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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

A recursive algorithm for Zernike polynomials

The analysis of a function defined on a rotationally symmetric system, with either a circular or annular pupil is discussed. In order to numerically analyze such systems it is typical to expand the given function in terms of a class of orthogonal polynomials. Because of their particular properties, the Zernike polynomials are especially suited for numerical calculations. Developed is a recursive algorithm that can be used to generate the Zernike polynomials up to a given order. The algorithm is recursively defined over J where R(J,N) is the Zernike polynomial of degree N obtained by orthogonalizing the sequence R(J), R(J+2), ..., R(J+2N) over (epsilon, 1). The terms in the preceding row - the (J-1) row - up to the N+1 term is needed for generating the (J,N)th term. Thus, the algorith generates an upper left-triangular table. This algorithm was placed in the computer with the necessary support program also included.

Davenport, J. W.↗
NASA NTRS1983

Numerical computations on one-dimensional inverse scattering problems

An approximate method to determine the index of refraction of a dielectric obstacle is presented. For simplicity one dimensional models of electromagnetic scattering are treated. The governing equations yield a second order boundary value problem, in which the index of refraction appears as a functional parameter. The availability of reflection coefficients yield two additional boundary conditions. The index of refraction by a k-th order spline which can be written as a linear combination of B-splines is approximated. For N distinct reflection coefficients, the resulting N boundary value problems yield a system of N nonlinear equations in N unknowns which are the coefficients of the B-splines.

Dunn, M. H.↗
NASA NTRS1983

Remote sensing investigations of wetland biomass and productivity for global biosystems research

Monitoring biomass of wetlands ecosystems can provide information on net primary production and on the chemical and physical status of wetland soils relative to anaerobic microbial transformation of key elements. Multispectral remote sensing techniques successfully estimated macrophytic biomass in wetlands systems. Regression models developed from ground spectral data for predicting Spartina alterniflora biomass over an entire growing season include seasonal variations in biomass density and illumination intensity. An independent set of biomass and spectral data were collected and the standing crop biomass and net primary productivity were estimated. The improved spatial, radiometric and spectral resolution of th LANDSAT-4 Thematic Mapper over the LANDSAT MSS can greatly enhance multispectral techniques for estimating wetlands biomass over large areas. These techniques can provide the biomass data necessary for global ecology studies.

Harkisky, M.↗
NASA NTRS1983

Solar-simulator-pumped atomic iodine laser kinetics

The literature contains broad ranges of disagreement in kinetic data for the atomic iodine laser. A kinetic model of a solar-simulator-pumped iodine laser is used to select those kinetic data consistent with recent laser experiments at the Langley Research Center. Analysis of the solar-simulator-pumped laser experiments resulted in the following estimates of rate coefficients: for alkyl radical (n-C3F7) and atomic iodine (I) recombination, 4.3 x 10 to the 11th power (1.9) + or - cu cm/s; for n-C3F7I stabilized atomic iodine recombination (I + I) 3.7 x 10 to the -32nd power (2.3) + or -1 cm to the 6th power/s; and for molecular iodine (I2) quenching, 3.1 x 10 to the -11th power (1.6) + or - 1 cu cm/s. These rates are consistent with the recent measurements.

Wilson, H. W.↗
NASA NTRS1983

The AlGaAs single-mode stability

Single-mode spectral behavior with aging in constricted double heterojunction (CDH) lasers was studied. The CDH lasers demonstrated excellent reliability ( or = 1 million years extrapolated room-temperature MTTF) and single-mode operation after 10,000 hours of 70 C aging. The deleterious effects of laser-fiber coupling on the spectra of the diodes were eliminated through the use of wedge-shaped fibers. A novel high-power large optical cavity (LOC)-type laser was developed: the terraced-heterostructure (TH)-LOC laser, which provides the highest power into a single-mode (i.e., 50 mW CW) ever reported.

