Search NASA⌕ Search

SEARCH · Search NASA

Results for “Bandwidth”

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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Estimation of the differential pulse propagation times in two-color laser ranging systems

Two-color laser ranging systems can be used to determine the atmospheric delay by measuring the difference in propagation times between two optical pulses transmitted at two different wavelengths. In this paper, the performance of a cross-correlation technique for estimating the differential propagation time is analyzed by considering both speckle and shot noise. For the flat diffuse targets, the differential timing accuracy is highly dependent on the receiver bandwidth and the characteristics of the time-resolved speckle. At low signal levels, when the receiver bandwidth is chosen properly, the performance of the correlation estimator is comparable with that of the maximum-likelihood estimator. At high signal levels, however, speckle places a fundamental limit on the performance of the correlation estimator. For the cube-corner reflector arrays, timing performance is dominated by partially developed speckle. The differential propagation time cannot by resolved to better than the pulse widths of the received signals.

Im, K. E.↗
NASA NTRS1986

Optimal coherent image amplification by two-wave coupling in photorefractive BaTiO3

Coherent image amplification by two-wave coupling in a crystal of photorefractive BaTiO3 is analyzed. This amplifier is optimized with respect to such operational characteristics as gain versus amplified image quality and space-bandwidth product. Experimental results that demonstrate coherent image amplification of 4000, space-bandwidth product of 1,000,000, and gray-level dynamic range of greater than 100 are presented.

Fainman, Y.↗
NASA NTRS1986

Flight investigation of a multivariable model-following control system for rotorcraft

A high-bandwidth, multivariable, explicit model-following control system for advanced rotorcraft has been developed and evaluated on the NASA Ames CH-47B fly-by-wire helicopter. This control system has expanded the in-flight simulation capabilities of the CH-47B to support research efforts directed at the next generation of superaugmented helicopters. A detailed, analytical model of the augmented CH-47B has also been developed to support the flight tests. Analysis using this theoretical model was used to expose fundamental limitations caused by the basic vehicle characteristics and original control system implementation that had affected the performance of the model-following control system. Improvements were made to the nominal control system design to compensate for large time delays created by the higher-orderd dynamics of the aircraft and its control system. With these improvements, high bandwidth control and excellent model-following performance were achieved. Both analytical and flight-test results for the lateral axis are presented and compared. In addition, frequency-domain techniques are employed for documenting the system performance.

Hilbert, K. B.↗
NASA NTRS1985

The Serendip II design

Serendip II is an automated system designed to perform a real time search for narrow band radio signals in the spectra of sources in a regularly scheduled, non-Seti, astronomical observing program. Because Serendip II is expected to run continuously without requiring dedicated observing time, it is hoped that a large portion of the sky will be surveyed at high sensitivity and low cost. Serendip II will compute the power spectrum using a 65,536 channel fast Fourier transform processor with a real time bandwidth of 128 KHz and 2 Hz per channel resolution. After searching for peaks in a 100 KHz portion of the radio telescope's IF band, Serendip II will move to the next 100 KHz portion using a programmable frequency synthesizer; when the whole IF band has been scanned, the process will start again. Unidentified peaks in the power spectra are candidates for further study and their celestial coordinates will be recorded along with the time and power, IF and RF frequency, and bandwidth of the peak.

Werthimer, D.↗
NASA NTRS1986

Designing Delta-DOR acquisition strategies to determine highly elliptical earth orbits

Delta-DOR acquisition strategies are designed for use in determining highly elliptical earth orbits. The requirements for a possible flight demonstration are evaluated for the Charged Composition Explorer spacecraft of the Active Magnetospheric Particle Tracer Explorers. The best-performing strategy uses data spanning the view periods of two orthogonal baselines near the same orbit periapse. The rapidly changing viewing geometry yields both angular position and velocity information, but each observation may require a different reference quasar. The Delta-DOR data noise is highly dependent on acquisition geometry, varying several orders of magnitude across the baseline view periods. Strategies are selected to minimize the measurement noise predicted by a theoretical model. Although the CCE transponder is limited by S-band and a small bandwidth, the addition of Delta-DOR to coherent Doppler and range improves the one-sigma apogee position accuracy by more than an order of magnitude. Additional Delta-DOR accuracy improvements possible using dual-frequency (S/X) calibration, increased spanned bandwidth, and water-vapor radiometry are presented for comparison. With these benefits, the residual Delta-DOR data noise is primarily due to quasar position uncertainties.

