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At least 847 records · Page 47

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NASA NTRS1990

A procedure for the extraction of airglow features in the presence of strong background radiation

A technique is developed that can be used to derive the total intensity of band emissions from twilight airglow measurements when the basic spectral signature of the band to be considered is known. The method is designed to automatically extract total band or line intensities of a signal imbedded in background radiation several orders of magnitude greater in brightness. It is shown that the technique developed can reliably measure the intensity of both weak and strong band and line emissions in the presence of strong twilight background radiation. The method of extraction is shown as part of a general purpose spectral analysis program written in VAX FORTRAN. This extraction procedure has been used successfully on emissions of Fel, Ca(+), N2(+) (1N) (0-0) and (0-1), OH in the near UV. OI red (630nm) and green (558nm) lines in the visible, and the OH Meinel bands and O(+) (2P) 732 nm in the near IR.

Swift, W. R.↗

NASA NTRS1990

Development of a validation model for the defense meteorological satellite program's special sensor microwave imager

The Defence Meteorological Space Program's (DMSP) Special Sensor Microwave/Imager (SSM/I), an operational wind speed algorithm was developed. The algorithm is based on the D-matrix approach which seeks a linear relationship between measured SSM/I brightness temperatures and environmental parameters. D-matrix performance was validated by comparing algorithm derived wind speeds with near-simultaneous and co-located measurements made by off-shore ocean buoys. Other topics include error budget modeling, alternate wind speed algorithms, and D-matrix performance with one or more inoperative SSM/I channels.

Swift, C. T.↗

NASA NTRS1991

An electrically scanned thinned array radiometer for Earth remote sensing

Interferometric aperture synthesis is being investigated as an alternative to real aperture measurements of the brightness temperature of the earth from low earth orbit. Aperture synthesis allows for the realization of space borne microwave systems with attractive spatial resolution characteristics. Radio astronomers have performed aperture synthesis by cross-correlating spatially separated antenna element pairs for some time. The application of this technique to the observation of a distributed source, as opposed to a point source in radio astronomy, presents a novel and challenging application of aperture synthesis. The demonstration of this geophysical application of aperture synthesis was carried out by constructing and flight testing a phototype instrument, the Electrically Scanned Thinned Array Radiometer (ESTAR). The subsequent data analysis and image reconstruction yielded an image of the eastern shore of Virginia. In the past year, more data of the same region were taken and analyzed, as well as for other regions of Virginia, Pennsylvania, and Maine. Improvements were made in the image reconstruction technique and in the system electronics for the current and upcoming instruments (ESTAR00 and ESTAR0, respectively), particularly in terms of calibration instrumentation and techniques.

Griffis, Andrew J.↗

NASA NTRS1991

Airborne oceanographic lidar participation in the US Joint Global Ocean Flux Study (JGOFS)

The U.S. JGOFS is part of a major international field experiment which the National Science Foundation (NSF) is conducting during the decade. It will combine ship and aircraft data collected in both the Atlantic and Pacific oceans. The primary data collected by the Airborne Oceanographic Lidar (AOL) is laser-induced chlorophyll fluorescence together with the concurrent laser-induced water Raman backscatter and phycoerythrin fluorescence. Passive solar-induced up-welling radiances and down-welling solar irradiance are also acquired. Results from the 1989 Atlantic mission as well as plans for use of the new long-range Wallops P-3 during the upcoming Equatorial Pacific mission are presented.

Hoge, Frank E.↗

NASA NTRS1991

Applying kinematic GPS to airborne laser remote sensing

Results from the application of differential carrier phase tracking of the GPS constellation of satellites to precise positioning of an aircraft are presented. These aircraft positions are utilized in the process of providing geodetic quality position information for airborne laser footprint location on the surface of the earth. The combination of these techniques provides a rapid, cost effective means for collecting geodetic and topographic data, including sea surface topography. Operational techniques and constraints, along with result from recently collected data, are presented.

Krabill, William B.↗

NASA NTRS1990

Determination of cloud liquid water content using the SSM/I

As part of a calibration/validation effort for the special sensor microwave/imager (SSM/I), coincident observations of SSM/I brightness temperatures and surface-based observations of cloud liquid water were obtained. These observations were used to validate initial algorithms and to derive an improved algorithm. The initial algorithms were divided into latitudinal-, seasonal-, and surface-type zones. It was found that these initial algorithms, which were of the D-matrix type, did not yield sufficiently accurate results. The surface-based measurements of channels were investigated; however, the 85V channel was excluded because of excessive noise. It was found that there is no significant correlation between the SSM/I brightness temperatures and the surface-based cloud liquid water determination when the background surface is land or snow. A high correlation was found between brightness temperatures and ground-based measurements over the ocean.

