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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Open-loop nanosecond-synchronization for wideband satellite communications

A synchronization technique for use with an onboard processing satellite communication system is discussed. The satellite oscillator is used both as the system time reference and as the frequency source for all downlink carriers and data clocks. Downlink timing is established at each system earth terminal through a combination of carrier and data-clock tracking and a downlink timing epoch signal consisting of one bit per TDMA data burst. Uplink timing is established by an open-loop range prediction process using precision ephemerides calculated and distributed by the central control station. Overall timing accuracy of the uplink signal at the satellite receiver of + or - 7 nanoseconds permits unambiguous identification of each data bit position in a 128 Mbps TDMA burst. This is accomplished by means of simple, inexpensive terminal hardware using available crystal oscillators for time/frequency references and digital synthesis techniques that may be implemented in digital LSI chips.

Holmes, W. M., Jr.↗
NASA NTRS1984

An evaluation of the directed flow graph methodology

The applicability of the Directed Graph Methodology (DGM) to the design and analysis of special purpose image and signal processing hardware was evaluated. A special purpose image processing system was designed and described using DGM. The design, suitable for very large scale integration (VLSI) implements a region labeling technique. Two computer chips were designed, both using metal-nitride-oxide-silicon (MNOS) technology, as well as a functional system utilizing those chips to perform real time region labeling. The system is described in terms of DGM primitives. As it is currently implemented, DGM is inappropriate for describing synchronous, tightly coupled, special purpose systems. The nature of the DGM formalism lends itself more readily to modeling networks of general purpose processors.

Snyder, W. E.↗
NASA NTRS1986

An Ada implementation for fault detection, isolation and reconfiguration using a fault-tolerant processor

The design and implementation, in Ada, of the Fault Detection, Isolation, and Reconfiguration (FDIR) Manager for the triply redundant, tightly synchronized, Fault Tolerant Processor (FTP) are covered. It also examines the suitability of Ada, in the context of the FTP, for real time control tasks. The operational concepts behind the FTP are explained, and the structure of the resultant Ada code is discussed.

Greeley, Gregory L.↗
NASA NTRS1991

An interferometric spectral mapper for the Cassini mission to Saturn and Titan

The research reported resulted in a detailed proposal for the Cassini Mission that addressed fundamental atmospheric structure and composition questions for both Saturn and Titan. Due to the brief time available between the initiation of the research and the proposal date, it was necessary to develop the science goals in close synchronicity with the instrument design. A major goal of the program was to acquire, implement, and test SWIR and MWIR arrays suitable for use with the DIGITAL ARRAY SCANNED INTERFEROMETERS, (DASI's), for the extended goal of developing DASI's for a range of spectral mapping research in solar system exploration. These goals were all reached or exceeded in the course of this work.

Smith, William Hayden↗
NASA NTRS2005

Ground System for Solar Dynamics Observatory (SDO) Mission

NASA s Goddard Space Flight Center (GSFC) has recently completed its Critical Design Review (CDR) of a new dual Ka and S-band ground system for the Solar Dynamics Observatory (SDO) Mission. SDO, the flagship mission under the new Living with a Star Program Office, is one of GSFC s most recent large-scale in-house missions. The observatory is scheduled for launch in August 2008 from the Kennedy Space Center aboard an Atlas-5 expendable launch vehicle. Unique to this mission is an extremely challenging science data capture requirement. The mission is required to capture 99.99% of available science over 95% of all observation opportunities. Due to the continuous, high volume (150 Mbps) science data rate, no on-board storage of science data will be implemented on this mission. With the observatory placed in a geo-synchronous orbit at 36,000 kilometers within view of dedicated ground stations, the ground system will in effect implement a "real-time" science data pipeline with appropriate data accounting, data storage, data distribution, data recovery, and automated system failure detection and correction to keep the science data flowing continuously to three separate Science Operations Centers (SOCs). Data storage rates of approx. 45 Tera-bytes per month are expected. The Mission Operations Center (MOC) will be based at GSFC and is designed to be highly automated. Three SOCs will share in the observatory operations, each operating their own instrument. Remote operations of a multi-antenna ground station in White Sands, New Mexico from the MOC is part of the design baseline.

