Engineering topics
Publications and source records attributed to Williams, D..
New Galileo SSI imaging of Io from orbits C30 I33 will be presented. The aging Galileo spacecraft continues to produce spectacular new results, including the tallest volcanic plume yet found on Io. Additional information is contained in the original extended abstract.
The Galileo spacecraft has been periodically monitoring volcanic activity on Io since June 1996, making it possible to chart the evolution of individual eruptions.
Explore the source record for details and available documents.
Galileo's recent high-resolution imaging provides new insights into the nature of Io's colorful surface, shedding light on the composition and origin of pyroclastic deposits and suggesting that Io's mysterious green spots are due to coating or alteration of silicate lavas.
The Center/TRACON Automation System (CTAS), under development at NASA Ames Research Center, is designed to assist controllers with the management and control of air traffic transitioning to/from congested airspace. This paper focuses on the transition from the en route environment, to high-density terminal airspace, under a time-based arrival-metering constraint. Two flight tests were conducted at the Denver Air Route Traffic Control Center (ARTCC) to study trajectory-prediction accuracy, the key to accurate Decision Support Tool advisories such as conflict detection/resolution and fuel-efficient metering conformance. In collaboration with NASA Langley Research Center, these test were part of an overall effort to research systems and procedures for the integration of CTAS and flight management systems (FMS). The Langley Transport Systems Research Vehicle Boeing 737 airplane flew a combined total of 58 cruise-arrival trajectory runs while following CTAS clearance advisories. Actual trajectories of the airplane were compared to CTAS and FMS predictions to measure trajectory-prediction accuracy and identify the primary sources of error for both. The research airplane was used to evaluate several levels of cockpit automation ranging from conventional avionics to a performance-based vertical navigation (VNAV) FMS. Trajectory prediction accuracy was analyzed with respect to both ARTCC radar tracking and GPS-based aircraft measurements. This paper presents detailed results describing the trajectory accuracy and error sources. Although differences were found in both accuracy and error sources, CTAS accuracy was comparable to the FMS in terms of both meter-fix arrival-time performance (in support of metering) and 4D-trajectory prediction (key to conflict prediction). Overall arrival time errors (mean plus standard deviation) were measured to be approximately 24 seconds during the first flight test (23 runs) and 15 seconds during the second flight test (25 runs). The major source of error during these tests was found to be the predicted winds aloft used by CTAS. Position and velocity estimates of the airplane provided to CTAS by the ATC Host radar tracker were found to be a relatively insignificant error source for the trajectory conditions evaluated. Airplane performance modeling errors within CTAS were found to not significantly affect arrival time errors when the constrained descent procedures were used. The most significant effect related to the flight guidance was observed to be the cross-track and turn-overshoot errors associated with conventional VOR guidance. Lateral navigation (LNAV) guidance significantly reduced both the cross-track and turn-overshoot error. Pilot procedures and VNAV guidance were found to significantly reduce the vertical profile errors associated with atmospheric and aircraft performance model errors.
This paper describes a type of cosmic ray detector suitable for the construction of a small, low power space-based instrument to detect the energy and isotope of energetic nuclei, such as those produced by solar flares. The detector is similar to previous types of silicon PIN detectors where the fully depleted body of the wafer comprises intrinsic region of the PIN structure. The novel aspect of this detector is that the one surface is divided into a two-dimensional array of pixels, and that the collected holes are divided between a row and a column collector in each pixel, yielding both dimensions of position information from this side of the detector.
The Committee on Earth Observations Satellites (CEOS) is outlined and the dynamics of the expanded CEOS with its increased focus on activities related to global climate change are examined. The participation of affiliate organizations at the Dec. 1991 Plenary is highlighted. The roles and current activities of constituted CEOS Working Groups on Data and on Sensor Calibration and Performance Validation are examined. The status of CEOS deliberations in Earth observation data policy is examined. The expanded CEOS is believed to be well suited to carry forward and help further the legacy already well established in the myriad of Earth observation activities associated with the 1992 International Space Year.
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.
The absorption of solar photons controls the composition of the middle atmosphere. Concern about the effects of man's industrial activities on his environment has led to a number of long-term programs related to stratospheric photochemistry. An important part of these programs represents studies of the variability of the sun's solar spectral irradiance. A program to measure the solar ultraviolet flux, both within and outside the earth's atmosphere, has been in progress for a number of years. This program has the objective to develop techniques which would provide an absolute accuracy of at least 5 percent. The amount of satellite instrument degradation is determined on the basis of a comparison of data provided by these instruments with data obtained in connection with rocket flights. It was found that a significant decrease in the solar irradiance below 190 nm occurred between 1980 and 1981.
