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At least 451 records · Page 25

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NASA NTRS1995

High Precision Piezoelectric Linear Motors for Operations at Cryogenic Temperatures and Vacuum

The Jet Propulsion Laboratory evaluated the use of an electromechanical device for optically positioning a mirror system during the pre-project phase of the Pluto-Fast-Flyby (PFF) mission. The device under consideration was a piezoelectric driven linear motor functionally dependent upon a time varying electric field which induces displacements ranging from submicrons to millimeters with positioning accuracy within nanometers. Using a control package, the mirror system provides image motion compensation and mosaicking capabilities. While this device offers unique advantages, there were concerns pertaining to its operational capabilities for the PFF mission. The issues include irradiation effects and thermal concerns. A literature study indicated that irradiation effects will not significantly impact the linear motor's operational characteristics. On the other hand, thermal concerns necessitated an in depth study.

piezoelectric motors↗

NASA NTRS2011

Characterization of Photon-Counting Detector Responsivity for Non-Linear Two-Photon Absorption Process

Sub-band absorption at 1550 nm has been demonstrated and characterized on silicon Geiger mode detectors which normally would be expected to have no response at this wavelength. We compare responsivity measurements to singlephoton absorption for wavelengths slightly above the bandgap wavelength of silicon (approx. 1100 microns). One application for this low efficiency sub-band absorption is in deep space optical communication systems where it is desirable to track a 1030 nm uplink beacon on the same flight terminal detector array that monitors a 1550 nm downlink signal for pointingcontrol. The currently observed absorption at 1550 nm provides 60-70 dB of isolation compared to the response at 1064 nm, which is desirable to avoid saturation of the detector by scattered light from the downlink laser.

Geiger mode detector↗

NASA NTRS1988

Optical matrix-vector processing for computational fluid dynamics

An optical processor to solve partial differential equations for computational fluid dynamics applications is considered. This application is new and original for optical processors. The algorithms that are used are optical realizations of the Newton-Raphson method for nonlinear equations and a new optical LU direct decomposition and Gauss-Seidel iterative solution to the resultant linear algebraic equations. These algorithms are used to solve Burger's equation (a specific form of the momentum equation in fluid dynamics). The nonlinear equations provide 1-D velocity data at each time step. Simulation results of optical processing with these algorithms on computational fluid dynamics data is included.

Perlee, Caroline J.↗

NASA NTRS2008

Magnetic Compensation for Second-Order Doppler Shift in LITS

The uncertainty in the frequency of a linear-ion-trap frequency standard (LITS) can be reduced substantially by use of a very small magnetic inhomogeneity tailored to compensate for the residual second-order Doppler shift. An effect associated with the relativistic time dilatation, one cause of the second-order Doppler shift, is ion motion that is attributable to the trapping radio-frequency (RF)electromagnetic field used to trap ions. The second-order Doppler shift is reduced by using a multi-pole trap; however it is still the largest source of systematic frequency shift in the latest generation of LITSs, which are among the most stable clocks in the world. The present compensation scheme reduces the frequency instability of the affected LITS to about a tenth of its previous value. The basic principles of prior generation LITSs were discussed in several prior NASA Tech Briefs articles. Below are recapitulated only those items of basic information necessary to place the present development in context. A LITS includes a microwave local oscillator, the frequency of which is stabilized by comparison with the frequency of the ground state hyperfine transition of 199Hg+ ions. The comparison involves a combination of optical and microwave excitation and interrogation of the ions in a linear ion trap in the presence of a nominally uniform magnetic field. In the current version of the LITS, there are two connected traps (see figure): (1) a quadrupole trap wherein the optical excitation and measurement take place and (2) a 12-pole trap (denoted the resonance trap), wherein the microwave interrogation takes place. The ions are initially loaded into the quadrupole trap and are thereafter shuttled between the two traps. Shuttling ions into the resonance trap allows sensitive microwave interrogation to take place well away from loading interference. The axial magnetic field for the resonance trap is generated by an electric current in a finely wound wire coil surrounded by magnetic shields. In the quadrupole and 12-pole traps, the potentials are produced by RF voltages applied to even numbers (4 and 12, respectively) of parallel rods equally spaced around a circle. The polarity of the voltage on each rod is opposite that of the voltage on the adjacent rod. As a result, the amplitude of the RF trapping field is zero along the centerline and increases, with radius, to a maximum value near the rods.

