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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Synchronized computational architecture for generalized bilateral control of robot arms

A master six degree of freedom Force Reflecting Hand Controller (FRHC) is available at a master site where a received image displays, in essentially real time, a remote robotic manipulator which is being controlled in the corresponding six degree freedom by command signals which are transmitted to the remote site in accordance with the movement of the FRHC at the master site. Software is user-initiated at the master site in order to establish the basic system conditions, and then a physical movement of the FRHC in Cartesean space is reflected at the master site by six absolute numbers that are sensed, translated and computed as a difference signal relative to the earlier position. The change in position is then transmitted in that differential signal form over a high speed synchronized bilateral communication channel which simultaneously returns robot-sensed response information to the master site as forces applied to the FRHC so that the FRHC reflects the feel of what is taking place at the remote site. A system wide clock rate is selected at a sufficiently high rate that the operator at the master site experiences the Force Reflecting operation in real time.

Szakaly, Zoltan F.↗
NASA NTRS1990

Estimation and filtering for high-precision GPS positioning applications

Estimation of GPS satellite orbits and other parameters for high-accuracy geodetic and tracking applications is carried out with a multi-satellite batch sequential pseudo-epoch state process noise filter. Both square-root information filtering and UD factorized covariance filtering and smoothing formulations are implemented in the software to enhance numerical stability. The filtering algorithms have been arranged to take advantage of sparse matrices and other characteristics of the GPS measurement scenarios. The filter includes unique error evaluation capabilities to assess effects from mismodeling. Process noise plays a key role in the orbit determination for stochastic behavior of transmitter/receiver clocks, atmospheric-induced delay fluctuations, and unmodeled spacecraft accelerations. Present GPS orbit accuracy is better than 1 m, with ground baselines determined simultaneously to relative accuracy of 1.5 parts in 10 to the 8th over several thousand km distance.

Lichten, Stephen M.↗
NASA NTRS1990

Efficient parametric analysis of performance measures for communication networks

Efficient techniques for estimating performance measures in communication networks in a steady-state or transient setting are developed. These techniques may be used in a simulation environment or in connection with real-time observations. For Markov chain models, the recently proposed standard clock approach is extended, and a class of real-time algorithms for simultaneously generating multiple sample paths under different parameter sets is presented. Attention is focused on the link crash time estimation problem, where a 'crash' is defined as the first time a buffer overflows, given some initial conditions. An algorithm for estimating crash times under various traffic shocks is derived, where all estimates are obtained in parallel to an actual network's normal operation. An algorithm for crash time estimation of a G/D/1 link model is also derived using a different (perturbation-analysis-based) approach. Finally, extensive simulation results are provided to validate the proposed algorithms and compare them to brute-force simulation.

Cassandras, Christos G.↗
NASA NTRS1990

New tests of the gravitational redshift effect

Tests of the gravitational redshift effect provide a way to check the validity of the Einstein Equivalence Principle (EEP) and, more specifically, of general relativity. If the EEP is valid, then the redshift should be the same for different clocks. Also, according to general relativity, the redshift should depend upon only the total mass of a gravitating body without reference to its detailed composition. These predictions have been tested mainly in the gravitational field of the earth. It is now possible to measure, with space probes, the redshift effect to good accuracy in the vicinity of other bodies in the solar system, in particular at the massive outer planets. The present and future status of these experiments is discussed.

Krisher, Timothy P.↗
NASA NTRS1991

Wire transfer of charge packets using a CCD-BBD structure for charge-domain signal processing

A structure for the virtual transfer of charge packets across metal wires is described theoretically and is experimentally verified. The structure is a hybrid of charge-coupled device (CCD) and bucket-brigade device (BBD) elements and permits the topological crossing of charge-domain signals in low power signal processing circuits. A test vehicle consisting of 8-, 32-, and 96-stage delay lines of various geometries implemented in a double-poly, double-metal foundry process is used to characterize the wire-transfer operation. Transfer efficiency ranging between 0.998 and 0.999 is obtained for surface n-channel devices with clock cycle times in the range from 40 ns to 0.3 ms. Transfer efficiency as high as 0.9999 is obtained for buried n-channel devices. Good agreement is found between experiment and simulation.

