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Caldwell, D.

Publications and source records attributed to Caldwell, D..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

An Assessment of the Differences Between Spatial Resolution and Grid Size for the SMAP Enhanced Soil Moisture Product over Homogeneous Sites

Satellite-based passive microwave remote sensing typically involves a scanning antenna that makes measurements at irregularly spaced locations. These locations can change on a day to day basis. Soil moisture products derived from satellite-based passive microwave remote sensing are usually resampled to a fixed Earth grid that facilitates their use in applications. In many cases the grid size is finer than the actual spatial resolution of the observation, and often this difference is not well understood by the user. Here, this issue was examined for the Soil Moisture Active Passive (SMAP) enhanced version of the passive-based soil moisture product, which has a grid size of 9-km and a nominal spatial resolution of 33-km. In situ observations from core validation sites were used to compute comparison metrics. For sites that satisfied the established reliability and scaling criteria, the impact of validating the 9-km grid product with in situ data collected over a 9-km versus a 33-km domain was very small for the sites studied (0.039 cu. m/cu. m unbiased root mean square difference for the 9-km case versus 0.037 cu. m/cu. m for the 33-km case). This result does not mean that the resolution of the product is 9-km but that for the conditions studied here the soil moisture estimated from in situ observations over 9-km is a close approximation of the soil moisture estimated from in situ observations over the 33-km resolution. The implication is that using the enhanced SMAP product at its grid resolution of 9-km should not introduce large errors in most applications.

Soil moisture↗
NASA NTRS2009

Development of X-Ray Optics for the International X-Ray Observatory (IXO)

The International X-ray Observatory requires mirror assemblies with unprecedented characteristics that cannot be provided by existing optical technologies. In the past several years, the project has supported a vigorous mirror technology development program. This program includes the fabrication of lightweight mirror segments by slumping commercially available thin glass sheets, the support and mounting of these thin mirror segments for accurate metrology, the mounting and attachment of these mirror segments for the purpose of X-ray tests, and development of methods for aligning and integrating these mirror segments into mirror assemblies. This paper describes our efforts and developments in these areas.

Zhang, William W.↗
NASA NTRS2003

The Kepler Mission: A Search for Terrestrial Planets - Development Status

We have embarked on a mission to detect terrestrial planets. The space mission has been optimized to search for earth-size planets (0.5 to 10 earth masses) in the habitable zone (HZ) of solar-like stars. Given this design, the mission will necessarily be capable of not only detecting Earth analogs, but a wide range of planetary types and characteristics ranging from Mercury-size objects with orbital periods of days to gas-giants in decade long orbits that have undeniable signatures even with only one transit detected. The mission is designed to survey the full range of spectral-type dwarf stars. The approach is to detect the periodic signal of transiting planets. Three or more transits of a star exceeding a combined threshold of eight sigma with a statistically consistent period, brightness change and duration provide a rigorous method of detection. From the relative brightness change the planet size can be calculated. From the period the orbital size can be calculated and its location relative to the HZ determined. Presented here are: the mission goals, the top level system design requirements derived from these goals that drive the flight system design, a number of the trades that have lead to the mission concept, expected photometric performance dependence on stellar brightness and spectral type based on the system 'noise tree' analysis. Updated estimates are presented of the numbers of detectable planets versus size, orbit, stellar spectral type and distances based on a planet frequency hypothesis. The current project schedule and organization are given.

Koch, David↗
NASA NTRS1999

Photometric Observations of 6000 Stars in the Cygnus Field

A small photometer to detect transits by extrasolar planets has been assembled and is being tested at Lick Observatory on Mt. Hamilton, California. The Vulcan photometer is constructed from a 30 cm focal length, F/2.5 AeroEktar reconnaissance lens and Photometrics PXL16800 CCD camera. A spectral filter is used to confine the pass band from 480 to 763 mn. It simultaneously monitors 6000 stars brighter than 12th magnitude within a single star field in the galactic plane. When the data are folded and phased to discover low amplitude transits, the relative precision of one-hour samples is about 1 part per thousand (10 x l0(exp -3)) for many of the brighter stars. This precision is sufficient to find jovian-size planets orbiting solar-like stars, which have signal amplitudes from 5 to 30 x l0(exp -3) depending on the inflation of the planet and the size of the star. Based on the frequency of giant inner-planets discovered by Doppler-velocity method, one or two planets should be detectable in a rich star field. The goal of the observations is to obtain the sizes of giant extrasolar planets in short-period orbits and to combine these with masses determined from Doppler velocity measurements to determine the densities of these planets. A further goal is to compare the measured planetary diameters with those predicted from theoretical models. From August 10 through September 30 of 1998, a forty nine square degree field in the Cygnus constellation centered at RA and DEC of 19 hr 47 min, +36 deg 55 min was observed. Useful data were obtained on twenty-nine nights. Nearly fifty stars showed some evidence of transits with periods between 0.3 and 8 days. Most had amplitudes too large to be associated with planetary transits. However, several stars showed low amplitude transits. The data for several transits of each of these two stars have been folded and been folded into 30 minute periods. Only Cygl433 shows any evidence of a flattened bottom that is expected when a small object transits a much larger primary. However when high-resolution spectra were obtained for both stars, the stars were found to be double-lined binaries so similar in size as to have indistinguishable transit depths. The low amplitude of the transits is explained if the stellar orbital planes are tipped approximately 5 degrees from the line of sight causing both binaries to show grazing transits. The two absorption lines, due to the H(sub beta) feature in each star, are apparent and indicate the presence of a binary system with similar components.