Burt, Eric↗

NASA NTRS2018

NASA's Black Marble Nighttime Lights Product Suite

NASA's Black Marble nighttime lights product suite (VNP46) is available at 500 meters resolution since January 2012 with data from the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB) onboard the Suomi National Polar-orbiting Platform (SNPP). The retrieval algorithm, developed and implemented for routine global processing at NASA's Land Science Investigator-led Processing System (SIPS), utilizes all high-quality, cloud-free, atmospheric-, terrain-, vegetation-, snow-, lunar-, and stray light-corrected radiances to estimate daily nighttime lights (NTL) and other intrinsic surface optical properties. Key algorithm enhancements include: (1) lunar irradiance modeling to resolve non-linear changes in phase and libration; (2) vector radiative transfer and lunar bidirectional surface anisotropic reflectance modeling to correct for atmospheric and BRDF (Bidirectional Reflectance Distribution Function) effects; (3) geometric-optical and canopy radiative transfer modeling to account for seasonal variations in NTL; and (4) temporal gap-filling to reduce persistent data gaps. Extensive benchmark tests at representative spatial and temporal scales were conducted on the VNP46 time series record to characterize the uncertainties stemming from upstream data sources. Initial validation results are presented together with example case studies illustrating the scientific utility of the products. This includes an evaluation of temporal patterns of NTL dynamics associated with urbanization, socioeconomic variability, cultural characteristics, and displaced populations affected by conflict. Current and planned activities under the Group on Earth Observations (GEO) Human Planet Initiative are aimed at evaluating the products at different geographic locations and time periods representing the full range of retrieval conditions.

Suomi-NPP↗

NASA NTRS2015

Modeling the Effects of Spaceflight on the Posterior Eye in VIIP

Purpose: Visual Impairment and Intracranial Pressure (VIIP) syndrome is a new and significant health concern for long-duration space missions. Its etiology is unknown, but is thought to involve elevated intracranial pressure (ICP)that induces connective tissue changes and remodeling in the posterior eye (Alexander et al. 2012). Here we study the acute biomechanical response of the lamina cribrosa (LC) and optic nerve to elevations in ICP utilizing finite element (FE) modeling. Methods: Using the geometry of the posterior eye from previous axisymmetric FE models (Sigal et al. 2004), we added an elongated optic nerve and optic nerve sheath, including the pia and dura. Tissues were modeled as linear elastic solids. Intraocular pressure and central retinal vessel pressures were set at 15 mmHg and 55 mmHg, respectively. ICP varied from 0 mmHg (suitable for standing on earth) to 30 mmHg (representing severe intracranial hypertension, thought to occur in space flight). We focused on strains and deformations in the LC and optic nerve (within 1 mm of the LC) since we hypothesize that they may contribute to vision loss in VIIP. Results: Elevating ICP from 0 to 30 mmHg significantly altered the strain distributions in both the LC and optic nerve (Figure), notably leading to more extreme strain values in both tension and compression. Specifically, the extreme (95th percentile) tensile strains in the LC and optic nerve increased by 2.7- and 3.8-fold, respectively. Similarly, elevation of ICP led to a 2.5- and 3.3-fold increase in extreme (5th percentile) compressive strains in the LC and optic nerve, respectively. Conclusions: The elevated ICP thought to occur during spaceflight leads to large acute changes in the biomechanical environment of the LC and optic nerve, and we hypothesize that such changes can activate mechanosensitive cells and invoke tissue remodeling. These simulations provide a foundation for more comprehensive studies of microgravity effects on human vision, e.g. to guide biological studies in which cells and tissues are mechanically loaded in a ranger elevant for microgravity conditions.

gravitational physiology↗

NASA NTRS1981

E-beam generated holographic masks for optical vector-matrix multiplication

An optical vector matrix multiplication scheme that encodes the matrix elements as a holographic mask consisting of linear diffraction gratings is proposed. The binary, chrome on glass masks are fabricated by e-beam lithography. This approach results in a fairly simple optical system that promises both large numerical range and high accuracy. A partitioned computer generated hologram mask was fabricated and tested. This hologram was diagonally separated outputs, compact facets and symmetry about the axis. The resultant diffraction pattern at the output plane is shown. Since the grating fringes are written at 45 deg relative to the facet boundaries, the many on-axis sidelobes from each output are seen to be diagonally separated from the adjacent output signals.

