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Nelson, R.

Publications and source records attributed to Nelson, R..

34 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

First observation of a mass independent isotopic fractionation in a condensation reaction

Thiemens and Heidenreich (1983) first demonstrated that a chemically produced mass independent isotopic fractionation process could produce an isotopic composition which is identical to that observed in Allende inclusions. This raised the possibility that the meteoritic components could be produced by chemical, rather than nuclear processes. In order to develop a mechanistic model of the early solar system, it is important that relevant reactions be studied, particularly, those which may occur in the earliest condensation reactions. The isotopic results for isotopic fractionations associated with condensation processes are reported. A large mass independent isotopic fractionation is observed in one of the experiments.

Thiemens, M. H.↗
NASA NTRS1993

The Coherent Backscattering Opposition Effect: Measurements at Very Small Phase Angles

This oral presentation explains that measurements of the opposition surge (the nonlinear increase in reflectance seen in particulate materials when observed at small phase angles) are the first ever made using the JPL long-arm goniometer, which permits very small phase angle measuremnets to be made.

long-arm goniometer phase angles coherent backscat↗
NASA NTRS1993

(abstract) Hermes Global Orbiter: Mission to Mercury

The Hermes Global Orbiter is a proposed Discovery-class mission. Hermes will be launched aboard a Delta II rocket in 1999 and will be placed in an elliptical polar orbit about Mercury. Remote sensing measurements of the planet's surface, atmosphere, and magnetosphere will be performed. Key mission goals include mapping the entire surface at 1 km resolution, characterizing the surface composition, texture and topography, searching for water ice at the poles, characterizing the atmosphere, and constraining the interior structure.

Mercury Hermes orbiter remote sensing surface atmo↗
NASA NTRS1993

AVHRR-LAC estimates of forest area in Madagascar, 1990

Three AVHRR-LAC data sets acquired in September 1990 and January 1991 were used to map the forest resources of Madagascar. The island was partitioned into four strata to include: (1) the western hardwoods, (2) the central grasslands, (3) the eastern rainforest, and (4) spiny forest. Each stratum was classified separately using AVHRR-LAC data in conjunction with 1984-1988 Landsat-MSS photoproducts. The results of AVHRR classification indicate that approximately 11 percent of the island is covered by forest. Estimates of forest area, by stratum, are as follows: western hardwoods, 6697 sq km; central grasslands, 2830 sq km; eastern rainforest 34,167 sq km; and spiny forest, 17,224 sq km. The total forest area on the 587,041 sq km island is estimated to be 60,918 sq km. The AVHRR forest map was compared to a mid-1970s land cover map which was developed using Landsat-MSS photoproducts. The average class agreement between the mid 1970s ground reference map and the 1990 AVHRR-LAC map was 78.2 percent, the overall accuracy was 81.1 percent. Much of the per-pixel disagreement between the ground reference and AVHRR maps involved areas identified as forest in the 1970s and as nonforest in 1990.

Nelson, R.↗
NASA NTRS1989

Oxygen isotopic fractionation in the condensation of refractory smokes

This paper reports preliminary measurements of the oxygen isotopic fractionation that occurs during the condensation of refractory metal oxide smokes. Fractionation occurred at temperatures in excess of 500 K during and/or subsequent to reaction of SiH4, Fe(CO)5, Al(CH3)3, and O2 in a H2 carrier gas. Observed isotopic fractionation was as high as -15 per mil/amu. The magnitude of the isotopic fractionation suggests that the process is kinetically controlled since the effect is greatly in excess of that expected for equilibrium isotopic exchange.

Nelson, R.↗
NASA NTRS1988

Generic supervisor: A knowledge-based tool for control of space station on-board systems

The concept of a generic module for management of onboard systems grew out of the structured analysis effort for the Space Station software. Hierarchical specification of subsystems software revealed that nontrivial supervisory elements are required at all levels. The number of supervisors (and subsequent software) required to implement the hierarchical control over onboard functions comprise a large portion of the Space Station software. Thus, a generic knowledge based supervisory module significantly reduces the amount of software developed. This module, the Generic Supervisor, depends on its knowledge of control to provide direction for subordinates and feedback to superiors within a specific subsystem area. The Generic Supervisor provides an adaptable and maintainable control system. A portion of the Space Station Environmental Control and Life Support System (ECLSS) was implemented as a hierarchy of supervisors. This prototype implementation demonstrates the feasibility of a generic knowledge based supervisor, and its facility to meet complex mission requirements.

Carnes, J. R.↗
NASA NTRS1987

Bidirectional reflectance properties of planetary surface materials

Laboratory measurements using a spectrogoniometer to separate the effects of surficial texture and albedo in the characterization of planetary surface materials are discussed. An investigation of the surface of Io is discussed. A number of technical improvements to the goniometer are summarized.

Buratti, B.↗
NASA NTRS1986

Hydrous alteration of amorphous silicate smokes - First results

Results of the initial studies of the hydrous alteration of amorphous Mg-SiO smokes indicate that, although these materials readily adsorb water, the silicate structure is much more stable at 360 K than expected. Drastic changes in the relative absorption strengths of the 10- and 20-micron 'silicate' features that appear quite rapidly at 750 K were observed; these observations might have important implications for the interpretation of cometary dust spectra. Observations of the development of 3.4-3.5 micron features possibly due to hydrocarbons in the spectra of processed Mg-SiO smokes have raised the exciting possibility that these amorphous condensates could act as Fischer-Tropsch type catalysts to produce hydrocarbons in the primitive solar nebula.

