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At least 865 records · Page 48

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NASA NTRS1990

A numerical method for prediction of compressible turbulent flows with closure models

A new computer code to solve the time averaged Navier-Stokes equations is developed. Many of the state-of-the-art numerical techniques and algorithms have been tested and implemented in the program in order to achieve a better numerical accuracy and code efficiency. Various turbulence models are tested for a wide range of flows. The initial focus has been on two-equation eddy-viscosity models, which are the most advanced available in current compressible flow codes. The long term goal will be to test Reynolds-Stress models and to explore their performance in the high Mach number range. Although testing and improvement of turbulence models for supersonic and hypersonic flows is the primary objective of this research, part of the effort has been devoted to analyzing the vortex breakdown phenomena using new computer programs. Some preliminary results on the breakdown of a vortex flow in a tube are reported. Present calculations are restricted to two dimensional flow geometry.

Huang, P. G.↗

NASA NTRS1991

Measurement of subresolution terrain displacements using SPOT panchromatic imagery

It is shown that satellite-derived imagery can be used to measure subresolution horizontal terrain displacements associated with present-day earthquakes, sand dune migration, glacial motion, coastal sediment transport, and pre-eruptive volcanic processes. This use of these data allows the detection of change and the determination of rates of many environmental processes worldwide. The premise of the method is that areally extensive, subresolution spatial differences in ground patterns between images acquired at different times can be accurately measured to high precision and can be distinguished from systematic image differences, such as those due to sensing-system attitude variations. Thus far, a preliminary algorithm has been tested that shows that the approach is reasonable.

Crippen, Robert E.↗

NASA NTRS1993

Compression through decomposition into browse and residual images

Economical archival and retrieval of image data is becoming increasingly important considering the unprecedented data volumes expected from the Earth Observing System (EOS) instruments. For cost effective browsing the image data (possibly from remote site), and retrieving the original image data from the data archive, we suggest an integrated image browse and data archive system employing incremental transmission. We produce our browse image data with the JPEG/DCT lossy compression approach. Image residual data is then obtained by taking the pixel by pixel differences between the original data and the browse image data. We then code the residual data with a form of variable length coding called diagonal coding. In our experiments, the JPEG/DCT is used at different quality factors (Q) to generate the browse and residual data. The algorithm has been tested on band 4 of two Thematic mapper (TM) data sets. The best overall compression ratios (of about 1.7) were obtained when a quality factor of Q=50 was used to produce browse data at a compression ratio of 10 to 11. At this quality factor the browse image data has virtually no visible distortions for the images tested.

Novik, Dmitry A.↗

NASA NTRS1993

Building a generalized distributed system model

The key elements in the 1992-93 period of the project are the following: (1) extensive use of the simulator to implement and test - concurrency control algorithms, interactive user interface, and replica control algorithms; and (2) investigations into the applicability of data and process replication in real-time systems. In the 1993-94 period of the project, we intend to accomplish the following: (1) concentrate on efforts to investigate the effects of data and process replication on hard and soft real-time systems - especially we will concentrate on the impact of semantic-based consistency control schemes on a distributed real-time system in terms of improved reliability, improved availability, better resource utilization, and reduced missed task deadlines; and (2) use the prototype to verify the theoretically predicted performance of locking protocols, etc.

Mukkamala, R.↗

NASA NTRS1992

Subvisual-thin cirrus lidar dataset for satellite verification and climatological research

A polarization (0.694 microns wavelength) lidar dataset for subvisual and thin (bluish-colored) cirrus clouds is drawn from project FIRE (First ISCCP Regional Experiment) extended time observations. The clouds are characterized by their day-night visual appearance; base, top, and optical midcloud heights and temperatures; measured physical and estimated optical cloud thicknesses; integrated linear depolarization ratios; and derived k/2 eta ratios. A subset of the data supporting 30 NOAA polar-orbiting satellite overpasses is given in tabular form to provide investigators with the means to test cloud retrieval algorithms and establish the limits of cirrus detectability from satellite measurements under various conditions. Climatologically, subvisual-thin cirrus appear to be higher, colder, and more strongly depolarizing than previously reported multilatitude cirrus, although similar k/2 eta that decrease with height and temperature are found.

