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Results for “FREQUENCY STANDARDS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

The Space Station Freedom evolution-phase - Crew-EVA demand for robotic substitution by task primitive

Space Station Freedom represents a significant demand for automation and robotics services as substitutes for crew EVAs. Results are reported from a study aimed at identifying the demand for crew EVA and the crew-task primitive distributions derived for input to future robotic substitution studies. Generic EVA tasks are developed from historical EVA mission timelines, and a set of 70 task primitives defined. The generic task activities are partitioned into task setup, kernel, and tear-down, with standardized task times and frequencies. These standardized times are coupled with inputs from numerous mission data bases in a probabilistic simulation to obtain estimates of total crew-EVA task time demand by crew task primitive. The use of probabilistic model is found to be crucial for understanding, isolating, and addressing the large uncertainties in the EVA task kernels.

Smith, Jeffrey H.↗
NASA NTRS2001

Phase modulation for reduced vibration sensitivity in laser-cooled clocks in space

The standard interrogation technique in atomic beam clocks is square-wave frequency modulation (SWFM), which suffers a first order sensitivity to vibrations as changes in the transit time of the atoms translates to perceived frequency errors. Square-wave phase modulation (SWPM) interrogation eliminates sensitivity to this noise.

frequency standard frequency modulation phase modu↗
NASA NTRS1968

Communications elements research

Radio frequency techniques, microwave thermal noise standards, radio frequency breakdown in coaxial transmission lines, and coaxial cavity radiator development

Source record↗
NASA NTRS1968

Sidereal frequency generator Patent

System generating sidereal frequency signals from signals of standard solar frequency without use of mixing operations or feedback loops

Granata, R. L.↗
NASA NTRS1992

Application of magnitude estimation scaling to the assessment of subjective loudness response to simulated sonic booms

A laboratory study was conducted for the following reasons: (1) to investigate the application of magnitude estimation scaling for evaluating the subjective loudness of sonic booms; and (2) to compare the relative merits of magnitude estimation and numerical category scaling for sonic boom loudness evaluation. The study was conducted in the NASA LeRC's sonic boom simulator and used a total of 80 test subjects (48 for magnitude estimation and 32 for numerical category scaling). Results demonstrated that magnitude estimation was a practical and effective method for quantifying subjective loudness of sonic booms. When using magnitude estimation, the subjects made valid and consistent ratio judgments of sonic boom loudness irrespective of the frequency of presentation of the standard stimulus. Presentation of the standard as every fourth stimulus was preferred by the subjects and is recommended as the standard presentation frequency to be used in future tests.

S McDaniel↗
DOE OSTI2021

“Frequency” and the PMU standard

The Phasor Measurement Unit (PMU) has revolutionized power system management by providing high-resolution, time-stamped, spatial, and temporal measure¬ments of the state of power systems. However, the standards defining the measured quantities have ambiguities that cause uncertainty, irreproducibility, and loss of interoperability during periods of change where the measurements should have the greatest value. This paper examines PMU frequency measurement to highlight the problems. It develops some suggestions for the scope and the next evolutionary improvements to the documentary standards. The aim is to improve the meaningfulness and trustworthiness of dynamic power system measurements.

Frequency, rate of change of frequency, definition↗
NASA NTRS1984

Microwave Radiation Detector

Direct photon detector responds to microwave frequencies. Method based on trapped-ion frequency-generation standards proposed to detect radio-frequency (RF) radiation at 40.5 GHz. Technique used for directdetection (RF) communication, radar, and radio astronomy.

Lesh, J. R.↗
NASA NTRS1987

New approaches to some methodological problems of meteor science

Several low cost approaches to continuous radioscatter monitoring of the incoming meteor flux are described. Preliminary experiments were attempted using standard time frequency stations WWVH and CHU (on frequencies near 15 MHz) during nighttime hours. Around-the-clock monitoring using the international standard aeronautical beacon frequency of 75 MHz was also attempted. The techniques are simple and can be managed routinely by amateur astronomers with relatively little technical expertise. Time series analysis can now be performed using relatively inexpensive microcomputers. Several algorithmic approaches to the analysis of meteor rates are discussed. Methods of obtaining optimal filter predictions of future meteor flux are also discussed.

Meisel, David D.↗
NASA NTRS1974

The uses and limitations of HF standard broadcasts for time and frequency comparison

The most practical methods of using high-frequency (HF) broadcasts for frequency and time comparison are reviewed. Although standard broadcast and receiving equipment has improved vastly throughout the past fifty years, the HF propagation medium is no more stable today than it was a half century ago. Doppler shift resulting from changes in the effective height of the ionosphere typically limits the usable accuracy of received high frequencies to a few parts in 10 to the 7th power. At locations beyond groundwave range of the transmitter, uncertainties in path delay generally restrict the usable accuracy of HF time signals to the order of a millisecond. Signal-averaging techniques are sometimes employed to extract frequency or time signals from a noisy background.

