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At least 199 records · Page 11

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NASA NTRS1974

Convolutional code performance in planetary entry channels

The planetary entry channel is modeled for communication purposes representing turbulent atmospheric scattering effects. The performance of short and long constraint length convolutional codes is investigated in conjunction with coherent BPSK modulation and Viterbi maximum likelihood decoding. Algorithms for sequential decoding are studied in terms of computation and/or storage requirements as a function of the fading channel parameters. The performance of the coded coherent BPSK system is compared with the coded incoherent MFSK system. Results indicate that: some degree of interleaving is required to combat time correlated fading of channel; only modest amounts of interleaving are required to approach performance of memoryless channel; additional propagational results are required on the phase perturbation process; and the incoherent MFSK system is superior when phase tracking errors are considered.

Modestino, J. W.↗

NASA NTRS1974

The Skylab concentrated atmospheric radiation project - An overview

An overview is presented of the Skylab Concentrated Atmospheric Radiation Project (SCARP), which was conducted to determine the accuracy and applicability of different models of radiation transfer through air masses of differing characteristics including varying amounts of wet and dry aerosols. Sites surveyed included onshore and offshore areas in the vicinity of Houston, Texas, the White Sands Missile Range in New Mexico, and Phoenix, Arizona, which provided a variety of warm, hot, dry, wet, clean, and dirty atmospheres. A typical exercise is described, and problems encountered during the experiment are discussed. It is noted that ten models of atmospheric scattering were utilized and that available data is being compared with the models.

Whitehead, V. S.↗

NASA NTRS1975

Radar derived spatial statistics of summer rain. Volume 1: Experiment description

An experiment was performed at Wallops Island, Virginia, to obtain a statistical description of summer rainstorms. Its purpose was to obtain information needed for design of earth and space communications systems in which precipitation in the earth's atmosphere scatters or attenuates the radio signal. Rainstorms were monitored with the high resolution SPANDAR radar and the 3-dimensional structures of the storms were recorded on digital tape. The equipment, the experiment, and tabulated data obtained during the experiment are described.

Katz, I.↗

NASA NTRS1977

Microwave absorption characteristics of the clouds of Venus from Mariner 10 radio occultation

Measurements of received signal strength at S-band (13 cm) and X-band (4.8 cm) wavelengths during the radio occultation of Mariner 10 by Venus on February 5, 1974, are examined in order to study the structure and composition of the absorbing medium. The frequency excursions of the signals are determined and used to obtain the structure of the refractive index in the lower atmosphere. Profiles of excess signal attenuation due to atmospheric scattering and absorption are presented which indicate that the X-band signal experienced much more absorption and was extinguished at about 50 km, while the S-band signal penetrated to about 42 km. The optical-depth data are inverted by means of a discrete inversion method to obtain the absorption coefficient for each band as a function of height, and the resulting absorption-coefficient profiles are compared with the attenuation at vertical incidence modeled from planetary radar and passive microwave observations of Venus. The absorption coefficients at the two wavelengths are employed to estimate the liquid content and composition of the microwave-absorbing cloud particles.

Kliore, A. J.↗

NASA NTRS1978

Determination of aerosol content in the atmosphere from LANDSAT data

The author has identified the following significant results. A large set of LANDSAT 2 data, obtained at San Diego, showed excellent linear relationships, particularly for MSS 5 and MSS 6, between the radiance over the ocean and the atmospheric aerosol content. Two other data points obtained at Adrigole, Ireland, representing a different ocean and a different ground truth instrument, showed very good agreement with the San Diego data. It appeared that the technique could be used for global monitoring of the atmospheric aerosol content over the oceans. Results obtained at several inland bodies of water showed that MSS 4, MSS 5, and MSS 6 cannot be used due to the effect of water pollution generally present. However, the LANDSAT 1 results suggested that MSS 7, which operates at longer wavelengths, was not very sensitive to water pollution, and might be useful for inland measurements of aerosol content. Use of the longer wavelength would also minimize the effects of adjacent high albedo land, since atmospheric scattering was reduced at longer wavelengths.

Griggs, M.↗

NASA NTRS1978

Work on power-plant (air) plumes involving remote sensing of SO2

Acquisition of air quality and concurrent meteorological data was used for dispersion model development and plant siting needs of the Maryland power plants. One of the major instruments in these studies was the Barringer correlation spectrometer, a remote sensor, using atmospherically scattered sunlight that was used to measure the total amount of SO2 in a cross section of the plume. Correlation spectrometer and its role in this measurement program are described.

White, C. L., Jr.↗

NASA NTRS1978

A statistical technique for determining rainfall over land employing Nimbus-6 ESMR measurements

At 37 GHz, the frequency at which the Nimbus 6 Electrically Scanning Microwave Radiometer (ESMR 6) measures upwelling radiance, it was shown theoretically that the atmospheric scattering and the relative independence on electromagnetic polarization of the radiances emerging from hydrometers make it possible to monitor remotely active rainfall over land. In order to verify experimentally these theoretical findings and to develop an algorithm to monitor rainfall over land, the digitized ESMR 6 measurements were examined statistically. Horizontally and vertically polarized brightness temperature pairs (TH, TV) from ESMR 6 were sampled for areas of rainfall over land as determined from the rain recording stations and the WSR 57 radar, and areas of wet and dry ground (whose thermodynamic temperatures were greater than 5 C) over the Southeastern United States. These three categories of brightness temperatures were found to be significantly different in the sense that the chances that the mean vectors of any two populations coincided were less than 1 in 100.