Fossum, Eric R.↗
NASA NTRS1990

32 bit digital optical computer - A hardware update

Such state-of-the-art devices as multielement linear laser diode arrays, multichannel acoustooptic modulators, optical relays, and avalanche photodiode arrays, are presently applied to the implementation of a 32-bit supercomputer's general-purpose optical central processing architecture. Shannon's theorem, Morozov's control operator method (in conjunction with combinatorial arithmetic), and DeMorgan's law have been used to design an architecture whose 100 MHz clock renders it fully competitive with emerging planar-semiconductor technology. Attention is given to the architecture's multichannel Bragg cells, thermal design and RF crosstalk considerations, and the first and second anamorphic relay legs.

Guilfoyle, Peter S.↗
NASA NTRS1990

New advancements in charge-coupled device technology - Sub-electron noise and 4096 x 4096 pixel CCDs

This paper reports on two new advancements in CCD technology. The first area of development has produced a special purpose CCD designed for ultra low-signal level imaging and spectroscopy applications that require sub-electron read noise floors. A nondestructive output circuit operating near its 1/f noise regime is clocked in a special manner to read a single pixel multiple times. Off-chip electronics average the multiple values, reducing the random noise by the square-root of the number of samples taken. Noise floors below 0.5 electrons rms are reported. The second development involves the design and performance of a high resolution imager of 4096 x 4096 pixels, the largest CCD manufactured in terms of pixel count. The device utilizes a 7.5-micron pixel fabricated with three-level poly-silicon to achieve high yield.

Janesick, James R.↗
NASA NTRS1991

Linear ion trap based atomic frequency standard

In order to develop a trapped ion-based fieldable frequency standard with stability 1 x 10 to the -13th/sq rt tau for averaging times tau greater than 10,000 s, a hybrid RF/DC linear ion trap was developed which permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields. The authors have confined Hg-199(+) ions in this trap and have measured very high Q transitions with good SNRs. In preliminary measurements they obtained stabilities of 1.6 x 10 to the -13th/sq rt tau (tau between 50 and 800 s) with a 160-mHz wide atomic resonance linewidth and a signal-to-noise ratio of 40 for each measurement cycle. Atomic resonance lines as narrow as 30 mHz on the 40.5-GHz clock transition have been measured with no appreciable reduction in the ion signal. A stability of 7 x 10 to the -14th/sq rt tau is made possible by the signal-to-noise and line Q of this measured transition. Analysis of fundamental sources of frequency instability indicates that a long-term stability of 2 x 10 to the -16th is feasible for this device with existing technology for tau = 10 to the 6th s or more.

Prestage, John D.↗
NASA NTRS1990

Custom CMOS Reed Solomon coder for the Hubble Space Telescope

A VLSI coder is presented that can function either as an encoder or decoder for Reed-Solomon codes. VLSI is one approach to implementing high-performance Reed-Solomon decoders. There are three VLSI technologies that could be used: gate arrays, standard cells, and full custom. The first two approaches are relatively easy to implement, but are limited in both performance and density. Full-custom VLSI is used to achieve both circuit density and speed, and allows control of the amount of interconnect. Speed, which is a function of capacitance, which is a function of interconnect, is an important parameter in high-performance VLSI. A single 8.2 mm x 8.4 mm, 200,000 transistor CMOS chip implementation of the Reed-Solomon code required by the Hubble Space Telescope is reported. The chip features a 10-MHz sustained byte rate independent of error pattern. The 1.6-micron CMOS integrated circuit has complete decoder and encoder functions and uses a single data/system clock. Block lengths up to 255 bytes and shortened codes are supported with no external buffering. Erasure corrections and random error corrections are supported with programmable correction of up to 10 symbol errors. Correction time is independent of error pattern and the number of errors in the incoming message.