Botez, D.↗
NASA NTRS1983

The NASA/GSFC hydrogen maser program: A review of recent data

Data is presented on the phase and frequency stability, over time periods extending to one week, of the new NR field operable hydrogen masers developed by the Applied Physics Laboratory (APL) and the older NX and NP field operable hydrogen masers developed by Goddard Space Flight Center and maintained and upgraded by Bendix Field Engineering Corporation (BFEC). Data is presented on the NR masers in the laboratory showing frequency stabilities well into the 10 to the -15th power range and phase stabilities well into the 100 ps range for periods of up to one day. Data is presented on upgraded NP masers in the laboratory showing that the frequency stability has been improved substantially to virtually the NR level. VLBI data is presented on the phase difference between NX-2 at Owens Valley, California and NR-2 at Fort Davis, Texas for a one week period showing, after removal of a constant frequency drift, a 350 ps RMS phase stability.

Chiu, M.↗
NASA NTRS1983

Intercontinental time and frequency transfer using a global positioning system timing receiver

The Deep Space Network (DSN) has a requirement to maintain knowledge of the frequency offset between DSN stations within 3 x 10 to the -13th power and time offset within 10 microseconds. It is further anticipated that in the 1987-1990 era the requirement for knowledge of time offset between DSN stations will be less than 10 nanoseconds. The Jet Propulsion Laboratory (JPL) is using the Global Positioning System (GPS) Space Vehicles, as a development project, to transfer time and frequency over intercontinental distances between stations of the DSN and between the DSN and other agencies. JPL has installed GPS timing receivers at its tracking station near Barstow, California and at its tracking station near Madrid, Spain. The details of the experiment and the data are reported. There is a discussion of the ultimate capabilities of these techniques for meeting the functional requirements of the DSN.

Clements, P. A.↗
NASA NTRS1983

Radiation hardness of Efratom M-100 rubidium frequency standard

The effects of nuclear radiation on rubidium gas cell frequency standards and components are presented, including the results of recent tests where a continuously operating rubidium frequency standard (Effratom, Model M-100) was subjected to simultaneous neutron/gamma radiation. At the highest neutron fluence 7.5 10 to the 12th power n/sq cm and total dose 11 krad(Si) tested, the unit operated satisfactorily; the total frequency change over the 2 1/2 hour test period due to all causes, including repeated retraction from and insertion into the reactor, was less than 1 x 10 to the -10th power. The effects of combined neutron/gamma radiation on rubidium frequency standard physics package components were also studied, and the results are presented.

English, T. C.↗
NASA NTRS1982

A quantitative model of geomagnetic activity

A quantitative model of geomagnetic activity is developed and utilized to investigate the causes of the diurnal, seasonal, and IMF sector variations in the AL index records. This auroral index was chosen for study because of its high sensitivity to the strength of the westward electrojet and, hence, magnetospheric substorm activity. After the introduction of corrections for processes not related to substorms, ability of the function Bs to the 0.85th power x V squared to produce the observed variations in AL was examined. The annual variation of Bs was determined by superposing the contributions to Bs due to the inclination of the magnetic axis (Russell and McPherron, 1973) on an empirical mean distribution of Bz. V was assumed constant. The predicted values of ALc have been compared with observed averages for 9 years of solar cycle 20. The predicted annual variation of ALc for toward and away sectors are in good agreement with observation. While the predicted semi-annual component of ALc is in phase with observation, it is less than half the observed amplitude. The predicted diurnal variation of ALc for June is in satisfactory agreement with observation.