Sassen, Kenneth↗

NASA NTRS1993

Estimation of solar backscatter ultraviolet albedo using ground-based Umkehr measurements

A retrieval method was developed to estimate the solar backscatter ultraviolet (SBUV) satellite albedo for the ozone profiler wavelengths using ground-based ultraviolet measurements. For the present investigation the Umkehr was used as the ground-based ultraviolet measurement. Simulated SBUV data and Umkehr data theoretically computed from a priori ozone profiles observed by the SAGE II satellite were used to develop the retrieval algorithm and to test its capability. The test indicated that albedos for the SBUV ozone profiler wavelengths should allow estimates to a precision of +/- 5 percent or better, depending on the accuracy of the ultraviolet measurement. Retrievals using actual Umkehr observations were also performed to provide a preliminary look at the magnitude and annual variation of retrieved albedos. A case study was performed, comparing retrieved albedos with SBUV-measured albedos. The SBUV albedo change was seen to be approximately twice as large as the albedo changes estimated by the Umkehr method. Results of the investigation suggest that the method of estimation may be useful for determining the drift rate of the SBUV calibration.

Deluisi, John J.↗

NASA NTRS1992

A cloud and radiation model-based algorithm for rainfall retrieval from SSM/I multispectral microwave measurements

A hybrid statistical-physical retrieval scheme is explored which combines a statistical approach with an approach based on the development of cloud-radiation models designed to simulate precipitating atmospheres. The algorithm employs the detailed microphysical information from a cloud model as input to a radiative transfer model which generates a cloud-radiation model database. Statistical procedures are then invoked to objectively generate an initial guess composite profile data set from the database. The retrieval algorithm has been tested for a tropical typhoon case using Special Sensor Microwave/Imager (SSM/I) data and has shown satisfactory results.

Xiang, Xuwu↗

NASA NTRS1991

Optical correlator techniques applied to robotic vision

Vision processing is one of the most computationally intensive tasks required of an autonomous robot. The data flow from a single typical imaging sensor is roughly 60 Mbits/sec, which can easily overload current on-board processors. Optical correlator-based processing can be used to perform many of the functions required of a general robotic vision system, such as object recognition, tracking, and orientation determination, and can perform these functions fast enough to keep pace with the incoming sensor data. We describe a hybrid digital electronic/analog optical robotic vision processing system developed at Ames Research Center to test concepts and algorithms for autonomous construction, inspection, and maintenance of space-based habitats. We discuss the system architecture design and implementation, its performance characteristics, and our future plans. In particular, we compare the performance of the system to a more conventional all digital electronic system developed concurrently. The hybrid system consistently outperforms the digital electronic one in both speed and robustness.

Hine, Butler P., III↗

NASA NTRS1993

Studies and simulations of the DigiCipher system

During this period the development of simulators for the various high definition television (HDTV) systems proposed to the FCC was continued. The FCC has indicated that it wants the various proposers to collaborate on a single system. Based on all available information this system will look very much like the advanced digital television (ADTV) system with major contributions only from the DigiCipher system. The results of our simulations of the DigiCipher system are described. This simulator was tested using test sequences from the MPEG committee. The results are extrapolated to HDTV video sequences. Once again, some caveats are in order. The sequences used for testing the simulator and generating the results are those used for testing the MPEG algorithm. The sequences are of much lower resolution than the HDTV sequences would be, and therefore the extrapolations are not totally accurate. One would expect to get significantly higher compression in terms of bits per pixel with sequences that are of higher resolution. However, the simulator itself is a valid one, and should HDTV sequences become available, they could be used directly with the simulator. A brief overview of the DigiCipher system is given. Some coding results obtained using the simulator are looked at. These results are compared to those obtained using the ADTV system. These results are evaluated in the context of the CCSDS specifications and make some suggestions as to how the DigiCipher system could be implemented in the NASA network. Simulations such as the ones reported can be biased depending on the particular source sequence used. In order to get more complete information about the system one needs to obtain a reasonable set of models which mirror the various kinds of sources encountered during video coding. A set of models which can be used to effectively model the various possible scenarios is provided. As this is somewhat tangential to the other work reported, the results are included as an appendix.