Stanley, J. T.↗
NASA NTRS1971

An estimator for the standard deviation of a natural frequency. I.

A brief review of mean-square approximate systems is given. The case in which the masses are deterministic is considered first in the derivation of an estimator for the upper bound of the standard deviation of a natural frequency. Two examples presented include a two-degree-of-freedom system and a case in which the disorder in the springs is perfectly correlated. For purposes of comparison, a Monte Carlo simulation was done on a digital computer.

Schiff, A. J.↗
NASA NTRS2006

High-frequency ECG

The standard ECG is by convention limited to 0.05-150 Hz, but higher frequencies are also present in the ECG signal. With high-resolution technology, it is possible to record and analyze these higher frequencies. The highest amplitudes of the high-frequency components are found within the QRS complex. In past years, the term "high frequency", "high fidelity", and "wideband electrocardiography" have been used by several investigators to refer to the process of recording ECGs with an extended bandwidth of up to 1000 Hz. Several investigators have tried to analyze HF-QRS with the hope that additional features seen in the QRS complex would provide information enhancing the diagnostic value of the ECG. The development of computerized ECG-recording devices that made it possible to record ECG signals with high resolution in both time and amplitude, as well as better possibilities to store and process the signals digitally, offered new methods for analysis. Different techniques to extract the HF-QRS have been described. Several bandwidths and filter types have been applied for the extraction as well as different signal-averaging techniques for noise reduction. There is no standard method for acquiring and quantifying HF-QRS. The physiological mechanisms underlying HF-QRS are still not fully understood. One theory is that HF-QRS are related to the conduction velocity and the fragmentation of the depolarization wave in the myocardium. In a three-dimensional model of the ventricles with a fractal conduction system it was shown that high numbers of splitting branches are associated with HF-QRS. In this experiment, it was also shown that the changes seen in HF-QRS in patients with myocardial ischemia might be due to the slowing of the conduction velocity in the region of ischemia. This mechanism has been tested by Watanabe et al by infusing sodium channel blockers into the left anterior descending artery in dogs. In their study, 60 unipolar ECGs were recorded from the entire ventricular surface and were signal-averaged and filtered in the 30-250 Hz frequency range. The results showed that the decrease noted in the HF-QRS correlated linearly with the local conduction delay. The results suggest that HF-QRS is a potent indicator of disturbed local conduction. An alternative theory is that HF-QRS reflect the shape of the original electrocardiographic signal. Bennhagen et al showed that root mean square (RMS) voltage values of the depolarization signal correlate poorly with the signal amplitude but highly with the first and second derivatives, i.e. the velocity and the acceleration of the signal. It has also been suggested that the autonomic nervous system affects HF-QRS. For example, sitting up causes significant changes in HF-QRS in some leads compared to the supine position [Douglas et al., 2006]. Unpublished results indicate that familial dysautonomic patients (both vagal and sympathetic degeneration) have very little Reduced Amplitude Zones (RAZ) formation . Athletic individuals, especially elite athletes, who have vagally-mediated changes on the conventional ECG (i.e. early repolarization, bradycardia) have increased RAZ formation. Further electrophysiological studies are needed, however, to better understand the underlying mechanisms of HF-QRS. Several investigators have studied HF-QRS in different cardiac conditions, including acute myocardial ischemia and myocardial infarction (MI). However, in order for clinicians to confidently use HF-QRS as an adjunct to standard ECG, more knowledge about the characteristics of HF-QRS is needed.