Tann, Hun K.↗
NASA NTRS2009

Digital Synchronizer without Metastability

A proposed design for a digital synchronizing circuit would eliminate metastability that plagues flip-flop circuits in digital input/output interfaces. This metastability is associated with sampling, by use of flip-flops, of an external signal that is asynchronous with a clock signal that drives the flip-flops: it is a temporary flip-flop failure that can occur when a rising or falling edge of an asynchronous signal occurs during the setup and/or hold time of a flip-flop. The proposed design calls for (1) use of a clock frequency greater than the frequency of the asynchronous signal, (2) use of flip-flop asynchronous preset or clear signals for the asynchronous input, (3) use of a clock asynchronous recovery delay with pulse width discriminator, and (4) tying the data inputs to constant logic levels to obtain (5) two half-rate synchronous partial signals - one for the falling and one for the rising edge. Inasmuch as the flip-flop data inputs would be permanently tied to constant logic levels, setup and hold times would not be violated. The half-rate partial signals would be recombined to construct a signal that would replicate the original asynchronous signal at its original rate but would be synchronous with the clock signal.

Simle, Robert M.↗
NASA NTRS2005

Stroboscopic Interferometer for Measuring Mirror Vibrations

Stroboscopic interferometry is a technique for measuring the modes of vibration of mirrors that are lightweight and, therefore, unavoidably flexible. The technique was conceived especially for modal characterization of lightweight focusing mirror segments to be deployed in outer space; however, the technique can be applied to lightweight mirrors designed for use on Earth as well as the modal investigation of other optical and mechanical structures. To determine the modal structure of vibration of a mirror, it is necessary to excite the mirror by applying a force that varies periodically with time at a controllable frequency. The excitation can utilize sinusoidal, square, triangular, or even asynchronous waveforms. Because vibrational modes occur at specific resonant frequencies, it is necessary to perform synchronous measurements and sweep the frequency to locate the significant resonant modes. For a given mode it is possible to step the phase of data acquisition in order to capture the modal behavior over a single cycle of the resonant frequency. In order to measure interferometrically the vibrational response of the mirror at a given frequency, an interferometer must be suitably aligned with the mirror and adjustably phase-locked with the excitation signal. As in conventional stroboscopic photography, the basic idea in stroboscopic interferometry is to capture an image of the shape of a moving object (in this case, the vibrating mirror) at a specified instant of time in the vibration cycle. Adjusting the phase difference over a full cycle causes the interference fringes to vary over the full range of motion for the mode at the excitation frequency. The interference-fringe pattern is recorded as a function of the phase difference, and, from the resulting data, the surface shape of the mirror for the given mode is extracted. In addition to the interferometer and the mirror to be tested, the equipment needed for stroboscopic interferometry includes an arbitrary-function generator (that is, a signal generator), an oscilloscope, a trigger filter, and an advanced charge-coupled-device (CCD) camera. The optical components are positioned to form a pupil image of the mirror under test on the CCD chip, so that the interference pattern representative of the instantaneous mirror shape is imaged on the CCD chip.

Stahl, H. Philip↗
NASA NTRS1981

Timing signal propagates without phase shift

Continous monitoring of transmission delay corrects for phase shift. Nodes in Master/Slave Returnable Timing System (MSRTS) are arranged in hierarchy, with each node serving as master to several slave nodes. As signal at each slave is synchronized with original master, it serves as master to synchronize following slave nodes. System improves performance of phased microwave antenna arrays in solar-powered satellites and clock distribution systems in avionics and computers.