Holzer, R. E.↗
NASA NTRS1982

The Apollo 15 yellow impact glasses - Chemistry, petrology, and exotic origin

The Apollo 15 yellow impact glasses are characterized by moderate TiO2 (about 4.8%) and high abundances of the large ion lithophile elements (e.g., K, P, Hf, Th, REE). Since the chemistry of these glasses cannot be duplicated by any combination of local components presently known to occur at the Apollo 15 landing site, these yellow glasses seem to be exotic to that area. Chemical and petrologic constraints suggest that these samples were produced by impact melting of an immature mare regolith developed upon an unusual variety of mare basalt. It is speculated that the target basalts were the youngest lava flows known to exist on the moon (i.e., Eratosthenian-age lavas in Oceanus Procellarum and Mare Imbrium). Specific tests are proposed for evaluating this provocative hypothesis.

Delano, J. W.↗
NASA NTRS1982

Mass loss in Alpha Cygni - Synthetic H-Alpha profiles

Alpha Cygni (A2 Ia) is the brightest and best-studied A type supergiant. The star's position in the H-R diagram makes the determination of its mass loss rate extremely important. The present investigation is concerned with a semiempirical modeling of the H-alpha profile in Alpha Cyg in connection with the objective of estimating the mass loss rate and further constraining the physical state of the stellar wind. The synthetic H-alpha profiles considered are compared with the observed profiles in Alpha Cyg. It is concluded that a spherically symmetric, steady-state model, with the network of hydrogen transitions treated in detail, can fit the deeper observed profiles but not the shallower ones. The obtained results indicate that, within the context of the turbulent models, the mass loss rate of Alpha Cyg is (1.7 + or - 0.4) x 10 to the -7th solar mass per year.

Kunasz, P. B.↗
NASA NTRS1982

Further observational evidence for a coronal boundary line in the cool star region of the H-R diagram

Soft X-ray emission was detected from Gamma Hya and 37 Com, at levels of 6-8 x 10 to the -13th ergs/sq cm per sec, during 0.2-4.0 keV observations of eight G-K giant stars obtained with the Einstein Observatory's Imaging Proportional Counter. Upper limits of about 3 x 10 to the 13th ergs/sq cm per sec were determined for the remaining stars. These results are judged to strengthen the Ayres et al (1981) findings that stellar coronae whose temperature is greater than one million K are either very weak or entirely absent among red giants and supergiants, while a large range in coronal emission exists among the yellow giant and supergiant stars.

Haisch, B. M.↗
NASA NTRS1982

The atmospheres of Io and other satellites

Voyager measurements of gaseous SO2 in a hot spot region and of ions of sulfur, oxygen, and SO2(+) in the plasma torus, combined with the groundbased measurements of SO2 frost on the surface, indicate that SO2 is perhaps the dominant constituent of Io's atmosphere. Indeed the ionosphere of Io can be sustained by a pure SO2 atmosphere in thermal equilibrium with the surface. Upper limits on common volatiles H2O, CO2, NH3, and CH4 and exceedingly small, although S2 and Ar may still be present. Photochemistry of SO2 could lead to O2 as the major gas on the night side. Reasonable processes for loss to the torus include thermal escape, sputtering, and sweep-up of ions. The symmetry of the sodium cloud between eastern and western elongation is difficult to reconcile with any substantial, diurnally varying atmosphere. Europa, Ganymede, and Callisto may have oxygen atmospheres resulting from photolysis of water vapor. A pressure of approximately 10 to the -6th microbar is predicted, consistent with the Voyager upper limit for Ganymede. A much higher pressure might be present if the water vapor pressure is increased by a low albedo (Callisto) or sputtering (Europa).

Kumar, S.↗
NASA NTRS1982

The effect of monomolecular surface films on the microwave brightness temperature of the sea surface

It is pointed out that monomolecular surface films of biological origin are often encountered on the ocean surface, especially in coastal regions. The thicknesses of the monomolecular films are of the order of 3 x 10 to the -9th m. Huehnerfuss et al. (1978, 1981) have shown that monomolecular surface films damp surface waves quite strongly in the centimeter to decimeter wavelength regime. Other effects caused by films are related to the reduction of the gas exchange at the air-sea interface and the decrease of the wind stress. The present investigation is concerned with experiments which reveal an unexpectedly large response of the microwave brightness temperature to a monomolecular oleyl alcohol slick at 1.43 GHz. Brightness temperature is a function of the complex dielectric constant of thy upper layer of the ocean. During six overflights over an ocean area covered with an artificial monomolecular alcohol film, a large decrease of the brightness temperature at the L-band was measured, while at the S-band almost no decrease was observed.