Arnold, S. M.↗

NASA NTRS1982

An analysis of Seasat SAR for detecting geologic linears

The value of Seasat synthetic aperture radar (SAR) imagery for detecting geologic linears was assessed in a study of an 89,000 sq km section of New York's Adirondack Mountains. A photographic print of optically processed, 1:500,000 scale SAR imagery (one look direction) was analyzed visually, and the detected linears were compared to those recorded on a 1:250,000 scale geologic map. Eighty percent of the 4,170 km of mapped, geologic linears were detected with the SAR imagery. Moreover, nearly 6,900 km of unmapped linears were also detected. Of these, an estimated 90 percent could be observed on high altitude aerial photographs. The relationship between SAR image detection of linears and the different types of indicators (e.g., straight valleys or shorelines) is reported.

Yan, S.-Y.↗

NASA NTRS2000

Proton Damage in Linear and Digital Optocouplers

Optocoupler failures occurred on the Topex-Poseidon spacecraft after about two years of operation. Later work in the laboratory showed that the failures were due to extreme sensitivity of LEDs within the optocouplers to displacement damage from protons. Although earlier work had been done on displacement damage in light-emitting diodes, none of the devices studied previously had been heavily damaged at the low radiation levels where the optocouplers failed in space. Subsequent work has shown that LED damage varies over an extremely wide range, depending on the particular manufacturing technology. This paper discusses proton degradation of linear and digital optocouplers. One obvious way to harden optocoupler technologies is to select LEDs that are more resistant to displacement damage. A direct comparison is made of degradation of a commercial linear optocoupler from one manufacturer with a modified version of the same device with a different LED technology. Other factors, including degradation of optical photoresponse and transistor -ain are also discussed, along with basic comparisons of digital and analog optocouplers. Linear optocouplers are designed with somewhat different requirements than digital optocouplers, which not only affects their radiation response but also the interpretation of radiation test data.

Johnston, A. H.↗

NASA NTRS1993

Optical and Hubble Space Telescope ultraviolet spectropolarimetry of 3C 273 and PG 1114+445

New optical spectropolarimetry and broad-band measurements, made during periods when the linear polarization of the quasar 3C 273 was no greater than 1 pct, are combined with ultraviolet polarization measurements made with the Faint Object Spectrograph of the Hubble Space Telescope. Very weak UV polarization during the time of observation is seen in the HST measurements, which is consistent with the completely unpolarized UV continuum and emission lines. Optical spectropolarimetry, however, reveals polarized light, the source of which is consistent with a nonthermal component, diluted by other unpolarized emission sources, such as the 'big blue bump'. Optical spectropolarimetry of the radio-quiet quasar PG 1114+445 is also presented. The polarization of this QSO is thought to result from scattering by dust external to the narrow-line region. 3C 273 and PG 1114+445 are useful in illustrating the range of polarizing mechanisms which can operate in low-polarization QSOs.

Smith, Paul S.↗

NASA NTRS1967

System enables dimensional inspection of very large structures

Precision rotary table with an integrated optical tooling bar system enables accurate and rapid measurement of linear and angular dimensions on very large structures of any configuration. The structure is mounted on the turntable, which can be rotated to expose any desired surface.

Simpson, R. R.↗

NASA NTRS1982

Land remote sensing activities in the US

The LANDSAT D system, shuttle imaging radar instruments, the thermal infrared multispectral scanner system, the shuttle multispectral infrared radiometer, and the multispectral linear array program are described. These programs will develop advanced optical and microwave sensor technologies to support a broad spectrum of aircraft, shuttle and free flier missions for research in agriculture, geology, hydrology and geography. A broad based fundamental research effort coupled with the development of improved data acquisition and information extraction techniques is planned to improve fundamental understanding of the remote sensing process and fully exploit its capabilities.

Keller, S. W.↗

NASA NTRS1984

Optical analysis of the star-tracker telescope for Gravity Probe

A ray tracing modeling of the star tracker telescope for Gravity Probe was used to predict the character of the output signal and its sensitivity to fabrication errors. In particular, the impact of the optical subsystem on the requirement of 1 milliarc second signal linearity over a + or - 50 milliarc second range was examined. Photomultiplier and solid state detector options were considered. Recommendations are made.

