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At least 289 records · Page 16

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NASA NTRS1990

Spectral albedo and emissivity of CO2 in Martian polar caps - Model results

In this paper, a snow albedo model previously developed for terrestrial snow is extended to the case of CO2 snow on Mars. Pure CO2 snow is calculated to have high albedo at visible wavelengths but not as high as that of water snow. At any given wavelength, the primary variable controlling albedo and emissivity is the snow grain size, with albedo decreasing and emissivity increasing as grain size increases. Observations that red albedo is much higher than blue albedo in the Martian south polar cap indicates that the snow or the atmosphere is contaminated with red dust. The absorption coefficient of CO2 ice in the thermal infrared is two to three orders of magnitude smaller than that measured for H2O ice. CO2 snow emissivity is therefore much lower than H2O snow, varying substantially with wavelength and quite sensitive to grain size and emission angle. Factors tending to increase emissivity are large grain size, small emission angle, and large concentrations of dust or water.

Warren, Stephen G.↗

NASA NTRS1990

AVHRR and VISSR satellite instrument calibration results for both Cirrus and marine stratocumulus IFO periods

Accurate characterizations of some cloud parameters are dependent upon the absolute accuracy of satellite radiance measurements. Visible wavelength measurements from both the AVHRR and VISSR instruments are often used to study cloud characteristics. Both of these instruments were radiometrically calibrated prior to launch, but neither has an onboard device to monitor degradation after launch. During the FIRE/SRB cirrus Intensive Field Operation (IFO), a special effort was made to monitor calibration of these two instruments onboard the NOAA-9 and GOES-6 spacecraft. In addition, several research groups have combined their efforts to assess the long-term performance of both instruments. These results are presented, and a limited comparison is made with the ERBE calibration standard.

Whitlock, C. H.↗

NASA NTRS1990

Sensor requirements for Earth and planetary observations

Future generations of Earth and planetary remote sensing instruments will require extensive developments of new long-wave and very long-wave infrared detectors. The upcoming NASA Earth Observing System (EOS) will carry a suite of instruments to monitor a wide range of atmospheric and surface parameters with an unprecedented degree of accuracy for a period of 10 to 15 years. These instruments will observe Earth over a wide spectral range extending from the visible to nearly 17 micrometers with a moderate to high spectral and spacial resolution. In addition to expected improvements in communication bandwidth and both ground and on-board computing power, these new sensor systems will need large two-dimensional detector arrays. Such arrays exist for visible wavelengths and, to a lesser extent, for short wavelength infrared systems. The most dramatic need is for new Long Wavelength Infrared (LWIR) and Very Long Wavelength Infrared (VLWIR) detector technologies that are compatible with area array readout devices and can operate in the temperature range supported by long life, low power refrigerators. A scientific need for radiometric and calibration accuracies approaching 1 percent translates into a requirement for detectors with excellent linearity, stability and insensitivity to operating conditions and space radiation. Current examples of the kind of scientific missions these new thermal IR detectors would enhance in the future include instruments for Earth science such as Orbital Volcanological Observations (OVO), Atmospheric Infrared Sounder (AIRS), Moderate Resolution Imaging Spectrometer (MODIS), and Spectroscopy in the Atmosphere using Far Infrared Emission (SAFIRE). Planetary exploration missions such as Cassini also provide examples of instrument concepts that could be enhanced by new IR detector technologies.

Chahine, Moustafa T.↗

NASA NTRS1989

Spatial and spectral characterization of acid rain stress in Canadian Shield lakes

Results from this study demonstrate that a remote sensor can discriminate lake clarity based upon reflection. The basic hypothesis was that seasonal and multiyear changes in lake optical transparency are indicative of sensitivity to acidic deposition. In many acid-sensitive lakes optical transparency is controlled by the amount of dissolved organic carbon (DOC) present. DOC is a strong absorbing, nonscattering material which has the greatest impact at short visible wavelengths, including Thematic Mapper band 1. Acid-sensitive lakes have high concentrations of aluminum which have been mobilized by acidic components contained in the runoff. Aluminum complexing with DOC is considered to be the primary mechanism to account for observed increases in lake transparency in acid-sensitive lakes. Thus seasonal changes in the optical transparency of lakes should provide an indication of the stress due to acid deposition and loading.