A modification of a row/column scanning system was investigated in order to increase the speed and accuracy with which communication aids can be accessed with one or two switches. A selection algorithm was developed and programmed in BASIC to automatically select individuals with the characteristic difficulty in controlling time dependent control and display systems. Four systems were compared: (1) row/column directed scan (2 switches); (2) row/column auto scan (1 switch); (3) row auto scan (1 switch); and (4) column auto scan (1 switch). For this sample population, there were no significant differences among systems for scan time to select the correct target. The row/column auto scan system resulted in significantly more errors than any of the other three systems. Thus, the most widely prescribed system for severely physically disabled individuals turns out for this group to have a higher error rate and no faster communication rate than three other systems that have been considered inappropriate for this group.
Three single-target visual search tasks were used to evaluate a set of CRT symbols for a helicopter traffic display. The search tasks were representative of the kinds of information extraction required in practice, and reaction time was used to measure the efficiency with which symbols could be located and identified. The results show that familiar numeric symbols were responded to more quickly than graphic symbols. The addition of modifier symbols such as a nearby flashing dot or surrounding square had a greater disruptive effect on the graphic symbols than the alphanumeric characters. The results suggest that a symbol set is like a list that must be learned. Factors that affect the time to respond to items in a list, such as familiarity and visual discriminability, and the division of list items into categories, also affect the time to identify symbols.
The LANDSAT 4 thematic mapper sensor possesses approximately twice the spectral resolution, three times the spatial resolution, and four times the spectral sensitivity of the MSS sensor on earlier LANDSAT satellites. Spectral bands on the TM, particularly those at wavelengths of 1.6 and 2.2 micrometers are useful for: (1) distinguishing crops such as rice and soybeans; (2) surveying areas that are cultivated in strip crop fashion; (3) determining clay variations and abundances and rock classifications; and (4) differentiating nutrients and sediments found in coastal waters. The sensor can identify surface features 30 meters on a side, which roughly corresponds to a standard city block. Highway construction, land excavation, urban growth, and the health and extent of vegetation can be detected.
The concepts of an integrated design data base system (DBMS) as it might apply to an electronic design company are discussed. Data elements of documentation, project specifications, project tracking, firmware, software, electronic and mechanical design can be integrated and managed through a single DBMS. Combining the attributes of a DBMS data handler with specialized systems and functional data can provide users with maximum flexibility, reduced redundancy, and increased overall systems performance. Although some system overhead is lost due to redundancy in transitory data, it is believed the combination of the two data types is advisable rather than trying to do all data handling through a single DBMS.
The spectral reflectances at near-normal incidence for methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol are reported for the spectral range 6700-350 kaysers. The real and imaginary parts of the complex index of refraction of these liquids are obtained in the range 4000-400 kaysers by use of Kramers-Kronig phase-shift analysis. For all of the alcohols studied, the strength for the OH-stretch bands is directly proportional to the number of OH groups per unit volume; similar relations are established for CH- and CO-stretch bands. Absorption cross sections for stretch vibrations of the three groups are considered, and the role of characteristic group intensities in intensity spectroscopy is discussed.
Kramers-Kronig analysis is applied to measured values of spectral reflectance at near-normal incidence to determine the real and the imaginary parts of the complex index of refraction for methane, ethane, propane, n-butane, n-hexane, n-heptane, and n-decane in the liquid state. The results indicate that the strengths of the characteristic bands as measured by the integral of the imaginary part are roughly constant for all the liquid alkanes except for methane. The intensity of the CH valence vibration bands in the spectra of the alkanes except methane is directly proportional to the number of CH groups per unit volume. The relations for the intensity of the bands due to CH2 and CH3 deformations are examined. Characteristic band intensities of the type established for NH4(+) and SO4(2-) groups in solutions and crystals cannot be extended to the more closely coupled CH2 and CH3 groups in alkane molecules.
The dielectric constants of several soil samples were measured at frequencies of 5 and 19 GHz using the infinite transmission line method. The results of these measurements are presented and discussed with respect to soil types and texture structures. A comparison is made with other measurements at 1.4 GHz. At all three frequencies, the dependence of dielectric constant on soil moisture can be approximated by two straight lines. At low moisture, the slope is less than at high moisture level. The intersection of the two lines is believed to be a function of soil texture.
We have applied Kramers-Kronig analysis to reflection spectra to determine the optical constants of ammonium hydroxide and of aqueous solutions of ammonium chloride and bromide. From considerations of the absorption indices k(nu) we conclude that ammonium hydroxide consists of a solution of NH3 in water, in which NH3 molecules are hydrogen bonded to neighboring water molecules. The spectrum of ammonium hydroxide differs from the spectra of ammonium salts, in which bands characteristic of NH4(+) ions are prominent. The existence of ammonium hydroxide as an aerosol in planetary atmospheres is briefly discussed
Explore the source record for details and available documents.