Kantak, A. V.↗
NASA NTRS1984

Landsat thematic mapper attitude data processing

The Landsat 4 and 5 satellites carry a new, high resolution, seven band thematic mapper imaging instrument. The spacecraft also carry two types of attitude sensors: a gyroscopic internal reference unit (IRU) which senses angular rate from dc to about 2 Hz, and an AC-coupled angular displacement sensor (ADS) measuring angular deviation above 2 Hz. A description of the derivation of the crossover network used to combine and equalize the IRU and ADS data is made. Also described are the digital data processing algorithms which produce the time history of the satellites' attitude motion including the finite impulse response (FIR) implementation of G and F filters; the resampling (interpolation/decimation) and synchronization of the IRU and ADS data; and the axis rotations required as a result of the on-board sensor locations on three orthogonal axes.

Sehn, G. J.↗
NASA NTRS1987

Evolution of the Southern Oscillation as observed by the Nimbus-7 ERB experiment

The Nimbus-7 satellite has been in a 955-km, sun-synchronous orbit since October 1978. The Earth Radiation Budget (ERB) experiment has taken approximately 8 years of high-quality data during this time, of which seven complete years have been archived at the National Space Science Data Center. A final reprocessing of the wide-field-of-view channel dataset is underway. Error analyses indicate a long-term stability of 1 percent better over the length of the data record. As part of the validation of the ERB measurements, the archived 7-year Nimbus-7 ERB dataset is examined for the presence and accuracy of interannual variations including the Southern Oscillation signal. Zonal averages of broadband outgoing longwave radiation indicate a terrestrial response of more than 2 years to the oceanic and atmospheric manifestations of the 1982-83 El Nino/Southern Oscillation (ENSO) event, especially in the tropics. This signal is present in monthly and seasonal averages and is shown here to derive primarily from atmospheric responses to adjustments in the Pacific Ocean. The calibration stability of this dataset thus provides a powerful new tool to examine the physics of the ENSO phenomena.

Ardanuy, Philip E.↗
NASA NTRS1993

Synchronizing A Television Camera With An External Reference

Improvement in genlock subsystem consists in incorporation of controllable delay circuit into path of composite synchronization signal obtained from external video source. Delay circuit helps to eliminate potential jitter in video display and ensures setup requirements for digital timing circuits of video camera satisfied.

Rentsch, Edward M.↗
NASA NTRS1994

Coherent-phase or random-phase acceleration of electron beams in solar flares

Time structures of electron beam signatures at radio wavelengths are investigated to probe correlated versus random behavior in solar flares. In particular we address the issue whether acceleration and injection of electron beams is coherently modulated by a single source, or whether the injection is driven by a stochastic (possibly spatially fragmented) process. We analyze a total of approximately = 6000 type III bursts observed by Ikarus (Zurich) in the frequency range of 100-500 MHz, during 359 solar flares with simultaneous greater than or = 25 keV hard X-ray emission, in the years 1890-1983. In 155 flares we find a total of 260 continuous type III groups, with an average number of 13 +/- 9 bursts per group, a mean duration of D = 12 +/- 14 s, a mean period of P = 2.0 +/- 1.2 s, with the highest burst rate at a frequency of nu = 310 +/- 120 MHz. Pulse periods have been measured between 0.5 and 10 s, and can be described by an exponential distribution, i.e., N(P) varies as e (exp -P/1.0s). The period shows a frequency dependence of P(nu)=46(exp-0.6)(sub MHz)s for different flares, but is invariant during a particular flare. We measure the mean period P and its standard deviation sigma (sub p) in each type III group, and quantify the degree of periodicity (or phase-coherence) by the dimensionless parameter sigma (sub p)P. The representative sample of 260 type III burst groups shows a mean periodicity of sigma (sub p/P) = 0.37 +/- 0.12, while Monte Carlo simulations of an equivalent set of truly random time series show a distinctly different value of sigma (sub p)P = 0.93 +/- 0.26. This result indicates that the injection of electron beams is coherently modulated by a particle acceleration source which is either compact or has a global organization on a timescale of seconds, in contrast to an incoherent acceleration source, which is stochastic either in time or space. We discuss the constraints on the size of the acceleration region resulting from electron beam propagation delays and from Alfvenic synchronization during a pulse period. We discuss two periodic preocesses in flares, which potentially control quasi-periodic particle acceleration: (1) MHD oscillations, and (2) current sheets with oscillatory dynamics.