Sayood, K.↗

NASA NTRS1993

Geomagnetic field models incorporating physical constraints on the secular variation

This proposal has been concerned with methods for constructing geomagnetic field models that incorporate physical constraints on the secular variation. The principle goal that has been accomplished is the development of flexible algorithms designed to test whether the frozen flux approximation is adequate to describe the available geomagnetic data and their secular variation throughout this century. These have been applied to geomagnetic data from both the early and middle part of this century and convincingly demonstrate that there is no need to invoke violations of the frozen flux hypothesis in order to satisfy the available geomagnetic data.

Constable, Catherine↗

NASA NTRS1992

Previous model comparisons

The first model comparison workshop examined 14 mathematical models. Issues discussed included: transport methodologies; transport of nuclear test debris; transport algorithms; source gases; NO(y), Cl(x), HO(x), and O3; photolysis rates; diurnal averaging; and rainout. The second model comparison meeting examined 16 models. More real model-model intercomparisons were undertaken and several topics were addressed including: photochemistry and radiation; transport; current atmosphere 1980; and assessment runs. The models computed different values for photolysis rates even when O3 and O2 were fixed, thus more time should be spent at future model comparison meeting on radiation codes.

Jackman, Charles H.↗

NASA NTRS1994

Array distribution in data-parallel programs

We consider distribution at compile time of the array data in a distributed-memory implementation of a data-parallel program written in a language like Fortran 90. We allow dynamic redistribution of data and define a heuristic algorithmic framework that chooses distribution parameters to minimize an estimate of program completion time. We represent the program as an alignment-distribution graph. We propose a divide-and-conquer algorithm for distribution that initially assigns a common distribution to each node of the graph and successively refines this assignment, taking computation, realignment, and redistribution costs into account. We explain how to estimate the effect of distribution on computation cost and how to choose a candidate set of distributions. We present the results of an implementation of our algorithms on several test problems.

Chatterjee, Siddhartha↗

NASA NTRS1995

A microcontroller-based three degree-of-freedom manipulator testbed

A wheeled exploratory vehicle is under construction at the Mars Mission Research Center at North Carolina State University. In order to serve as more than an inspection tool, this vehicle requires the ability to interact with its surroundings. A crane-type manipulator, as well as the necessary control hardware and software, has been developed for use as a sample gathering tool on this vehicle. The system is controlled by a network of four Motorola M68HC11 microcontrollers. Control hardware and software were developed in a modular fashion so that the system can be used to test future control algorithms and hardware. Actuators include three stepper motors and one solenoid. Sensors include three optical encoders and one cable tensiometer. The vehicle supervisor computer provides the manipulator system with the approximate coordinates of the target object. This system maps the workspace surrounding the given location by lowering the claw, along a set of evenly spaced vertical lines, until contact occurs. Based on this measured height information and prior knowledge of the target object size, the system determines if the object exists in the searched area. The system can find and retrieve a 1.25 in. diameter by 1.25 in. tall cylinder placed within the 47.5 sq in search area in less than 12 minutes. This manipulator hardware may be used for future control algorithm verification and serves as a prototype for other manipulator hardware.

Brown, Robert Michael, Jr.↗

NASA NTRS1994

Hubble Space Telescope photometry of the central regions of Virgo cluster elliptical galaxies. 2: Isophote shapes

The isophotal shapes of a magnitude limited sample of Virgo ellipticals are presented. These are derived from high resolution Hubble Space Telescope (HST) photometry. The absence of atmospheric seeing and accurate knowledge of the Point Spread Function (PSF) allows us to perform an accurate deconvolution. Model galaxies were constructed to test the deconvolution algorithms used, and showed that we can accurately recover isophotal shape parameters down to 0.5 sec. From the isophotal parameters we can classify the galaxies in two subsamples: disky and non-disky galaxies. In three of these disky galaxies we found evidence for a nuclear stellar disk in the inner 1.5 sec. In addition these galaxies also have an outer disk, that seems to break up inside 2 sec - 3 sec. In the two galaxies for which there is kinematic evidence from the literature of a decoupled core, we found no indication for such subsystem from the isophotal shape analysis. In 80% of these early type galaxies there are indications for dust. For eight of these galaxies the dust has not been detected before.