Alpers, W.↗
NASA NTRS1982

A new class of radio pulsars

A new class of spun-up radio pulsars with short periods, long apparent ages, and pulsed optical, X-ray, and gamma ray fluxes significantly below those expected for canonical pulsars with similar periods is reported. Scenarios for the formation of such a pulsar, in which a neutron star accretes from a Keplerian accretion disk initially fed by a companion, are discussed. Because of their long spin-down lifetimes, such pulsars would probably be observable if their birth rate exceeded even 10 to the -4th that of the canonical ones. It is proposed that the recently discovered millisecond pulsar 4C21.53 belongs to this class, together with the binaries PSRs 1913+16, 0820+02, and 0655+64, as well as possibly several isolated pulsars such as PSRs 1952+29 and 1804-08.

Alpar, M. A.↗
NASA NTRS1982

Persistent X-ray emission from a gamma-ray burst source

A quiescent X-ray source detected with the Einstein X-ray Observatory in a location consistent with that of an intense gamma ray burst is shown to be also consistent with the location of the 1928 optical transient, the likely optical counterpart of the gamma ray burst source GBS0117-29. The system appears to be underluminous in X-rays by a factor of 10; possible reasons for this are discussed. The observed X-ray flux would require an accretion rate of about 10 to the -14th (d/1 kpc/)-squared solar masses per year, which is probably too low to be consistent with published nuclear flash models for gamma bursts, unless the distance is substantially greater than about 1 kpc or the burst recurrence time is greater than about 50 yrs, or the accretion rate is highly variable. Such a long recurrence time appears to be inconsistent with the detection of the optical burst.

Grindlay, J. E.↗
NASA NTRS1982

On the spectra of type-III solar radio bursts observed at low frequencies

The spectra of strong bursts observed at low frequencies by OGO-5 during 1968-1970 are presented. They usually exhibit an intense main peak between 100 kHz and 1 MHz, and sometimes a less intense secondary peak between 1 and 3.5 MHz. Main peaks of 10 to the -12th W per sq m per Hz or more were obtained in very strong events, but because of antenna calibration problems those could be one or two orders of magnitude too high. Recently published work supports the finding that type III bursts at low frequencies can be at least four orders of magnitude more intense than at ground-based frequencies of observation. It is found that the energy received at the earth increases with decreasing frequency approximately as f to the -n, where n is between 3 and 4.

Alvarez, H.↗
NASA NTRS1983

The X-ray surface brightness of Kepler's supernova remnant

The first X-ray images of Kepler's supernova remnant (SN Ophiuchi 1604) are presented, and consequences for SNR models are discussed. Observations made with the Einstein Observatory Imaging Proportional Counter and High Resolution Imager show the remnant to be circular, with a strong shell brighter in the north than in the south. A flux of 1.2 x 10 to the -10th ergs/sq cm per sec was measured in the 0.15-4.5 keV region, which corresponds to an X-ray luminosity of 1.0 x 10 to the 36th ergs/sec at a distance of 5 kpc and an interstellar medium density of 2.8 x 10 to the 21st/sq cm. The X-ray observations do not allow the determination of whether the SNR is in the adiabatic or free expansion phase, but in either case it is shown that the mean ISM density must be greater than about 0.1/cu cm. In addition, the density of the X-ray emitting gas must be high, and its electron temperature must be fairly low. The high ISM densities derived for Kepler's SNR and other SNRs thus suggest an atypical ISM, possibly influenced by mass lost from the pre-supernova star.

White, R. L.↗