Aschwanden, Markus J.↗
NASA NTRS1993

Preliminary Stratigraphic Basis for Geologic Mapping of Venus

The age relations between geologic formations have been studied at 36 1000x1000 km areas centered at the dark paraboloid craters. The geologic setting in all these sites could be characterized using only 16 types of features and terrains (units). These units form a basic stratigraphic sequence (from older to younger: (1) Tessera (Tt); (2-3) Densely fractured terrains associated with coronae (COdf) and in the form of remnants among plains (Pdf); (4) Fractured and ridged plains (Pfr); (5) Plains with wrinkle ridges (Pwr); (6-7) Smooth and lobate plains (Ps/Pl); and (8) Rift-associated fractures (Fra). The stratigraphic position of the other units is determined by their relation with the units of the basic sequence: (9) Ridge bells (RB), contemporary with Pfr; (10-11) Ridges of coronae and arachnoids annuli (COar/Aar), contemporary with wrinkle ridges of Pwr; (12) Fractures of coronae annuli (COaf) disrupt Pwr and Ps/Pl; (13) Fractures (F) disrupt Pwr or younger units; (14) Craters with associated dark paraboloids (Cdp), which are on top of all volcanic and tectonic units except the youngest episodes of rift-associated fracturing and volcanism; (15-16) Surficial streaks (Ss) and surficial patches (Sp) are approximately contemporary with Cdp. These units may be used as a tentative basis for the geologic mapping of Venus including VMAP. This mapping should test the stratigraphy and answer the question of whether this stratigraphic sequence corresponds to geologic events which were generally synchronous all around the planet or whether the sequence is simply a typical sequence of events which occurred in different places at diffferent times.

Basilevsky, A. T.↗
NASA NTRS1999

Communications, Signal Processing, and Telemetering Research Program Review

A Communications, Signal Processing, and Telemetering Research Program Review was held on February 23, 1999. Research conducted under the grant was presented and reviewed, for progress, and for possible technology transfers. The research reviewed was in the following areas: (1) Bandwidth-efficient Modulation and nonlinear equalization; (2) Investigation of an architecture for parallel signal processing applicable to communications problems; (3)Coded partial response over satellites; (4) synchronization at Low SNR; (5) Serial concatenated convolutional codes and some implementation issues on high rate turbo codes; (6) Flight experiments; (7) Real time doppler tracking; (8) Space protocol testing; (9) Lightweight optical communications without carrying a laser in space. The presentations are given by the graduate students who performed the research.

