a single layer of turbulence was observed in the upper mesosphere by both experiments. There is very good agreement between the observed radar echo power and the radar scattering cross section calculated from the rocket data when these are interpreted in the context of isotropic turbulence. The inner and outer scales of turbulence have been calculated from both the radar and the rocket data, and good agreement is found. The radar data show indications of large-scale vortices in the layer of irregularities. Rocket data show that the inner scale of turbulence in the upper mesosphere is a few tens of meters and that the Jicamarca radar Bragg wavelength (3 m) is well within the viscous subrange of turbulence in this altitude range. The spectral index in the inertial subrange is close to -5/3, changing to about - 7 at higher wave numbers. Energy dissipation rate in the layer was calculated to be 0.05 W/kg, in good agreement with previous estimates.

Royrvik, O.↗

NASA NTRS1986

XeF lidar measurements of density and temperature in the middle atmosphere

In recent years, it has been widely recognized that vertically propagating internal gravity waves in the middle atmosphere play an important role in determining the large scale wind field by their momentum transport and deposition. However, very little is known about them in the upper stratosphere and lower mesosphere because of the lack of adequate techinques to observe small scale motions. A XeF excimer laser was used for the Rayleigh lidar observation. The characteristics of the XeF lidar system are given. The wavelength of the XeF laser is in the region that is ozone absorption free, and the Rayleigh scattering cross section is 5 times larger than in 532 nm. The advantages of using the XeF laser as against the YAG laser have been discussed in detail. The density and temperature profiles between the altitude range of 30 and 70 km measured with this lidar system are illustrated. The accuracy is less than + or - 3% and 10k at 60 km for an observation time of 15 min. The results on the gravity waves by the observation through about one year since Apr. 1985 is presented.

Shibata, T.↗

NASA NTRS1986

Gamma-ray burst spectra from photon-deficient Compton scattering by nonthermal electrons

Consideration is given to a model of gamma-ray burst sources based on repeated Compton scatterings of soft photons by relativistic nonthermal electrons. Relativistic electrons which are continuously produced in the source radiate the total power L supplied to them. Higher order Compton scatterings, which occur when L(soft) is much less than L make the model distinct. The spectrum having an X-ray energy index of about one results from the superposition of the spectral components from several orders of Compton scattering; the change of the slope at several hundred keV arises from the Klein-Nishina decline of the scattering cross section.

Zdziarski, Andrzej A.↗

NASA NTRS1986

Local variation of haze thickness on the equatorial region of Venus

The horizontal distribution of haze particles in the equatorial region of Venus was investigated using selected polarimetry maps obtained by the Orbiter Cloud Photopolarimeter (OCP) aboard the Pioneer Venus Orbiter, with five of these maps having been acquired on five consecutive terrestrial days. Nine sets of OCP polarimetry maps (PM) similar to PM 16 employed by Kawabata et al. (1980) were selected, and 935 nm data points were extracted to construct a cloud-haze model, from which the haze fraction f(h) within the cloud and the corresponding optical thickness tau(h) were determined for the period from December 1978 to October 1980. The results indicate that, in this period, the upper cloud layer of Venus had haze layers with tau(h) of about 0.02. Furthermore, the submicron particles present within the principal cloud contributed approximately 6 percent to the total scattering cross section per unit volume. Although no systematic time variations in the derived values of tau(h) and f(h) were noticeable, the consecutive five day observations indicated a presence of a large-scale horizontal inhomogeneity in haze particle distribution.

Kawabata, Kiyoshi↗