Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “frequency selection”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Two systems analyses of SETI

The problem of receiving and identifying a single microwave signal transmitted by extraterrestrial intelligent beings is analyzed in the cases where the signal is designed to catch our attention and the signal is designed for internal purposes of another civilization. Six variables which yield uncertainty as to the exact signal which should be searched for are described: polarization, modulation, flux level, direction, frequency (including bandwidth and drift rate), and time. It is shown that if all reasonable variations of these parameters are to be examined sequentially for 1000 seconds, the search would take over a million times longer than the age of the Universe. Ways to simplify the search are considered, including widening the frequency bin, selecting specific targets, cutting the observation time, using a Fourier transform device for data processing, and building larger antennas as well as better low-noise receivers.

Machol, R. E.↗
NASA NTRS1979

Description of the ISEE satellite-to-satellite kilometric wavelength interferometer system

The International Sun-Earth Explorer (ISEE) spacecraft 1 and 2 carry receivers for detecting electromagnetic waves with kilometric wavelengths. For selected receiver frequencies from 30 kHz to 2 MHz, a 10-kHz bandwidth channel is single-sideband mixed down to baseband. These analog data and a reference frequency, which is a submultiple of the local oscillator frequency, are transmitted to ground stations and tape recorded along with the precise time and frequency information. Cross correlation of these tape recorded signals constitutes a satellite-to-satellite interferometer with a fringe spacing of 0.4 to 41 arcmin (at a range of 10,000 to 100 km spacing between spacecraft for 250 kHz in frequency) and with a time-delay resolution of 32 microsec for a 10-kHz bandwidth, which gives an angular resolution of 3 min to 6 deg of arc. For kilometric radiation at earth, sources in the size range of 25 to 2500 km can be identified (from 20 earth radii) and located in relative position ranging from 0.02 to 2 earth radii, depending on spacecraft spacing. Reception and analysis of solar and Jovian bursts may also be possible.

Shawhan, S. D.↗
NASA NTRS1981

Development of a synthetic aperture radar design approach for wide-swath implementation

The first phase of a study program to develop an advanced synthetic aperture radar design concept is presented. Attributes of particular importance for the system design include wide swath coverage, reduced power requirements, and versatility in the selection of frequency, polarization and incident angle. The multiple beam configuration provides imaging at a nearly constant angle of incidence and offers the potential of realizing a wide range of the attributes desired for an orbital imaging radar for Earth resources applications.

Jean, B. R.↗
NASA NTRS1980

An improved output feedback control of flexible large space structures

A special output feedback control design technique for flexible large space structures is proposed. It is shown that the technique will increase both the damping and frequency of selected modes for more effective control. It is also able to effect integrated control of elastic and rigid-body modes and, in particular, closed-loop system stability and robustness to modal truncation and parameter variation. The technique is seen as marking an improvement over previous work concerning large space structures output feedback control.

Lin, Y. H.↗
NASA NTRS1982

Remote sensing of temperature profiles in the atmosphere

Methods of regularized and generalized cross validation can be used to estimate the atmosphere's temperature, moisture, and wind structure from a finite number m noisy measurements by meteorological satellites on the intensity of upwelling radiation in selected channel frequencies. The inversion of the equation of radiative transfer is discussed for data obtained by TIROS N satellite.

Osullivan, F.↗
NASA NTRS1983

Effects of specimen resonances on acoustic-ultrasonic testing

The effects of specimen resonances on acoustic ultrasonic (AU) nondestructive testing were investigated. Selected resonant frequencies and the corresponding normal mode nodal patterns of the aluminum block are measured up to 75.64 kHz. Prominent peaks in the pencil lead fracture and sphere impact spectra from the two transducer locations corresponded exactly to resonant frequencies of the block. It is established that the resonant frequencies of the block dominated the spectral content of the output signal. The spectral content of the output signals is further influenced by the transducer location relative to the resonant frequency nodal lines. Implications of the results are discussed in relation to AU parameters and measurements.