Source record↗
NASA NTRS1999

The Effect of Spaceflight on Bone Cell Cultures

Understanding the response of bone to mechanical loading (unloading) is extremely important in defining the means of adaptation of the body to a variety of environmental conditions such as during heightened physical activity or in extended explorations of space or the sea floor. The mechanisms of the adaptive response of bone are not well defined, but undoubtedly they involve changes occurring at the cellular level of bone structure. This proposal has intended to examine the hypothesis that the loading (unloading) response of bone is mediated by specific cells through modifications of their activity cytoskeletal elements, and/or elaboration of their extracellular matrices. For this purpose, this laboratory has utilized the results of a number of previous studies defining molecular biological, biochemical, morphological, and ultrastructural events of the reproducible mineralization of a primary bone cell (osteoblast) culture system under normal loading (1G gravity level). These data and the culture system then were examined following the use of the cultures in two NASA shuttle flights, STS-59 and STS-63. The cells collected from each of the flights were compared to respective synchronous ground (1G) control cells examined as the flight samples were simultaneously analyzed and to other control cells maintained at 1G until the time of shuttle launch, at which point they were terminated and studied (defined as basal cells). Each of the cell cultures was assayed in terms of metabolic markers- gene expression; synthesis and secretion of collagen and non-collagenous proteins, including certain cytoskeletal components; assembly of collagen into macrostructural arrays- formation of mineral; and interaction of collagen and mineral crystals during calcification of the cultures. The work has utilized a combination of biochemical techniques (radiolabeling, electrophoresis, fluorography, Western and Northern Blotting, and light microscopic immunofluorescence) and structural methods (conventional and high voltage electron microscopy, inununocytochemistry, stereomicroscopy, and 3D image reconstruction). The studies have provided new knowledge of aspects of bone cell development and structural regulation, extracellular matrix assembly, and mineralization during spaceflight and under normal gravity. The information has contributed to insights into the means in general by which cells respond and adapt to different conditions of gravity (loading). The data may as well have suggested an underlying basis for the observed loss of bone by vertebrates, including man, in microgravity; and these scientific results may have implications for understanding bone loss following fracture healing and extended periods of inactivity such as during long-term bedrest.

Landis, William J.↗
NASA NTRS2002

Timekeeping for the Space Technology 5 (ST-5) Mission

Space Technology 5, or better known as ST-5, is a space technology development mission in the New Millennium Program (NMP) and NASA s first experiment in the design of miniaturized satellite constellations. The mission will design, integrate and launch multiple spacecraft into an orbit high above the Earth s protective magnetic field known as the magnetosphere. Each spacecraft incorporates innovative technology and constellation concepts which will be instrumental in future space science missions. A total of three ST-5 spacecraft will be launched as secondary payloads into a highly elliptical geo-synchronous transfer orbit, and will operate as a 3-element constellation for a minimum duration of 90 days. In order to correlate the time of science measurements with orbit position relative to the Earth, orbit position in space (with respect to other objects in space) and/or with events measured on Earth or other spacecraft, accurate knowledge of spacecraft and ground time is needed. Ground time as used in the USA (known as Universal Time Coordinated or UTC) is maintained by the U.S. Naval Observatory. Spacecraft time is maintained onboard within the Command and Data Handling (C&DH) system. The science requirements for ST-5 are that spacecraft time and ground time be correlatable to each other, with some degree of accuracy. Accurate knowledge of UTC time on a spacecraft is required so that science measurements can be correlated with orbit position relative to the Earth, orbit position in space and with events measured on Earth or other spacecraft. The most crucial parameter is not the clock oscillator frequency, but more importantly, how the clock oscillator frequency varies with time or temperature (clock oscillator drift). Even with an incorrect clock oscillator frequency, if there were no drift, the frequency could be assessed by comparing the spacecraft clock to a ground clock during a few correlation events. Once the frequency is accurately known, it is easy enough to make a regular adjustment to the spacecraft clock or to calculate the correct ground time for a given spacecraft clock time. The oscillator frequency, however, is temperature dependent, drifts with age and is affected by radiation; hence, repeated correlation measurements are required.

Raphael, Dave↗
NASA NTRS1989

The case for extraterrestrial causes of extinction

The dramatic increase in our knowledge of large-body impacts that have occurred in Earth's history has led to strong arguments for the plausibility of meteorite impact as a cause of extinction. Proof of causation is often hampered, however, by our inability to demonstrate the synchronism of specific impacts and extinctions. A central problem is range truncation: the last reported occurrences of fossil taxa generally underestimate the true times of extinction. Range truncation, because of gaps in sedimentation, lack of preservation, or lack of discovery, can make sudden extinctions appear gradual and gradual extinctions appear sudden. Also, stepwise extinction may appear as an artefact of range truncation. These effects are demonstrated by experiments performed on data from field collections of Cretaceous ammonities from Zumaya (Spain). The challenge for future research is to develop a new calculus for treating biostratigraphic data so that fossils can provide more accurate assessments of the timing of extinctions.

Non-NASA Center↗