Showing papers by "Jet Propulsion Laboratory published in 1993"

Active Pixel Sensors: Are CCD's Dinosaurs?

Abstract: Charge-coupled devices (CCDs) are presently the technology of choice for most imaging applications. In the 23 years since their invention in 1970, they have evolved to a sophisticated level of performance. However, as with all technologies, we can be certain that they will be supplanted someday. In this paper, the Active Pixel Sensor (APS) technology is explored as a possible successor to the CCD. An active pixel is defined as a detector array technology that has at least one active transistor within the pixel unit cell. The APS eliminates the need for nearly perfect charge transfer--the Achilles' heel of CCDs. This perfect charge transfer makes CCD's radiation 'soft,' difficult to use under low light conditions, difficult to manufacture in large array sizes, difficult to integrate with on-chip electronics, difficult to use at low temperatures, difficult to use at high frame rates, and difficult to manufacture in non-silicon materials that extend wavelength response. With the active pixel, the signal is driven from the pixel over metallic wires rather than being physically transported in the semiconductor. This paper makes a case for the development of APS technology. The state of the art is reviewed and the application of APS technology to future space-based scientific sensor systems is addressed.

A search for life on Earth from the Galileo spacecraft

Abstract: In its December 1990 fly-by of Earth, the Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium; together, these are strongly suggestive of life on Earth. Moreover, the presence of narrow-band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence. These observations constitute a control experiment for the serach for extraterrestrial life by modern interplanetary spacecraft.

Surface Ices and the Atmospheric Composition of Pluto

Abstract: Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.

Ices on the Surface of Triton

Abstract: The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Triton's surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.

Radio Emission from the Heliopause Triggered by an Interplanetary Shock

Abstract: A strong heliospheric radio emission event has been detected by Voyagers 1 and 2 in the frequency range of 2 to 3 kilohertz. This event started in July 1992 and is believed to have been generated at or near the heliopause by an interplanetary shock that originated during a period of intense solar activity in late May and early June 1991. This shock produced large plasma disturbances and decreases in cosmic ray intensity at Earth, Pioneers 10 and 11, and Voyagers 1 and 2. The average propagation speed estimated from these effects is 600 to 800 kilometers per second. After correction for the expected decrease in the shock speed in the outer heliosphere, the distance to the heliopause is estimated to be between 116 and 177 astronomical units.

VLA observations of a complete sample of core-dominated radio sources

Abstract: Maps with high dynamic range are presented of a well-defined sample of powerful core-dominated radio sources, all of which have 5-GHz core flux densities >1 Jy. The maps were made at a frequency of 1.64 GHz from combined VLA and B configuration data. A novel technique for producing high dynamic range maps of variable sources from data taken at different epochs is described. It is found that, on average, BL Lac objects are not more core-dominated than the quasars in the sample. This provides no support for the view that BL Lacs are those quasars seen at such small angles to the line of sight that their relativistically beamed core emission swamps that from other components. The results are consistent with most BL Lac objects being the beamed cores of low-luminosity (FRI) radio galaxies

The opposition effect of the moon: the contribution of coherent backscatter.

Abstract: The opposition effect, the sharp surge in brightness of an astronomical object observed near zero phase angle, which has been known for more than a century, has generally been explained by shadow hiding. The reflectances of several Apollo lunar soil samples have been measured as a function of phase angle in linearly and circularly polarized light. All samples exhibited a decrease in the linear polarization ratio and an increase in the circular polarization ratio in the opposition peak. This provides unequivocal proof that most of the lunar opposition effect is caused by coherent backscatter, not shadow hiding. This result has major implications for the interpretation of photometric observations of bodies in the solar system, including the Earth.

Stratospheric transport from the tropics to middle latitudes by planetary-wave mixing

Abstract: TRANSPORT of air from the troposphere to the stratosphere takes place mainly in the tropics1. By studying satellite records of the dispersal of volcanic aerosols from tropical eruptions, Trepte and Hitchman2 concluded that there is a barrier inhibiting the transport of stratospheric air from the tropics to middle latitude, raising the question of how stratospheric material that has been transported from the troposphere is subsequently conveyed to higher latitudes. Here we present global maps of nitrous oxide and water mixing ratios obtained by the Upper Atmosphere Research Satellite. We see strong latitudinal gradients in these trace species, confirming the existence of a barrier to transport. But superimposed on this background structure we also see planetary-scale ‘tongues’ of tropical stratospheric air extending out into middle latitudes, and time sequences show irreversible mixing from the tropics into middle latitudes. Such episodes could be responsible for transporting significant quantities of stratospheric air across the tropical barrier.