Bosch, Frank C. Van Den↗

NASA NTRS1993

Efficient Three-Dimensional Direct Simulation Monte Carlo for Complex Geometry Problems

The simulation of flowfields in the transition flow regime is notoriously difficult with high demands on computer resources (CPU time and storage) and user expertise/labor. This paper describes a new, efficient code which has been developed to simulate high Knudsen number flowfields in three dimensions about bodies of arbitrarily complex geometry. The algorithm has been tested over a wide range of conditions, from free molecular to near-continuum flow regimes, for slender and blunt bodies, for re-entry vehicles and spacecraft. A series of validation tests have been conducted using both wind-tunnel measurements and flight data.

Rault, Didier F. G.↗

NASA NTRS1996

The effects of cloud inhomogeneities upon radiative fluxes, and the supply of a cloud truth validation dataset

The ASTER polar cloud mask algorithm is currently under development. Several classification techniques have been developed and implemented. The merits and accuracy of each are being examined. The classification techniques under investigation include fuzzy logic, hierarchical neural network, and a pairwise histogram comparison scheme based on sample histograms called the Paired Histogram Method. Scene adaptive methods also are being investigated as a means to improve classifier performance. The feature, arctan of Band 4 and Band 5, and the Band 2 vs. Band 4 feature space are key to separating frozen water (e.g., ice/snow, slush/wet ice, etc.) from cloud over frozen water, and land from cloud over land, respectively. A total of 82 Landsat TM circumpolar scenes are being used as a basis for algorithm development and testing. Numerous spectral features are being tested and include the 7 basic Landsat TM bands, in addition to ratios, differences, arctans, and normalized differences of each combination of bands. A technique for deriving cloud base and top height is developed. It uses 2-D cross correlation between a cloud edge and its corresponding shadow to determine the displacement of the cloud from its shadow. The height is then determined from this displacement, the solar zenith angle, and the sensor viewing angle.

Welch, Ronald M.↗

NASA NTRS1995

The detector response matrices of the burst and transient source experiment (BATSE) on the Compton Gamma Ray Observatory

The detector response matrices for the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) are described, including their creation and operation in data analysis. These response matrices are a detailed abstract representation of the gamma-ray detectors' operating characteristics that are needed for data analysis. They are constructed from an extensive set of calibration data coupled with a complex geometry electromagnetic cascade Monte Carlo simulation code. The calibration tests and simulation algorithm optimization are described. The characteristics of the BATSE detectors in the spacecraft environment are also described.

Pendleton, Geoffrey N.↗

NASA NTRS1998

Construction and Operation of 1-5 Micron Array Camera (Minne-Cam) for the University of Minnesota Observatories

The 1-5 Micron Imaging Infrared (IR) Camera (MinneCam) for the University of Minnesota (UM) observatories has been constructed. It is presently housed at the Mt. Lemmon Observing Facility (MLOF) in Arizona for its inaugural observing run. MinneCam is testing and comparing various observational approaches, and will produce data sets that can be used to define and test data analysis algorithms. The camera is being used in support of studies to define strategies for using NASA's Space Infrared Telescope Facility (SIRTF) for observations of comets and other targets of opportunity. A long term goal is to modify the instrument to accommodate imaging polarimetry. This instrument is presently putting the University of Minnesota observatories in competition with other major IR observatories and is allowing us to develop state-of-the-art instrumentation for the U of M's proposed 2.4 meter telescope proposed jointly with NOAO and the University of Colorado. The camera has provided important research opportunities for the Minnesota IR group, including other faculty and graduate students in the Department of Astronomy.

Gehrz, Robert D.↗