Tragardh, Elin↗
NASA NTRSDate not supplied

A Compact Dual-Band Bowtie Antenna for RF and ISM bands Operation

Traditionally, bowtie antennas have been known to exhibit wide impedance characteristics, omnidirectional radiation patterns, and linear polarization. There is a broad range of applications from medical imaging, archaeological survey, and Ground Penetrating Radar (GPR) to trackers and sensor networks where wideband bowtie antenna designs are required for their operations. Broadening the bandwidth of bowtie antenna requires widening the flare angle of the bowtie arms, which consequently results in a large surface area that may not be suitable for space-constrained applications. Moreover, drawback attributes of the wideband bowtie designs feature inconsistent radiation pattern across the bandwidth and low signal-to-noise (SNR) ratios. As it is known, the SNR would be improved in dual- or multi-band antennas due to their reduced bandwidth. To this end, dual-band/multi-band antennas are preferred over wideband antennas in applications where more than a single frequency of interest is present. Previously, a two-port double-dipole elements was reported, whose arms were orthogonally interleaved to facilitate operation in both the standard WLAN frequency bands (J. M. Steyn and et. al, Progress in Electromagnetic Research, Vol. – 10 pp. 151-161). Even though the antenna is not very compact it does exhibit good cross-polarization, moderate gain in both frequency bands. Another dual-band bowtie antenna which excites two bands using a single transmission line was reported (Wen Chao Zheng and et. al, IEEE Trans. Antennas propag., 2014). The design was compact and did not need multi-port feeding network. In this paper, a dual-band compact bowtie antenna operating at 900 MHz (RF band) and 2.45 GHz (ISM band) using a single excitation port is introduced. It is printed on a 1.54mm thick dielectric substrate (εr = 3.38). The antenna consists of two sets of bowtie arms, a microstrip transmission line to feed the bowtie arms, and a ground plane acting as a reflector to partially reduce the back radiation. One of the bowtie arms of each frequency is printed on the top layer and the other arm, which is mirror imaged, is printed on the bottom layer of the substrate. The length of the bowtie controls the resonance frequency of the antenna and the flare angle controls the bandwidth of the antenna. The microstrip transmission line, connected to a 50 Ω SMA probe, feeds the bowtie antenna. The compact antenna can be used for both RF and ISM band applications. The bowtie arms at the lower frequency band are miniaturized by elongating their electrical lengths. The influence of miniaturizing the bowtie arms and the supporting partial ground plane is observed in the reduced peak gain and degraded front-to-back ratio. These are partly neutralized using four quarter-wave choke-slots in the ground plane with two on each side of the feeding transmission line. The proposed antenna is numerically investigated and finalized by the finite-element based full-wave EM solver, ANSYS HFSS. The miniaturization has reduced the ground plane size by ~45% and the arms size by ~41%. In addition to the size reduction benefits, the antenna shows reasonable peak gain and front-to-back ratio in both the bands. The corresponding results will be presented and discussed at the conference.

Saininad Naik↗
NASA NTRS2013

Next Generation JPL Ultra-Stable Trapped Ion Atomic Clocks

Over the past decade, trapped ion atomic clock development at the Jet Propulsion Laboratory (JPL) has focused on two directions: 1) new atomic clock technology for space flight applications that require strict adherence to size, weight, and power requirements, and 2) ultra-stable atomic clocks, usually for terrestrial applications emphasizing ultimate performance. In this paper we present a new ultra-stable trapped ion clock designed, built, and tested in the second category. The first new standard, L10, will be delivered to the Naval Research Laboratory for use in characterizing DoD space clocks.

Naval Research Laboratory (NRL)↗
DOE OSTI2020

A variable X-ray chopper system for phase-sensitive detection in synchrotron X-ray scanning tunneling microscopy

An ultra-high-vacuum compatible X-ray chopper system has been designed, constructed and integrated into the XTIP beamline at the Advanced Photon Source at Argonne National Laboratory. The XTIP beamline can operate at soft X-ray energies from 400 eV to 1900 eV while providing a focused beam down to about 10 µm × 10 µm into the synchrotron X-ray scanning tunneling microscopy (SX-STM) endstation instrument. The X-ray chopper is a critical component for separating topographic information from chemical information in SX-STM through phase-sensitive current detection. Depending on the experimental needs, the modulation frequency can be controlled from 100 Hz to 10 kHz. In addition, the chopper system is fully bakeable and can achieve a base pressure of 10 -10 mbar. Facilities for active water cooling have been designed, but passive cooling through copper braids has been shown to be sufficient at standard chopping frequencies. Using an Fe/Al 2 O 3 /CoAl(111) sample, the separation of the SX-STM current into a chemical component and a stable feedback signal is demonstrated.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Misclassification in Workers’ Telecommuting Frequency Choices Using a Generalized Extreme Value Model

Telecommuting frequency is a response variable collected in travel surveys and is, therefore, prone to errors leading to mismeasurements or misclassification. Misclassification of explanatory variables is a common risk when using statistical modeling techniques. We define “misclassification” as a response reported or recorded in the wrong category; for example, a variable is recorded as a 1 when it should be 0. Here, in this context, this study aims to develop a statistical model to analyze telecommuting data which accounts for potential misclassification errors by building on existing literature in econometrics. The empirical analysis was undertaken using the 2017 National Household Travel Survey (NHTS) and the general extreme value (GEV) models available in the literature. Specifically, the frequency of telecommuting days was analyzed using the negative binomial (NB) model recast as the multinomial logit (MNL) model. By nature—and consistent with other studies—NHTS data are prone to errors that can be classified as intentional or unintentional misinformation provided by the person being interviewed. Ignoring these errors while modeling telecommuting frequencies using standard discrete count models can result in biased parameter estimates. The misclassification parameter was calculated for both over-reporting and under-reporting scenarios. The misclassification errors can be as high as 14% over-reported and 10% under-reported, particularly for the neighboring values. Statistical fit comparison between the models shows that models that ignore misclassification have worse data fit and biased parameter estimates with significant policy implications.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