Rodgers, E.↗

NASA NTRS1979

Skylab-earth laser-beacon experiment - Results and analysis

Photographs of a ground-based laser beacon were taken by astronauts on board the Skylab spacecraft. Unexpected large and elliptical laser-beam images were recorded. The photographic results are presented and discussed in terms of possible atmospheric scattering mechanisms. Current beacon experiments with the Landsat spacecraft are also discussed.

Bufton, J. L.↗

NASA NTRS1979

Zodiacal light surface brightness measurements by Atmosphere Explorer-C

The visible airglow photometer on the Atmosphere Explorer-C satellite has been used to map the zodiacal light surface brightness at the wavelengths monitored by the instrument: 3371, 4278, 5200, 5577, 6300, and 7319 A. The study constitutes a survey over this wavelength range, covering most of the celestial sphere from altitudes above the atmospheric emissions and free from atmospheric scattering and attenuation. The intensity variations reveal enhancements near elongations of 130 deg and possibly near 60 deg at all wavelengths. The intensity of the zodiacal light near the ecliptic pole is found to be about 30 S10 units. The color ratio with respect to the sun is found to be redder than the sun (0.7) at all elongations.

Torr, M. R.↗

NASA NTRS1979

Water vapor differential absorption lidar development and evaluation

A ground-based differential absorption lidar (DIAL) system is described which has been developed for vertical range-resolved measurements of water vapor. The laser transmitter consists of a ruby-pumped dye laser, which is operated on a water vapor absorption line at 724.372 nm. Part of the ruby laser output is transmitted simultaneously with the dye laser output to determine atmospheric scattering and attenuation characteristics. The dye and ruby laser backscattered light is collected by a 0.5-m diam telescope, optically separated in the receiver package, and independently detected using photomultiplier tubes. Measurements of vertical water vapor concentration profiles using the DIAL system at night are discussed, and comparisons are made between the water vapor DIAL measurements and data obtained from locally launched rawinsondes. Agreement between these measurements was found to be within the uncertainty of the rawinsonde data to an altitude of 3 km. Theoretical simulations of this measurement were found to give reasonably accurate predictions of the random error of the DIAL measurements. Confidence in these calculations will permit the design of aircraft and Shuttle DIAL systems and experiments using simulation results as the basis for defining lidar system performance requirements

Browell, E. V.↗

NASA NTRS1980

Ultraviolet albedos of Uranus and Neptune

Observations on Uranus, Neptune, and four G-type stars which were obtained with the five channel ultraviolet spectrometer aboard the Astronomical Netherlands Satellite are reported. The paper uses these data to derive geometric albedos between 1800 and 3300 A for Uranus and between 2200 and 3300 A for Neptune. It is shown that the albedos are in good agreement with ground measurements near 3300 A and with OAO 2 measurements near 2500 A. In addition, it is noted that the albedos are smaller than theoretical predictions for deep pure H2 Rayleigh-Raman scattering atmospheres, which confirms earlier predictions that additional sources of ultraviolet absorption are required in the upper atmosphere of these planets. Consideration is given to the possibility that the Uranian atmosphere contains an extra absorber, noting that the identification of this absorber is difficult from broad-band spectrophotometric measurements.

Savage, B. D.↗

NASA NTRS1980

Remote sensing of particulate concentrations in water

Determination of residual radiance is essential in remote sensing measurements of suspended solids in the absence of real-time ground-truth data. Some typical background corrections are presented for Lake Superior and the spectral distribution of the residual radiance is determined from three major categories of turbidity in the lake. The results tested with Landsat 2 digital tape data indicate that for large bodies of water general information on atmospheric scattering, water clarity, and optical properties of suspended solids enables one to estimate the concentrations of particulates to within + or - 0.5 mg/liter in the absence of real-time ground-truth data.

Sydor, M.↗

NASA NTRS1980

The application of artificial electron beams to magnetospheric research

Scientific and technical progress made in the utilization of electron beams injected from large sounding rockets for the investigation of magnetospheric structure and dynamics and plasma physics is reviewed. Consideration is given to the problem of vehicle neutralization during the generation of electron beams in the ionosphere and the stability and electromagnetic wave emissions of electron beams propagating in space. Optical effects observed during the two Hess artificial aurora experiments, the Minnesota Echo IV and V experiments, the Zarnitsa-1 artificial aurora experiment and the Precede-Excede experiments are discussed, together with atmospheric scattering effects observed during Echo I, II and III, and results of the three experimental programs which have detected and analyzed beams after passage through the distant magnetosphere (the second Hess experiment, ARAKS and the Electron Echo experiments) are presented. Finally, plans for future research are outlined, and a chronology and complete bibliography of the programs conducted to date are included.