Alishouse, John C.↗

NASA NTRS1991

Top/bottom multisensor remote sensing of Arctic sea ice

Results are presented on the Aircraft/Submarine Sea Ice Project experiment carried out in May 1987 to investigate concurrently the top and the bottom features of the Arctic sea-ice cover. Data were collected nearly simultaneously by instruments aboard two aircraft and a submarine, which included passive and active (SAR) microwave sensors, upward looking and sidescan sonars, a lidar profilometer, and an IR sensor. The results described fall into two classes of correlations: (1) quantitative correlations between profiles, such as ice draft (sonar), ice elevation (laser), SAR backscatter along the track line, and passive microwave brightness temperatures; and (2) qualitative and semiquantitative correlations between corresponding areas of imagery (i.e., passive microwave, AR, and sidescan sonar).

Comiso, J. C.↗

NASA NTRS1991

Ordering and phase transitions in random-field Ising systems

An exact analysis of the Ising model with infinite-range interactions in a random field and a local mean-field theory in three dimensions is carried out leading to a phase diagram with several coexistence surfaces and lines of critical points. The results show that the phase diagram depends crucially on whether the distribution of random fields is symmetric or not. Thus, Ising-like phase transitions in a porous medium (the asymmetric case) are in a different universality class from the conventional random-field model (symmetric case).

Maritan, Amos↗

NASA NTRS1991

Concurrent remote sensing of Arctic sea ice from submarine and aircraft

In May 1987 a concurrent remote sensing study of Arctic sea ice from above and below was carried out. A submarine equipped with sidescan and upward looking sonar collaborated with two remote sensing aircraft equipped with passive microwave, synthetic aperture radar (SAR), a laser profilometer and an infrared radiometer. By careful registration of the three tracks it has been possible to find relationships between ice type, ice morphology and thickness, SAR backscatter and microwave brightness temperatures. The key to the process has been the sidescan sonar's ability to identify ice type through differences in characteristic topography. Over a heavily ridged area of mainly multiyear ice there is a strong positive correlation between SAR backscatter and ice draft or elevation. It was also found that passive and active microwave complement each other in that SAR has a high contrast between open water and multiyear ice, while passive microwave has a high contrast between open water and first-year ice.

Wadhams, P.↗

NASA NTRS1991

High efficiency pump for space helium transfer

A centrifugal pump was developed for the efficient and reliable transfer of liquid helium in space. The pump can be used to refill cryostats on orbiting satellites which use liquid helium for refrigeration at extremely low temperatures. The pump meets the head and flow requirements of on-orbit helium transfer: a flow rate of 800 L/hr at a head of 128 J/kg. The overall pump efficiency at the design point is 0.45. The design head and flow requirements are met with zero net positive suction head, which is the condition in an orbiting helium supply Dewar. The mass transfer efficiency calculated for a space transfer operation is 0.99. Steel ball bearings are used with gas fiber-reinforced teflon retainers to provide solid lubrication. These bearings have demonstrated the longest life in liquid helium endurance tests under simulated pumping conditions. Technology developed in the project also has application for liquid helium circulation in terrestrial facilities and for transfer of cryogenic rocket propellants in space.

Hasenbein, Robert↗

NASA NTRS1991

Frequency dependence of electromagnetic bias in radar altimeter sea surface range measurements

Range measurements made by satellite radar altimeters experience a bias toward the troughs of ocean waves. A series of aircraft flights during February-April 1989 measured this electromagnetic (EM) bias at three radar frequencies and the UV under a variety of wind and wave conditions, and provided the first airborne open-ocean measurements at the 13.6-GHz and 5.3-GHz operating frequencies of the NASA altimeter on the TOPEX/Poseidon satellite. The data suggest that the mean EM bias decreases linearly with increasing radar frequency between 5.3 and 36 GHz, according to the expression: EM bias (percent of significant wave height) = (3.0-0.0617 F)(1 +/-0.5), where F is in gigahertz. EM bias is fairly constant over a mesoscale region on a given day but can fluctuate significantly from one day to another. It shows a strong increase at all radar frequencies with increasing wind speed, although other sea state conditions, such as the wind direction relative to the wave direction, are also factors.

Walsh, E. J.↗

NASA NTRS1991

Imaging the earth's magnetosphere - Effects of plasma flow and temperature

The effects of Doppler shifting on the line centers of the magnetospheric O(+) cross section are investigated, and the resulting structure of the scattering rate as a function of bulk density is explained. Whereas the Doppler shifting frequently results in a decrease of the scattering rate, it is demonstrated that for certain drift speeds the overlap of the cross section and the solar intensity profile can lead to an increased rate, thus enhancing the relative brightness of the image above that obtained when v(p) is zero. Simulated images of the magnetosphere are obtained which are used to show quantitively how the magnetospheric image responds to variations in plasma drift speed and temperature. Changes in the brightness of the magnetospheric images also depend on the variability of the solar flux at 83.4 nm. In regions where there are plasma drifts, the brightness in the image is governed by the structure of the scattering rate, assuming a fixed temperature.