Three-component scattering model to describe polarimetric SAR data

Abstract: Classification, decomposition and modeling of polarimetric SAR data has received a great deal of attention in the recent literature. The objective behind these efforts is to better understand the scattering mechanisms which give rise to the polarimetric signatures seen in SAR image data. In this paper, a new approach is described, which involves the fit of a combination of three simple scattering mechanisms to the polarimetric SAR observations. The mechanisms are volume scatter from a cloud of randomly oriented dipoles, even-bounce scatter from a pair of orthogonal surfaces with different dielectric constants and Bragg scatter from a moderately rough surface. This composite scattering model is used to describe the polarimetric backscatter from naturally occurring scatterers. Results are presented of application of this new algorithm to different types of scene, including multi-frequency polarimetric SAR images of a tropical rain forest, a boreal forest, a pine forest, geologic targets, urban areas and agricultural fields. Fitting the model to polarimetric SAR data of the tropical rain forest for example, allows clear discrimination between flooded and non-flooded forest. The model can be used to estimate the overall contribution from each of the three basic scattering mechanisms for each SAR image pixel.

Latitudinal variation of solar wind corotating stream interaction regions: Ulysses

Abstract: Ulysses' initial transit tot high heliographic latitudes at a heliocentric distance of approximately 5 AU has revealed systematic effects in the latitudinal evolution of corotating interaction regions (CIRs). At a latitude corresponding roughly to, but slightly less than, the inferred tilt of the coronal streamer belt and embedded heliospheric current sheet, the strong forward shocks commonly associated with CIRs at lower latitudes disappeared almost entirely; however, the reverse shocks associated with these CIRs persisted to latitudes approximately 10 deg above the streamer belt. Systematic meridional flow deflections observed in association with the forward and reverse waves bounding the CIRs demonstrate that the above effect is the result of the fact that the forward waves propagate to lower latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a three-dimensional model of corotating solar wind flows that originate in a tilted dipole geometry back at the Sun.

Chemical loss of ozone in the Arctic polar vortex in the winter of 1991- 1992

Abstract: In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

Use of Spectralon as a diffuse reflectance standard for in-flight calibration of earth-orbiting sensors

Abstract: Spectralon, a commercially available diffuse reflectance material made from polytetrafluoroethylene (PTFE), is being evaluated for the multiangle imaging spectroradiometer (MISR), currently under development for the Earth Observing System. Results of a series of environmental exposure tests indicate that no degradation of the optical properties was apparent following proton bombardment, and stability through UV illumination was satisfactory, provided simple cleaning and handling procedures were implemented. A buildup of several thousand volts of static charge was found to develop while simulating a rare pass through an auroral storm.

Optical properties and environmental stability of oxide coatings deposited by reactive sputtering

Abstract: Refractory metal-oxide coatings are deposited by reactive dc magnetron sputtering in an oxygen environment. The optical constants and the environmental stability of silicon oxide, aluminium oxide, hafnium oxide, zirconium oxide, tantalum oxide, titanium oxide, and a blend of hafnium oxide with silicon oxide are investigated. Properties of both single-layer and multilayer interference filters are examined.

Infrared readout electronics for space-science sensors: state of the art and future directions

Abstract: A review of infrared readout electronics for space science sensors is presented. General requirements for scientific IR FPA readout are discussed. Specific approaches to the unit cell electronics are described with respect to operation, complexity, noise and other operating parameters. Recent achievements in IR FPA readout electronics are reviewed. Implementation technologies for realization of IR FPA readout electronics are discussed. Future directions for addressing NASA and other scientific users' needs are suggested.