Zissa, D. E.↗

NASA NTRS1989

The double-nucleus galactic merger Mkn 463

Deep CCD imaging in broadband and forbidden O III wavelengths of the twin-nucleus active galaxy Markarian 463 is presented. These data show that the system has triple curved tails indicative of a strong tidal interaction and probable merger. Colors, morphology, and their implications are discussed. The forbidden O III line emission is seen to be extended and linear, perhaps filling twin cones with apex at the eastern optical nucleus. It is argued that this indicates nonisotropic radiation from an obscured AGN. Spatially resolved spectroscopy on and off the nuclei reveals that the radial velocities of the gas lie in a small range, unlike some other multiple nucleus systems. The nature of the emission-line regions is discussed, along with the evolutionary state of the system indicated by the present data and IR and radio data.

Hutchings, J. B.↗

NASA NTRS1990

The nature of the optical variations of Seyfert galaxy 3C 120

Results are presented from 61 years of optical observations of the Seyfert galaxy 3C 120. A previously published model of the 3C 120 light curve, derived from power spectrum analysis, is found to be valid for historical as well as current data. It is concluded that the optical variations of 3C 120 can be separated into a linear component, a sinusoidal component, and rapid, high-amplitude flares. Possible sources of the regular variations observed in 3C 120 are also suggested in the context of accretion models and other theoretical models.

Webb, James R.↗

NASA NTRS1992

Sea surface mean square slope from Ku-band backscatter data

A surface mean-square-slope parameter analysis is conducted for 14-GHz airborne radar altimeter near-nadir, quasi-specular backscatter data, which in raw form obtained by least-squares fitting of an optical scattering model to the return waveform show an approximately linear dependence over the 7-15 m/sec wind speed range. Slope data are used to draw inferences on the structure of the high-wavenumber portion of the spectrum. A directionally-integrated model height spectrum that encompasses wind speed-dependent k exp -5/2 and classical Phillips k exp -3 power laws subranges in the range of gravity waves is supported by the data.

Jackson, F. C.↗

NASA NTRS1998

Validation of POLDER/ADEOS data using a ground-based lidar network: Preliminary results for semi-transparent and cirrus clouds

At mid and tropical latitudes, cirrus clouds are present more than 50% of the time in satellites observations. Due to their large spatial and temporal coverage, and associated low temperatures, cirrus clouds have a major influence on the Earth-Ocean-Atmosphere energy balance through their effects on the incoming solar radiation and outgoing infrared radiation. At present the impact of cirrus clouds on climate is well recognized but remains to be asserted more precisely, for their optical and radiative properties are not very well known. In order to understand the effects of cirrus clouds on climate, their optical and radiative characteristics of these clouds need to be determined accurately at different scales in different locations i.e. latitude. Lidars are well suited to observe cirrus clouds, they can detect very thin and semi-transparent layers, and retrieve the clouds geometrical properties i.e. altitude and multilayers, as well as radiative properties i.e. optical depth, backscattering phase functions of ice crystals. Moreover the linear depolarization ratio can give information on the ice crystal shape. In addition, the data collected with an airborne version of POLDER (POLarization and Directionality of Earth Reflectances) instrument have shown that bidirectional polarized measurements can provide information on cirrus cloud microphysical properties (crystal shapes, preferred orientation in space). The spaceborne version of POLDER-1 has been flown on ADEOS-1 platform during 8 months (October 96 - June 97), and the next POLDER-2 instrument will be launched in 2000 on ADEOS-2. The POLDER-1 cloud inversion algorithms are currently under validation. For cirrus clouds, a validation based on comparisons between cloud properties retrieved from POLDER-1 data and cloud properties inferred from a ground-based lidar network is currently under consideration. We present the first results of the validation.

Chepfer, H.↗

NASA NTRS1994

Enhanced Reverse Saturable Absorption and Optical Limiting in Heavy-Atom Substituted Phthalocyanines

The reverse saturable absorption and optical limiting response of metal phthalocyaninies can be enhanced by using the heavy-atom effect. Phthalocyanines containing heavy metal atoms, such as In, Sn, and Pb show nearly a factor of two enhancement in the ratio of effective excited-state to ground-state absorption cross sections compared to those containing lighter atoms, such as Al and Si. In an f/8 optical geometry, homogeneous solutions of heavy metal phthalocyanines, at 30% linear transmission, limit 8-ns, 532-nm laser pulses to less than or equal to 3 (micro)J (the energy for 50% probability of eye damage) for incident pulses up to 800 (micro)J.

metal phthalocyanines↗