Marshall, Elizabeth J.↗

NASA NTRS1991

Southern Hemisphere tropospheric aerosol backscatter measurements - Implications for a laser wind system

To identify the aerosol size fraction that is responsible for the 10.6-micron backscatter and the morphological composition of these particles, values of backscatter coefficient at 0.532 micron, 0.694 micron, and 10.6 microns were determined both by direct measurement and by Mie computation from measured microphysical properties of aerosols over eastern Australia between 4 deg and 41 deg S. The values of backscatter coefficient determined from direct measurements at 10.6 microns were found to agree well with values obtained indirectly from calculations based on measured aerosol particle size distributions and properties. It was also found that backscatter at 10.6 microns and at visible wavelengths were sensitive to different regions of the aerosol size spectrum, with considerable day-to-day variability observed at all wavelengths.

Gras, J. L.↗

NASA NTRS1990

In-situ atmospheric water-vapor retrieval in support of AVIRIS validation

A comparison is made between two ground-based atmospheric water-vapor measurement techniques, each of which uses data from a solar-pointing radiometer. One technique uses visible wavelength channels to retrieve aerosol loading, surface pressure readings for Rayleigh scattering analyses, and the 0.94 micron channel to extract water vapor from the residual of total versus scattering opacity. The other technique requires only the ratio of channels centered at 0.94 and 0.87 micron. Results are given for the April 13, 1989 AVIRIS in-flight calibration.

Bruegge, Carol J.↗

NASA NTRS1991

Light scattering by lunar-like particle size distributions

A fundamental input to models of light scattering from planetary regoliths is the mean phase function of the regolith particles. Using the known size distribution for typical lunar soils, the mean phase function and mean linear polarization for a regolith volume element of spherical particles of any composition were calculated from Mie theory. The two contour plots given here summarize the changes in the mean phase function and linear polarization with changes in the real part of the complex index of refraction, n - ik, for k equals 0.01, the visible wavelength 0.55 micrometers, and the particle size distribution of the typical mature lunar soil 72141. A second figure is a similar index-phase surface, except with k equals 0.1. The index-phase surfaces from this survey are a first order description of scattering by lunar-like regoliths of spherical particles of arbitrary composition. They form the basis of functions that span a large range of parameter-space.

Goguen, Jay D.↗

NASA NTRS1991

Derivation of mid-infrared (5-25 microns) optical constraints of some silicates and palagonite

This study was initially conceived in order to aid in the interpretation of Martian surface and atmospheric aerosol mineralogy. As a result, the minerals included are biased toward samples which represent hydrated and hydroxylated silicates. Due to their physical particle size, clays and other materials, such as palagonite, cannot be prepared using typical preparation techniques. Yet in some cases, such as for Mars, these are the materials of perhaps the greatest interest. In order to obtain a suitable sample of these less cohesive materials for the laboratory measurements, a KBr pellet die was used and a pellet of the pure sample was prepared. For all clays and the palagonite, a pellet with highly reflective surfaces at visible wavelengths was produced. The reflectivities of all samples were determined, and to derive the optical constants of a materials as a function of wavelength, dispersion analysis was used which describes the real and imaginary indices of refraction as the contributions due to a sum of classical oscillators and relates them via Frensel's equations for non-normal incidence, the the measured near-normal reflectivity. The final values that were determined represent averages of several model fits to each data set using the same number of oscillators by varying the high frequency dielectric constant.

Roush, T. L.↗

NASA NTRS1991

Spectral reflectance of thin snow

A radiative transfer model was used to calculate the spectral reflectance of thin snow overlying dark soil. Model results show that directional hemispherical reflectance depends on density, grain size, and solar and viewing geometries. Measurements of thin snow spectral Bidirectional Reflectance Distribution Function (BRDF) show that the reflectance in the visible wavelengths is reduced when the sensor is near nadir yet, at a viewing zenith of 75 degrees the same snowpack will appear to be optically thick. If the snow is sufficiently thin and snow grains are large, the spectral signature of the substrate may influence snow reflectance. This phenomenon was also detected in reflectance data collected using the ground based PIDAS (Portable Instantaneous Display and Analysis Spectrometer). Concurrent airborne data from the AVIRIS instrument (Advanced Visible/Infrared Imaging Spectrometer) show effects of thin snow and mixed pixels for areas of thin and patchy spring snow.

Nolin, Anne W.↗

NASA NTRS1992

Infrared imaging of Mars for volatile distribution and seasonal variability between 2.4 and 5.1 microns