Grain mantles in the Taurus dark cloud

Abstract: We have obtained low-resolution (λ/Δλ ∼ 160) 3-μm spectra of a selection of field stars lying behind the Taurus dark cloud. Several of these stars have abnormal extinction laws, which are best characterized by A K ≤ 0.27E(J−K) and A V ≤ 5.4 E(J−K). We find a very good correlation between A V and τ 3.05 (the H 2 O ice feature), confirming a previous result, and between A V and τ 3.45 (the long-wavelength wing), yielding the ratios τ 3.05 /A V = 0.059 ± 0.003 and τ 3.45 /A V = 0.0077 ± 0.0005. The threshold extinctions for the H 2 O ice band and the long-wavelength wing are found to be the same, within the uncertainties, at A v0 = 2.6. The fact that these thresholds are the same means that the long-wavelength wing cannot be a result of a highly refractory hydrocarbon residue

Irradiance variance of optical waves through atmospheric turbulence by numerical simulation and comparison with experiment

Abstract: Irradiance variance for optical propagation through atmospheric turbulence is calculated by numerical simulation. Let l0 be the inner scale, Rf be the Fresnel scale, and β02 be the weak-fluctuation irradiance variance at zero inner scale. Then results in the strong-focusing regime just past the peak can be summarized by σI2 = 1.74 − 0.092β0 + 0.60(l0/Rf) for a plane wave and σI2 = 3.02 − 0.35β0 + 5.56(l0/Rf) for a point source. These numerical results are in excellent agreement with experimental results.

The abundance of sulfur dioxide below the clouds of venus

Abstract: We present a new method for determining the abundance of sulfur dioxide below the clouds of Venus. Absorption by the 3nu3 band of SO2 near 2.45 microns has been detected in high-resolution spectra of the night side of Venus recorded at the Canada-France Hawaii telescope in 1989 and 1991. The inferred SO2 abundance is 130 +/- 40 ppm at all observed locations and pertains to the 35-45 km region. These values are comparable to those measured by the Pioneer Venus and Venera 11/12 entry probes in 1978. This stability stands in contrast to the apparent massive decrease in SO2 observed at the cloud tops since these space missions. These results are consistent with laboratory and modeling studies of the SO2 destruction rates in the lower atmosphere of Venus. The new spectroscopic technique presented here allows a remote monitoring of the SO2 abundance below the clouds, a likely tracer of Venusian volcanism.

Ultraviolet stability and contamination analysis of Spectralon diffuse reflectance material

Abstract: A detailed chemical analysis was carried out on Spectralon, a highly Lambertian, diffuse reflectance material. Results of this investigation unambiguously identified the presence of an organic (hydrocarbon) impurity intrinsic to the commercial material. This impurity could be removed by a vacuum bake-out procedure and was identified as the cause of optical changes (degradation) that occur in the material when exposed to UV light. It was found that when this impurity was removed, the Spectralon material was photochemically stable and maintained its reflectance properties even after extensive solar UV exposure.

Estimation of aerosol optical depth, pressure elevation, water vapor, and calculation of apparent surface reflectance from radiance measured by the airborne visible/infrared imaging spectrometer (AVIRIS) using a radiative transfer code

Abstract: The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is an imaging spectrometer that measures spatial images of the total up welling spectral radiance from 400 to 2500 nm at 10 nm spectral intervals. Quantitative research and application objectives for surface investigations require conversion of the measured radiance to surface reflectance or surface leaving radiance. To calculate apparent surface reflectance an estimation of aerosol optical depth is required for compensation of aerosol scattering and absorption across the spectral range. Determination of other atmospheric characteristics such as atmospheric water vapor and surface pressure is also required. In this paper we describe a set of algorithms to estimate aerosol optical depth, atmospheric water vapor, and surface pressure height from the AVIRIS measured radiance. Based upon these determined atmospheric parameters we described an algorithm to calculated apparent surface reflectance from the AVIRIS measured radiance using a radiative transfer code.

Counterstreaming suprathermal electron events upstream of corotating shocks in the solar wind beyond ∼2 Au: Ulysses

Abstract: Enhanced fluxes of suprathermal electrons are commonly observed upstream of corotating forward and reverse shocks in the solar wind at heliocentric distances beyond approximately 2 AU by the Los Alamos plasma experiment on Ulysses. The average duration of these events, which are most intense immediately upstream from the shocks and which fade with increasing distance from them, is approximately 2.4 days near 5 AU. These events are caused by the leakage of shock-heated electrons into the upstream region. The upstream regions of these shocks face back toward the Sun along the interplanetary magnetic field, so these leaked electrons commonly counterstream relative to the normal solar wind electron heat flux. The observations suggest that conservation of magnetic moment and scattering typically limit the sunward propagation of these electrons as beams to field-aligned distances of approximately 15 AU. Although it seems unlikely that these shock-associated events are an important source of counterstreaming events near 1 AU, remnants of the backstreaming beams may contribute importantly to the diffuse solar wind halo electron population there.