Garrido, D. E.↗

NASA NTRS1990

Photosynthetic accessory pigments - Evidence for the influence of phycoerythrin on the submarine light field

Upwelled water-leaving spectral radiances and the submarine light-field source are theorized to have the physical scattering and absorption effects of photosynthetic accessory pigments. A laser is employed to induce phycoerythrin pigment fluorescence, and the presence of the photosynthetic accessory pigment is measured in oceanic waters with a spectral fluorescence method. Experimental data from active-passive correlation spectroscopy demonstrate that water-leaving spectral radiances of about 600 nm are highly correlated with phycoerythrin pigment fluorescence. Because the data on the phycoerythrin and chlorophyll fluorescences are entered randomly in the correlation the results are considered unambiguous. An estimate of the photopigment is derived that agrees with measurements of its laser-induced fluorescence.

Hoge, Frank E.↗

NASA NTRS1991

Aperture synthesis concepts in microwave remote sensing of the earth

The application of aperture synthesis concepts, used for many years in radio astronomy to achieve high image resolution at a reasonable cost, to remote sensing technology is discussed. The electronically scanned thinned array radiometer (ESTAR) is put forward as a viable alternative to improve spatial resolution by an order of magnitude over what is presently achieved by microwave imaging systems that are collecting data from earth orbit. Future developments in airborne sensor technology and potential spacecraft application are described.

Swift, Calvin T.↗

NASA NTRS1992

Plasma dynamics on current-carrying magnetic flux tubes

A 1D numerical simulation is used to investigate the evolution of a plasma in a current-carrying magnetic flux tube of variable cross section. A large potential difference, parallel to the magnetic field, is applied across the domain. The result is that density minimum tends to deepen, primarily in the cathode end, and the entire potential drop becomes concentrated across the region of density minimum. The evolution of the simulation shows some sensitivity to particle boundary conditions, but the simulations inevitably evolve into a final state with a nearly stationary double layer near the cathode end. The simulation results are at sufficient variance with observations that it appears unlikely that auroral electrons can be explained by a simple process of acceleration through a field-aligned potential drop.

Swift, Daniel W.↗

NASA NTRS1991

A synthetic aperture microwave radiometer to measure soil moisture and ocean salinity from space

A concept is presented for a microwave radiometer in space to measure soil moisture and ocean salinity as part of an 'Earth Probe' mission. The measurements could be made using an array of stick antennas. The L-band channel (1.4 GHz) would be the primary channel for determining soil moisture, with the S-band (2.65-GHz) and C-band (5.0-GHz) channels providing ancillary information to help correct for the effects of the vegetation canopy and possibly to estimate a moisture profile. A preliminary study indicates that an orbit at 450 km would provide coverage of better than 95 percent of the earth every 3 days. A 10-km resolution cell (at nadir) requires stick antennas about 9.5-m long at L-band. The S-band and C-band sticks would be substantially shorter (5 m and 2.7 m, respectively).

Le Vine, D. M.↗

NASA NTRS1992

Particle simulations of driven collisionless magnetic reconnection at the dayside magnetopause

Particle simulations (including both ions and electrons) of the driven collisionless magnetic reconnection process at the earth's dayside magnetopause were carried out using a specially developed two-and-one-half-dimensional electromagnetic particle code with a driven inflow boundary and an open outflow boundary. The simulations were used to examine many features associated with the dayside magnetic reconnection process, including the acceleration and heating of particles and the generation of energetic particles and particle heat flux. In addition, the fluctuating electromagnetic field associated with a driven collisionless magnetic reconnection was examined and compared with the satellite observations of flux transfer events at the day-side magnetopause.

Ding, D. Q.↗

NASA NTRS1992

Random-anisotropy Blume-Emery-Griffiths model

The results are described of studies of a random-anisotropy Blume-Emery-Griffiths spin-1 Ising model using mean-field theory, transfer-matrix calculations, and position-space renormalization-group calculations. The interplay between the quenched randomness of the anisotropy and the annealed disorder introduced by the spin-1 model leads to a rich phase diagram with a variety of phase transitions and reentrant behavior. The results may be relevant to the study of the phase separation of He-3 - He-4 mixtures in porous media in the vicinity of the superfluid transition.

Maritan, Amos↗