Borucki, W.↗
NASA NTRS1989

Logistics resupply and emergency crew return system for Space Station Freedom

Sometime in the late 1990's, if all goes according to plan, Space Station Freedom will allow the United States and its cooperating partners to maintain a permanent presence in space. Acting as a scientific base of operations, it will also serve as a way station for future explorations of the Moon and perhaps even Mars. Systems onboard the station will have longer lifetimes, higher reliability, and lower maintenance requirements than seen on any previous space flight vehicle. Accordingly, the station will have to be resupplied with consumables (air, water, food, etc.) and other equipment changeouts (experiments, etc.) on a periodic basis. Waste materials and other products will also be removed from the station for return to Earth. The availability of a Logistics Resupply Module (LRM), akin to the Soviet's Progress vehicle, would help to accomplish these tasks. Riding into orbit on an expendable launch vehicle, the LRM would be configured to rendezvous autonomously and dock with the space station. After the module is emptied of its cargo, waste material from the space station would be loaded back into it. The module would then begin its descent to a recovery point on Earth. Logistics Resupply Modules could be configured in a variety of forms depending on the type of cargo being transferred. If the LRM's were cycled to the space station in such a way that at least one vehicle remained parked at the station at all times, the modules could serve double duty as crew emergency return capsules. A pressurized LRM could then bring two or more crew-persons requiring immediate return (because of health problems, system failure, or unavoidable catastrophes) back to Earth. Large cost savings would be accrued by combining the crew return function with a logistics resupply system.

Ahne, D.↗
NASA NTRS1985

Aerobic sulfur-oxidizing bacteria: Environmental selection and diversification

Sulfur-oxidizing bacteria oxidize reduced inorganic compounds to sulfuric acid. Lithotrophic sulfur oxidizer use the energy obtained from oxidation for microbial growth. Heterotrophic sulfur oxidizers obtain energy from the oxidation of organic compounds. In sulfur-oxidizing mixotrophs energy are derived either from the oxidation of inorganic or organic compounds. Sulfur-oxidizing bacteria are usually located within the sulfide/oxygen interfaces of springs, sediments, soil microenvironments, and the hypolimnion. Colonization of the interface is necessary since sulfide auto-oxidizes and because both oxygen and sulfide are needed for growth. The environmental stresses associated with the colonization of these interfaces resulted in the evolution of morphologically diverse and unique aerobic sulfur oxidizers.

Caldwell, D.↗
NASA NTRS1985

Microbial colonization and growth on metal sulfides and other mineral surfaces

To determine whether a bacterial film forms on sulfur minerals in situ, various sulfur containing and other minerals were incubated in Penitencia Creek. The rate of cell growth and attachment within the surface microenvironment of mineral surfaces was also determined. To determine whether surfaces enriched with soluble sulfur substrates (cysteine, glutathione, thioglycolate, sulfite, and thiosulfate) increased the rate of growth or attachment of natural communities, membrane enrichments were incubated. These rates were determined as described by Caldwell et al. (1981, 1983). The growth of Pseudomonas fluorescens, a heterotrophic sulfur oxidizer, was studied in batch cell suspensions and in continuous culture. In batch culture the cells were oxygen limited (growth rate 0.33 per hour under oxygen limitations and 0.52 per hour when vigorously aerated). Growth within the film was glucose limited. Several behavioral phenomena were observed for cells growing within the hydrodynamic boundary layer. Despite a flow of 10 cm per second in the environment, the bacteria were able to move freely in both directions within the hydrodynamic boundary layer.

Caldwell, D.↗