Geodetic measurement of deformation in the Ventura Basin Region, southern California

Abstract: We have measured the deformation in the Ventura basin region, southern California, with Global Positioning System (GPS) measurements carried out over 4.6 years between 1987 and 1992. The deformation within our network is spatically variable on scales of tens of kilometers, with strain rates reaching 0.6 +/- 1 micro-rad/yr in the east-central basin. Blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower (0.06 +/- 1 micro-rad/yr to the south). We also observed clockwise rotations of 1 deg - 7 deg/m.y. Shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and GPS measurements give consistent, though less precise, results. The geodetic rates of shortening across the basin and Western Transverse Ranges are lower than those estimated from geological observations, but the patterns of deformation from the two methods agree qualitatively.

Human engineering in stereoscopic viewing devices

Abstract: Introduction. Stereoscopic Properties of the Human Visual System. Methods of Viewing 3D Images. Double Camera Systems. Single Camera Systems. SpatiallySampling Cameras and Monitors. The Observer. Moving Objects in the Work Space. Reducing Depth Distortions for Converged Cameras. Setting Up a Stereoscopic Camera System. Index.

Recent solar extreme ultraviolet irradiance observations and modeling: A review

Abstract: For more than 90 years, solar extreme ultraviolet (EUV) irradiance modeling has progressed from empirical blackbody radiation formulations, through fudge factors, to typically measured irradiances and reference spectra was well as time-dependent empirical models representing continua and line emissions. A summary of recent EUV measurements by five rockets and three satellites during the 1980s is presented along with the major modeling efforts. The most significant reference spectra are reviewed and threee independently derived empirical models are described. These include Hinteregger's 1981 SERF1, Nusinov's 1984 two-component, and Tobiska's 1990/1991/SERF2/EUV91 flux models. They each provide daily full-disk broad spectrum flux values from 2 to 105 nm at 1 AU. All the models depend to one degree or another on the long time series of the Atmosphere Explorer E (AE-E) EUV database. Each model uses ground- and/or space-based proxies to create emissions from solar atmospheric regions. Future challenges in EUV modeling are summarized including the basic requirements of models, the task of incorporating new observations and theory into the models, the task of comparing models with solar-terrestrial data sets, and long-term goals and modeling objectives. By the late 1990s, empirical models will potentially be improved through the use of proposed solar EUV irradiance measurements and images at selected wavelengths that will greatly enhance modeling and predictive capabilities.

High-speed joint-transform optical image correlator using GaAs/AlGaAs semi-insulating multiple quantum wells and diode lasers

Abstract: We demonstrate a compact real-time optical image correlator using diode lasers and a semi-insulating GaAs/AlGaAs multiple-quantum-well (SI-MQW) device as the holographic element With only 3 mW of power incident upon the SI-MQW device, the correlation is obtained in 1 microsec and can be erased in as fast as 2 microsec, which presents the possibility of a system capable of 3 x 10 exp 5 correlations/s We also show that images can be stored for a controllable length of time (2-25 microsec), which presents possibilities for the use of the SI-MQW device as a data or image buffer memory

On loss functions which minimize to conditional expected values and posterior probabilities

Abstract: A loss function, or objective function, is a function used to compare parameters when fitting a model to data. The loss function gives a distance between the model output and the desired output. Two common examples are the squared-error loss function and the cross entropy loss function. Minimizing the mean-square error loss function is equivalent to minimizing the mean square difference between the model output and the expected value of the output given a particular input. This property of minimization to the expected value is formalized as P-admissibility. The necessary and sufficient conditions for P-admissibility, leading to a parametric description of all P-admissible loss functions, are found. In particular, it is shown that two of the simplest members of this class of functions are the squared error and the cross entropy loss functions. One application of this work is in the choice of a loss function for training neural networks to provide probability estimates. >