Recent advances in ground-based infrared imaging now allow for sub-arc second spectral imaging. Data collected at the NASA Infrared Telescope Facility using protocam, a 62 x 58 InSb array camera with a circular variable filter and a plate scale 0.2 arc-seconds/pixel, are discussed. These images are a first attempt at extended seasonal infrared coverage of Mars to look for seasonal variations. Currently, data collected in Jun. 1990 at Ls = 241 (southern spring) and in Jan. 1991 Ls = 360 (late southern summer) are being reduced and analyzed. The 3 micron bound water band is the strongest surface absorption feature on Mars in the infrared. Infrared spectroscopy can also be useful in the detection of ice and frost deposits, especially in the polar regions. While imaging and spectroscopy at visible wavelengths allows for the detection of condensates, infrared information is needed to distinguish between water and CO2 ice/frost deposits. In the Jun. images, the southern polar cap totally disappears in the 3.4 micron CO2 frost band and is bright in the 3.1 micron water ice band, indicating that water ice is not a detectable component of the southern polar cap at this season. Further investigations are currently under way to look for residual water ice after the disappearance of the seasonal south polar cap in the Jan. images. The Jun. images that were focused on had a sub-earth point located at 184 longitude, and 23.8 S latitude which put the center of the disk in the southern highland region between Elysium and Amazonis. Examination of the Jun. images show that there are four surface units identifiable: a CO2 frost deposit, a northern plains unit, an equatorial unit, and a southern highland unit. At this resolution there does not appear to be any latitudinal variations in the 3 micron band that is independent of the 2.4 micron albedo features. The northern planes unit and the southern highland unit have very similar 'color' in the three micron band as demonstrated by the rise out of the band, but different brightness levels. The equatorial unit has a distinct three micron color implying a compositional difference. The albedo features at 2.4 microns have decreased contrast at longer wavelengths, till they are unrecognizable at 4 microns.

Blaney, Diana L.↗

NASA NTRS1992

Doppler lidar wind measurement with the edge technique

The edge technique is a new and powerful method for measuring small frequency shifts. Range resolved lidar measurements of winds can be made with high accuracy and high vertical resolution using the edge technique to measure the Doppler shift of an atmospheric backscattered signal from a pulsed laser. The edge technique can be used at near-infrared or visible wavelengths using well developed solid state lasers and detectors with various edge filters. In the edge technique, the laser frequency is located on the steep slope of the spectral response function of a high resolution optical filter. Due to the steep slope of the edge, very small frequency shifts cause large changes in measured signal. The frequency of the outgoing laser pulse is determined by measuring its location on the edge of the filter. This is accomplished by sending a small portion of the beam to the edge detection setup where the incoming light is split into two channels - an edge filter and an energy monitor channel. The energy monitor signal is used to normalize the edge filter signal for magnitude. The laser return backscattered from the atmosphere is collected by a telescope and directed through the edge detection setup to determine its frequency (location on the edge) in a similar manner for each range element. The Doppler shift, and thus the wind, is determined from a differential measurement of the frequency of the outgoing laser pulse and the frequency of the laser return backscattered from the atmosphere. We have conducted simulations of the performance of an edge lidar system using an injection seeded pulsed Nd:YAG laser at 1.06 microns. The central fringe of a Fabry-Perot etalon is used as a high resolution edge filter to measure the shift of the aerosol return.

Korb, C. Laurence↗

NASA NTRS1991

Reflectivities of uniform and broken layered clouds

Differences between the reflectivities of uniform layered clouds and of broken clouds taken from the same layers of the Californian coast were investigated using 1 km data obtained by the AVHRRs on the NOAA-9 and -10 satellites during the FIRE Marine Stratocumulus Intensive Field Observations (June 29 - July 18, 1987). The reflectivities are compared at 0.63 micron, where cloud droplets are nonabsorbing, and at 3.7 micron, where the cloud droplets absorb and the single-scattering albedo is about 0.9. It was found that, at visible wavelengths, the reflectivities of broken clouds were only 80-85 percent of those of the same clouds where they formed regions of extensive uniform layers. The radiation not reflected was transmitted to the surface. The results suggest that the presatellite estimates of the planetary albedo may be too large because they failed to allow for the effects of cloud boundaries on cloud reflectivities.

Coakley, James A., Jr.↗

NASA NTRS1991

Observations of snow and ice features during the polar winter using moonlight as a source of illumination

Sunlight reflected off the surface of the moon provides sufficient illumination for nighttime imaging of the earth from those spacecraft on which the instruments are sensitive to low light levels. The Defense Meteorological Satellite Program (DMSP) series of satellites are at present the only systems capable of furnishing visible nocturnal images of the earth from space. Features having a high albedo in the visible wavelengths, such as snow and ice, can easily be identified on moonlit nights when the moon is in the gibbous or full phases. Little information exists about the distribution of leads, fractures, and polynya on a basin-wide scale in the winter pack ice, and knowledge of oceanic and atmospheric fluxes is hindered by the limited data available in winter. The DMSP data is a relatively untapped source for observing polar regions during the winter. Using nighttime imagery is a novel way to explore polar areas during the winter months when the sun is below the horizon for much of the time.