Williams, J. H., Jr.↗
NASA NTRS1982

Propagation of wide bandwidth signals through strongly turbulent ionized media

Analytic and numerical techniques are presented which directly address the problem of propagation of wide bandwidth signals through random ionized media. This work is applicable to the problems of satellite communication and space based radar observation through a disturbed ionospheric propagation channel that would result from a high altitude chemical release or nuclear detonation. An analytic solution is presented for the two-position, two-frequency mutual coherence function for spherical wave propagation in the strong scattering limit. This solution is used to derive simple expressions for the power impulse response function and to determine its relationship to the important parameters of decorrelation distance and coherence bandwidth which describe the disturbed propagation channel. Results for mean time delay and time delay jitter are presented and compared to direct simulation results and to other theoretical calculations. A numerical/analytical solution to the parabolic wave equation is presented in the form of a multiple phase-screen (MPS) propagation simulation. In this very general solution technique, the random medium is divided into a finite number of layers. The field fluctuation through each layer is obtained by replacing the layer by a centrally located thin phase-screen, whose statistical properties are determined from the statistics of the electron-density irregularities. The waveform then propagates from phase-screen to phase-screen via an exact solution to the Fresnel-Kirchnoff equation. For wide bandwidth waveforms, numerical solutions are obtained at a number of discrete frequencies centered about the carrier and then time-domain solutions are obtained by Fourier transform techniques. Detailed results are presented for a number of interesting cases including propagation of a 20 MHz bandwidth signal through a finite barium cloud at a carrier frequency of 100 MHz. One of the major uses of the MPS propagation simulation has been to provide realizations of the received signal after propagation through a disturbed channel. The MPS simulation obtains a general solution of the parabolic wave equation under both weak and strong scattering conditions. A second technique to directly obtain realizations of wide bandwidth waveforms is presented here. This technique is limited to the case of strong scattering but requires only a fraction of the computer resources needed for MPS signal generation. Detailed comparisons of the two signal generation techniques are presented.

Random Media↗
NASA NTRS1986

Comparison of medium frequency pulsed radar interferometer and correlation analysis winds, part 2

In order to test whether the chosen Doppler peaks represent localized scatters in motion, as opposed to some sort of integrated composite, an attempt was made to determine the change in position of single scatterers over a series of sequential records. A four-antenna system was employed which had 1 degree of freedom in phase. Due to limitations N-S linear transmission and E-W linear reception were used. The Doppler frequency peak selection criteria were that at least two of the four power spectra should have a local peak, and that normalized phase discrepancy, should be less than 0.3. The lack of success in tracking individual scatters seems to suggest a short lifetime. If this is the case, then the present experiment is not able to resolve the difference found between the correlation analysis true velocity and the interferometer value. On the other hand, it appears that the interferometer may be of some use in tracking waves.

Meek, C. E.↗
NASA NTRS1989

Design and simulated performance of a CARS spectrometer for dynamic temperature measurements using electronic heterodyning

A new design for generating CARS signals and for the detection and processing of these signals is presented and evaluated. The design is based on electronic heterodyning of the CARS spectrum of nitrogen at two selected narrowband frequencies, ratioing the resulting signal strengths, and comparing this ratio with a theoretically derived temperature scale. A reference cell is incorporated into the design for system calibration and for accurate temperature measurements. The spectrometer is found capable of measuring temperature in the submillisecond time scale with an accuracy of 10 percent in the 1000-2000 K temperature range. A typical result using the Hg(x)Cd(1-x)Te photomixer for T = 1500 K, Delta T = 50 K is a SNR of 21 dB and a data collection rate of 300 Hz.

Deen, M. Jamal↗
NASA NTRS1989

Pulsar-aided SETI experimental observations

The rotational frequencies of pulsars are used to select preferred radio frequencies for SETI. Pulsar rotational frequencies are converted into SETI frequencies in the 1-10 GHz Galactic radio window. Experimental observations using the frequencies are conducted for target stars closer than 25 parsecs, unknown targets in a globular cluster, and unknown targets in the Galaxy closer than 2.5 kpc. The status of these observations is discussed.

Heidmann, J.↗
NASA NTRS1989

Discovery and follow up of asteroids

After a summary of the increasing activity in steroid discovery during the past few years, the importance of carefully thought out observing strategy is discussed, in particular with regard to target selection, observing frequency, and the time distribution of observations. Problems of cataloging and orbit linkage are outlined, inasmuch as they affect individual observers and orbit computers, as well as the work of the Minor Planet Center. There is some discussion of appropriate two-way communication between observers and the Minor Planet Center.

Bowell, E.↗
NASA NTRS1993

Practical input optimization for aircraft parameter estimation experiments

The object of this research was to develop an algorithm for the design of practical, optimal flight test inputs for aircraft parameter estimation experiments. A general, single pass technique was developed which allows global optimization of the flight test input design for parameter estimation using the principles of dynamic programming with the input forms limited to square waves only. Provision was made for practical constraints on the input, including amplitude constraints, control system dynamics, and selected input frequency range exclusions. In addition, the input design was accomplished while imposing output amplitude constraints required by model validity and considerations of safety during the flight test. The algorithm has multiple input design capability, with optional inclusion of a constraint that only one control move at a time, so that a human pilot can implement the inputs. It is shown that the technique can be used to design experiments for estimation of open loop model parameters from closed loop flight test data. The report includes a new formulation of the optimal input design problem, a description of a new approach to the solution, and a summary of the characteristics of the algorithm, followed by three example applications of the new technique which demonstrate the quality and expanded capabilities of the input designs produced by the new technique. In all cases, the new input design approach showed significant improvement over previous input design methods in terms of achievable parameter accuracies.