Winckler, J. R.↗

NASA NTRS1980

Spectrophotometry of Pluto from 3500 to 7350 A

Spectra of Pluto have been obtained on six nights during February 1979 by the use of the Cassegrain Digicon spectrograph on the 2.1-m Struve reflector and the IDS spectrograph on the 2.7-m reflector of McDonald Observatory. These spectra, with nominal resolution of 6-7 A, have been reduced to relative fluxes. Relative albedos were then calculated using the solar irradiances of Arvesen et al. (1969). The spectra taken in the blue show no indication of the upturn in albedo at wavelengths less than 3800 A previously reported by Fix, et al. (1970). The lack of a UV upturn cannot be interpreted in terms of a Rayleigh scattering atmosphere unless the albedo of the underlying surface is known. From the lack of methane absorption at the wavelength of the 6190- or 7270-A methane bands, an upper limit of 1-3 m-am of gaseous CH4 is derived. The albedo curve has a constant slope between 3500 and 7300 A. The only other solar system body which has this feature is an S-type asteroid.

Barker, E. S.↗

NASA NTRS1982

Dayglow emissions of the O2 Herzberg bands and the Rayleigh backscattered spectrum of the earth

It is pointed out that numerous fluorescent emissions from the Herzberg bands of molecular oxygen lie in the spectral region 242-300 nm. This coincides with the wavelength range used by orbiting spectrometers that observe the Rayleigh backscattered spectrum of the earth for the purpose of monitoring the vertical distribution of stratospheric ozone. Model calculations suggest that Herzberg band emissions in the dayglow could provide significant contamination of the ozone measurements if the quenching rate of O2(A3Sigma) is sufficiently small. It is noted that this is especially true near 255 nm, where the most intense fluorescent emissions relative to the Rayleigh scattered signal are located and where past satellite measurements have shown a persistent excess radiance above that expected for a pure ozone absorbing and molecular scattering atmosphere. Very small quenching rates, however, are adequate to reduce the dayglow emission to negligible levels. Noting that available laboratory data have not definitely established the quenching on the rate of O2(A3Sigma) as a function of vibration level, it is emphasized that such information is required before the Herzberg band contributions can be evaluated with confidence.

Frederick, J. E.↗

NASA NTRS1982

Effects of varying soil moisture contents and vegetation canopies on microwave emissions

Results of NASA airborne passive microwave scans of bare and vegetated fields for comparison with ground truth tests are discussed and a model for atmospheric scattering of radiation by vegetation is detailed. On-board radiometers obtained data at 21, 2.8, and 1.67 cm during three passes over each of 46 fields, 28 of which were bare and the others having wheat or alfalfa. Ground-based sampling included moisture in five layers down to 15 cm in addition to soil temperature. The relationships among the brightness temperature and soil moisture, as well as the surface roughness and the vegetation canopy were examined. A model was developed for the dielectric coefficient and volume scattering for a vegetation medium. L- to C-band data were found useful for retrieving soil information directly. A surface moisture content of 5-35% yielded an emissivity of 0.9-0.7. The data agreed well with a combined multilayer radiative transfer model with simple roughness correction.

Burke, H.-H. K.↗

NASA NTRS1982

Modeling of vegetation canopy reflectance: Status, issues and recommended future strategy

Various technical issues related to mapping of vegetative type, condition and stage of maturity, utilizing remotely sensed spectral data are reviewed. The existing knowledge base of models, especially of radiative properties of the vegetation canopy and atmosphere, is reviewed to establish the state of the art for addressing the problem of vegetation mapping. Activities to advance the state of the art are recommended. They include working on canopy reflectance and atmospheric scattering models, and field measurements of canopy reflectance as well as of canopy components. Leaf area index (LAI) and solar radiation interception (SRI) are identified as the two most important vegetation variables requiring further investigation. It is recommended that activities related to sensing them or understanding their relationships with measurable variables, should be encouraged and supported.

Goel, N. S.↗

NASA NTRS1982

Study to assess the importance of errors introduced by applying NOAA 6 and NOAA 7 AVHRR data as an estimator of vegetative vigor: Feasibility study of data normalization

The use of NOAA AVHRR data to map and monitor vegetation types and conditions in near real-time can be enhanced by using a portion of each GAC image that is larger than the central 25% now considered. Enlargement of the cloud free image data set can permit development of a series of algorithms for correcting imagery for ground reflectance and for atmospheric scattering anisotropy within certain accuracy limits. Empirical correction algorithms used to normalize digital radiance or VIN data must contain factors for growth stage and for instrument spectral response. While it is not possible to correct for random fluctuations in target radiance, it is possible to estimate the necessary radiance difference between targets in order to provide target discrimination and quantification within predetermined limits of accuracy. A major difficulty lies in the lack of documentation of preprocessing algorithms used on AVHRR digital data.

Duggin, M. J.↗