Nuth, J. A.↗
NASA NTRS1986

Identifying deforestation in Brazil using multiresolution satellite data

The use of multiresolution satellite data to monitor deforestation on a continental/subcontinental scale is examined. MSS, local area coverage (LAC), global area coverage (GAC), and GOES data were applied to the study of deforestation in Rondonia, Brazil; the characteristics of these sensors are described. The probability thresholding and vegetation-index thresholding procedures used to process the data in order to differentiate between forest from nonforest are analyzed. The LAC, GAC, and GOES data are compared to MSS data. It is observed that GOES data is not useful for monitoring colonization projects due to excessive noise in the data; the GAC data is only applicable in large areas of contiguous forest clearing; the MSS data when available is applicable as a ground data reference source for differentiating cleared areas from primary; the LAC data are capable of delineating colonization clearings; and the probability thresholding procedure differentiates forest from nonforest more accurately than the vegetation-index procedure. The data reveal that the LAC data combined with the probability threshold procedure provide the best data-source/classification-procedure combination.

Nelson, R.↗
NASA NTRS1986

A determination of sampling intensity to characterize a Landsat MSS scene using two block sizes

Three Landsat MSS scenes were processed to empirically determine the sampling intensity needed to characterize various land cover types including water, conifer, and hardwood. The block sizes used as the sampling units were 497 by 500 pixels (picture elements) and 248 by 250 pixels. It is found that, for a given accuracy criterion, the sampling intensity is dependent on the abundance of the cover type of interest in the MSS scene. The results also indicate that, when using the smaller block size, a smaller percentage of the scene has to be classified to obtain a given level of accuracy.

Horning, N.↗
NASA NTRS1985

Performance comparisons between information extraction techniques using variable spatial resolution data

The decreased instantaneous field of view (IFOV) is one of the principal advances noted for the Thematic Mapper (TM) sensor. The 42.5 microradian IFOV of TM and the 710 km nominal orbit altitude result in a 30 m nominal spatial resolution at the earth surface. This is a considerable decrease in the projected pixel area when compared to the 79 m nominal spatial resolution of the Landsat Multispectral Scanner (MSS). An experiment was conducted which allowed a rigorous test of the influence of classifier design, with data spatial resolution of TM (30 m) and approximately that of the Landsat MSS (90 m), on classification performance for a particular TM scene. The experiment involved evaluation of the results for the per-point Gaussian maximum likelihood (GML) classifier and the supervised ECHO (Extraction and Classification of Homogeneous Objects) classifier.

Latty, R. S.↗
NASA NTRS1985

Reducing Landsat MSS scene variability

Landsat 1, 2, and 3 MSS data acquired for six different nonvegetated targets over a three-year period were used to determine which of five transformations was most useful for reducing between-scene variability. The following values were calculated from the MSS digital numbers (dn): (1) radiance; (2) reflectance; (3) reflectance corrected for changes in the earth/sun distance; (4) normalized dn (normalization equations proposed by ERIM researchers); and (5) band ratios. Results indicated that reflectance calculations were most effective overall for reducing interscene variability; ratio proved most useful on the bright targets.

Nelson, R.↗
NASA NTRS1984

Determining forest canopy characteristics using airborne laser data

A study is reported in which a profiling laser system flown at relatively low altitudes over a forested area was used to measure various forest canopy attributes, including tree heights. An analysis of the data obtained indicates that canopy closure is most strongly related to the penetration capability of the laser pulse, with the pulses attenuated more quickly in a dense canopy. Laser estimates of the average tree heights differ by less than 1 m from the photogrammetrically acquired values. It is concluded that the laser system is suitable for remotely sensing the vertical forest canopy profile. Elements of this profile are linearly related to crown closure and can be used to assess tree height.

Nelson, R.↗
NASA NTRS1982

Evaluation of temporal registration of Landsat scenes

The registration of Landsat images is important for multitemporal classification and for detecting change. Landsat data are now rectified to a ground coordinate system during preprocessing, hence scenes obtained over the same area are registered. The machine responsible for preprocessing the Landsat multispectral scanner data is the master data processor (MDP). This paper describes the rectification approach taken by the MDP, reviews the accuracy standards of the resultant product, and provides an assessment of the accuracy of the scene to scene registration of two Landsat images.

Nelson, R.↗
NASA NTRS1975

Evidence for an elemental sulfur component of the clouds from Venus spectrophotometry

The decrease in the reflectivity of Venus in the near-UV can be explained if the clouds contain particles of elemental sulfur in addition to sulfuric acid. The low-resolution McDonald-Pittsburgh spectrum can be fitted by two sulfur-containing, multiple-scattering cloud models: (1) a mixed cloud consisting of one particle of elemental sulfur of radius 10 microns for every 670 particles of sulfuric acid of radius 1 micron, and (2) a layered cloud of optical thickness tau = 1.0 consisting of one-micron particles of sulfuric acid overlying a thick cloud of elemental sulfur particles of radius 3.6 microns. Some of the sulfur is incompletely polymerized. The source of the sulfur is photo-dissociation of COS, although some may also be recycled from the lower atmosphere. The sulfur plays a crucial role in the planetary meteorology of Venus since it is responsible for the bulk of the absorption of solar energy.

Hapke, B.↗