Morelli, Eugene A.↗
NASA NTRS1993

Miniature modular microwave end-to-end receiver

An end-to-end microwave receiver system contained in a single miniature hybrid package mounted on a single heatsink is presented. It includes an input end connected to a microwave receiver antenna and an output end which produces a digital count proportional to the amplitude of a signal of a selected microwave frequency band received at the antenna and corresponding to one of the water vapor absorption lines near frequencies of 20 GHz or 30 GHz. The hybrid package is on the order of several centimeters in length and a few centimeters in height and width. The package includes an L-shaped carrier having a base surface, a vertical wall extending up from the base surface and forming a corner therewith, and connection pins extending through the vertical wall. Modular blocks rest on the base surface against the vertical wall and support microwave monolithic integrated circuits on top surfaces thereof connected to the external connection pins. The modular blocks lie end-to-end on the base surface so as to be modularly removable by sliding along the base surface beneath the external connection pins away from the vertical wall.

Sukamto, Lin M.↗
NASA NTRS1994

A comparative study of clock rate and drift estimation

Five different methods of drift determination and four different methods of rate determination were compared using months of hourly phase and frequency data from a sample of cesium clocks and active hydrogen masers. Linear least squares on frequency is selected as the optimal method of determining both drift and rate, more on the basis of parameter parsimony and confidence measures than on random and systematic errors.

Breakiron, Lee A.↗
NASA NTRS1994

A study of electric transmission lines for use on the lunar surface

The sources for electrical power on a lunar base are said to include solar/chemical, nuclear (static conversion), and nuclear (dynamic conversion). The transmission of power via transmission lines is more practical than power beaming or superconducting because of its low cost and reliable, proven technology. Transmission lines must have minimum mass, maximum efficiency, and the ability to operate reliably in the lunar environment. The transmission line design includes conductor material, insulator material, conductor geometry, conductor configuration, line location, waveform, phase selection, and frequency. This presentation oulines the design. Liquid and gaseous dielectrics are undesirable for long term use in the lunar vacuum due to a high probability of loss. Thus, insulation for high voltage transmission line will most likely be solid dielectric or vacuum insulation.

Gaustad, Krista L.↗
NASA NTRS2005

Design and Performance Evaluation of a UWB Communication and Tracking System for Mini-AERCam

NASA Johnson Space Center (JSC) is developing a low-volume, low-mass, robotic free-flying camera known as Mini-AERCam (Autonomous Extra-vehicular Robotic Camera) to assist the International Space Station (ISS) operations. Mini-AERCam is designed to provide astronauts and ground control real-time video for camera views of ISS. The system will assist ISS crewmembers and ground personnel to monitor ongoing operations and perform visual inspections of exterior ISS components without requiring extravehicular activity (EAV). Mini-AERCam consists of a great number of subsystems. Many institutions and companies have been involved in the R&D for this project. A Mini-AERCam ground control system has been studied at Texas A&M University [3]. The path planning and control algorithms that direct the motions of Mini-AERCam have been developed through the joint effort of Carnegie Mellon University and the Texas Robotics and Automation Center [5]. NASA JSC has designed a layered control architecture that integrates all functions of Mini-AERCam [8]. The research described in this report is part of a larger effort focused on the communication and tracking subsystem that is designed to perform three major tasks: 1. To transmit commands from ISS to Mini-AERCam for control of robotic camera motions (downlink); 2. To transmit real-time video from Mini-AERCam to ISS for inspections (uplink); 3. To track the position of Mini-AERCam for precise motion control. The ISS propagation environment is unique due to the nature of the ISS structure and multiple RF interference sources [9]. The ISS is composed of various truss segments, solar panels, thermal radiator panels, and modules for laboratories and crew accommodations. A tracking system supplemental to GPS is desirable both to improve accuracy and to eliminate the structural blockage due to the close proximity of the ISS which could at times limit the number of GPS satellites accessible to the Mini-AERCam. Ideally, the tracking system will be a passive component of the communication system which will need to operate in a time-varying multipath environment created as the robot camera moves over the ISS structure. In addition, due to many interference sources located on the ISS, SSO, LEO satellites and ground-based transmitters, selecting a frequency for the ISS and Mini-AERCam link which will coexist with all interferers poses a major design challenge. To meet all of these challenges, ultrawideband (UWB) radio technology is being studied for use in the Mini-AERCam communication and tracking subsystem. The research described in this report is focused on design and evaluation of passive tracking system algorithms based on UWB radio transmissions from mini-AERCam.

Barton, Richard J.↗
NASA NTRS2012

Alternating Magnetic Field Forces for Satellite Formation Flying

Selected future space missions, such as large aperture telescopes and multi-component interferometers, will require the precise positioning of a number of isolated satellites, yet many of the suggested approaches for providing satellites positioning forces have serious limitations. In this paper we propose a new approach, capable of providing both position and orientation forces, that resolves or alleviates many of these problems. We show that by using alternating fields and currents that finely-controlled forces can be induced on the satellites, which can be individually selected through frequency allocation. We also show, through analysis and experiment, that near field operation is feasible and can provide sufficient force and the necessary degrees of freedom to accurately position and orient small satellites relative to one another. In particular, the case of a telescope with a large number of free mirrors is developed to provide an example of the concept. We. also discuss the far field extension of this concept.

Youngquist, Robert C.↗