Whitaker, S.↗
NASA NTRS1990

Physics and astrophysics from a lunar base; Proceedings of the 1st NASA Workshop, Stanford, CA, May 19, 20, 1989

The present conference on physics and astrophysics from a lunar base encompasses space physics, cosmic ray physics, neutrino physics, experiments in gravitation and general relativity, gravitational radiation physics, cosmic background radiation, particle astrophysics, surface physics, and the physics of gamma rays and X-rays. Specific issues addressed include space-plasma physics research at a lunar base, prospects for neutral particle imaging, the atmosphere as particle detector, medium- and high-energy neutrino physics from a lunar base, muons on the moon, a search for relic supernovae antineutrinos, and the use of clocks in satellites orbiting the moon to test general relativity. Also addressed are large X-ray-detector arrays for physics experiments on the moon, and the measurement of proton decay, arcsec-source locations, halo dark matter and elemental abundances above 10 exp 15 eV at a lunar base.

Potter, A. E.↗
NASA NTRS1992

Improved Analog Synchronous Demodulator

Improved analog synchronous demodulator uses input filtering, differential amplification, and synchronized switching in such way output ripple suppressed without need for output filter, and output responds rapidly (within clock cycle) to change in level of demodulated signal.

Sutton, John F.↗
NASA NTRS1992

Control Circuit For Reed-Solomon Encoder

Control circuit designed for use with commercially available AHA4610 Reed-Solomon encoder. Needed to select depth of interleaving and to synchronize input and output blocks of data and parity bits with suitable clock signals. Circuit provides synchronizing and control signals for Reed-Solomon encoder. Encoder can operate with asynchronous input and output data streams at rates up to 80 Mb/s. Interleaving depth selectable, and accommodation to input data rate automatic.

Ross, Douglas↗
NASA NTRS1992

System Collects And Displays Demultiplexed Data

Electronic system collects, manipulates, and displays in real time results of manipulation of streams of data transmitted from remote scientific instrumentation. Interface circuit shifts data-and-clock signal from differential logic levels of multiplexer to single-ended logic levels of computer. System accommodates nonstandard data-transmission protocol. Software useful in applications where Macintosh computers used in real-time display and recording of data.

Reschke, Millard F.↗
NASA NTRS1992

Preventing Blooming In CCD Images

Clocking scheme for charge-coupled-device (CCD) imaging photodetector prevents smearing of bright spots and eliminates residual images. Also imposes charge-collecting electric field of optimum-full-well configuration, minimizes nonuniformities among picture elements, and keeps dark current low. Works under almost any lighting conditions.

Janesick, James↗
NASA NTRS1992

Digital Extraction Of Doppler Shift In An Advanced Receiver

Report describes demonstration of ability of Advanced Receiver II (ARX II) to extract Doppler shifts in radio telemetry signals from spacecraft by digital processing. Frequency of sampling clock fixed and derived from frequency-and-timing subsystem. Although this can result in noninteger number of samples per symbol, it enables digital extraction of Doppler shift. ARX II operates with older receiver called "Block IV". ARX II performed at least as well as, and in most cases better than, Block IV.

Hinedi, Sami M.↗
NASA NTRS1992

Open-Loop Acquisition Of Suppressed Carrier Signals

Three digital techniques for open-loop acquisition of suppressed carrier signals in presence of binary-phase-shift-keyed (BPSK) modulation representing non-return-to-zero (NRZ) data symbols. Based on assumption that offset, tau, between timing of symbols and clock in receiver unknown, but fixed fraction of symbol period, T. In all three, fast Fourier transforms (FFT's) performed on outputs of integrate-and-dump filters that are parts of Costas-type loops used to demodulate BPSK signals. Timing of integrate-and-dump filters constant. Outputs mixed to obtain an error signal.

Shah, Biren N.↗
NASA NTRS1992

Four-Sector Cylindrical Radio-Frequency Ion Trap

Proposed linear radio-frequency ion trap consists of closed metal cylinder partitioned into four equal cylindrical-sector electrodes and two circular end electrodes. Features include relatively large ion-storage capacity and shielding against external fields. Used in frequency-standard laboratories to confine 199Hg+ ions electrodynamically in isolation from external environment. Similar to device described in "Linear Ion Trap for Atomic Clock" (NPO-17758).

Melbourne, Ruthann K.↗
NASA NTRS1992

Numbers Of Degrees Of Freedom Of Allan-Variance Estimators

Report discusses formulas for estimation of Allan variances. Presents algorithms for closed-form approximations of numbers of degrees of freedom characterizing results obtained when various estimators applied to five power-law components of classical mathematical model of clock noise.

Greenhall, Charles A.↗