Foster, James L.↗

NASA NTRS1992

The luminosity of the Galactic center

High signal-to-noise 50- and 90-micron continuum maps of the central 5 x 4 arcmin of the Galaxy made from observations on the Kuiper Airborne Observatory are presented. Dust and luminosity distribution models which are consistent with the data are presented. These data are consistent with a flared disk of dust, centered on the position of Sgr A*, having a central cavity wth a radius of 1 pc. Strong evidence is found for a large amount of dust between the northern and eastern ionized arms located within the cavity. It is argued that this 'tongue' of dust is optically thick at UV and visible wavelengths. Both smooth and clumped dust distributions, consistent with the disk and central cavity, are considered. For the smooth distribution, the dust heating is found to be consistent with the data only for a central UV source of a few x 10 exp 6 solar luminosities.

Davidson, J. A.↗

NASA NTRS1991

Acousto-optic tunable filter imaging spectrometers

A remote sensing multispectral imaging instrument is being developed that uses a high resolution, fast programmable acoustooptic tunable filter (AOTF) as the spectral bandpass filter. A compact and fully computer controllable AOTF-based imaging spectrometer that operates in the visible wavelength range (0.5-0.8 microns) has been built and tested with success. A second imaging spectrometer operating in the near-infrared wavelength range (1.2-2.4 microns) is also under experimental investigation. The design criteria meeting various system issues, such as imaging quality, spectral response, and field of view (FOV), are discussed. An experiment using this AOTF imaging spectrometer breadboard is described.

Chao, Tien-Hsin↗

NASA NTRS1991

High resolution imaging with multilayer soft X-ray, EUV and FUV telescopes of modest aperture and cost

The development of multilayer reflective coatings now permits soft X-ray, EUV and FUV radiation to be efficiently imaged by conventional normal incidence optical configurations. Telescopes with quite modest apertures can, in principle, achieve images with resolutions which would require apertures of 1.25 meters or more at visible wavelengths. The progress is reviewed which has been made in developing compact telescopes for ultra-high resolution imaging of the sun at soft X-ray, EUV and FUV wavelengths, including laboratory test results and astronomical images obtained with rocket-borne multilayer telescopes. The factors are discussed which limit the resolution which has been achieved so far, and the problems which must be addressed to attain, and surpass the 0.1 arc-second level. The application of these technologies to the development of solar telescopes for future space missions is also described.

Walker, Arthur B. C., Jr.↗

NASA NTRS1993

Optical and scratch resistant properties of diamondlike carbon films deposited with single and dual ion beams

Amorphous diamondlike carbon (DLC) films were deposited using both single and dual ion beam techniques utilizing filament and hollow cathode ion sources. Continuous DLC films up to 3000 A thick were deposited on fused quartz plates. Ion beam process parameters were varied in an effort to create hard, clear films. Total DLC film absorption over visible wavelengths was obtained using a Perkin-Elmer spectrophotometer. An ellipsometer, with an Ar-He laser (wavelength 6328 A) was used to determine index of refraction for the DLC films. Scratch resistance and frictional and adherence properties were determined for select films. Applications for these films range from military to the ophthalmic industries.

Kussmaul, Michael T.↗

NASA NTRS1993

The influence of the hydrologic cycle on the extent of sea ice with climatic implications

Multi-temporal satellite images, field observations, and field measurements were used to investigate the mechanisms by which sea ice melts offshore from the Mackenzie River delta. Advanced Very High Resolution Radiometer (AVHRR) satellite data recorded in 1986 were analyzed. The satellite data were geometrically corrected and radiometrically calibrated so that albedo and temperature values could be extracted. The investigation revealed that sea ice melted approximately 2 weeks earlier offshore from the Mackenzie River delta than along coasts where river discharge is minimal or non-existent. There is significant intra-delta variability in the timing and patterns of ice melt. An estimation of energy flux indicates that 30 percent more of the visible wavelength energy and 25 percent more of the near-infrared wavelength energy is absorbed by water offshore of the delta compared to coastal areas with minimal river discharge. The analysis also revealed that the removal of sea ice involves the following: over-ice-flooding along the coast offshore from river delta channels; under-ice flow of 'warm' river water; melting and calving of the fast ice; and, the formation of a bight in the pack ice edge. Two stages in the melting of sea ice were identified: (1) an early stage where heat is supplied to overflows largely by solar radiation, and (2) a later stage where heat is supplied by river discharge in addition to solar radiation. A simple thermodynamic model of the thaw process in the fast ice zone was developed and parameterized based on events recorded by the satellite images. The model treats river discharge as the source of sensible heat at the base of the ice cover. The results of a series of sensitivity tests to assess the influence of river discharge on the near shore ice are presented.

Dean, Kenneson G.↗