Frauenholz, R. B.↗
NASA NTRS1986

Attitude control system synthesis for the Hoop/Column antenna using the LQG/LTR method

This paper investigates the application of the linear-quadratic-Gaussian (LQG)/loop transfer recovery (LTR) method to the problem of synthesizing a fine-pointing control system for a large flexible space anenna. The study is based on an antenna, which consists of three rigid-body rotational modes and the first ten elastic modes. A robust compensator design for achieving the required pointing performance in the presence of modeling uncertainties is obtained using the LQG/LTR method. For the Hoop/Column antenna, a satisfactory controller design meeting a desired bandwidth of .1 rad/sec and ensuring stability with unmodelled high frequency modes is obtained using only a collocated pair of 3-axis attitude sensors and torque actuators. This study also indicates that to achieve the desired performance bandwidth of 0.1 rad/sec. and to ensure stability in the presence of higher frequency elastic modes, the design model should include at least the first three flexible modes together with the rigid body modes.

Sundararajan, N.↗
NASA NTRS1987

Gain-Compensating Circuit For NDE and Ultrasonics

High-frequency gain-compensating circuit designed for general use in nondestructive evaluation and ultrasonic measurements. Controls gain of ultrasonic receiver as function of time to aid in measuring attenuation of samples with high losses; for example, human skin and graphite/epoxy composites. Features high signal-to-noise ratio, large signal bandwidth and large dynamic range. Control bandwidth of 5 MHz ensures accuracy of control signal. Currently being used for retrieval of more information from ultrasonic signals sent through composite materials that have high losses, and to measure skin-burn depth in humans.

Kushnick, Peter W.↗
NASA NTRS1987

Trellis-Coded Modulation for Fading Channels

In proposed communication system, digital signals transmitted efficiently over fading channel by combination of trellis coding and multiple-phase-shift keying with addition of asymmetry to signal set. Coding and modulation schemes not designed separately as in previous systems but integrated to yield bandwidth-efficient modulation and forward-error-correction coding. Helps to satisfy need for reliable high-quality transmission of voice and data between land-mobile units via satellites, where limitations of power and bandwidth imposed simultaneously. Also applicable to ionospheric communications between fixed or mobile units.

Simon, Marvin K.↗
NASA NTRS1985

Lightning mapper and the future

A trade-off analysis was completed that reveals how the lightning mapper detection efficiency will change as a function of interference filter bandwidth, pixel field of view, and telescope aperture. It is shown that the critical parameter on which we have minimum flexibility is filter band-width. The problem is that too narrow a filter bandwidth is incompatible with wide areal coverage. The trade-off analysis demonstrates that an 80 percent lightning detection efficiency will technically be relatively straight-forward, while a 90 percent detection efficiency will apparently be difficult to achieve. Three focal plane designs are currently under consideration. One would use a single large, solid state silicon integrating array with multiple output channels and off-the-focal-plane analog, time domain, background removing fibers. A second design would use the same technology, but the sensor would consist of up to four virtually independent focal plane arrays. This design reduces the areal coverage of each detector. Thus narrower interference filters could be utilized. Superior performance would be realized at a probable increase in cost. The final design would use a three-dimensional focal plane in order to perform background removal at the focal plane. Superior performance would be achieved along with reduced weight and power requirements. Unfortunately, this focal plane technology is still under development.

Christian, H. J.↗
NASA NTRS1986

Block 2A traveling-wave maser

Two 8.4GHz low-noise traveling-wave masers (TWMS) with effective input noise temperatures of 3.6 to 3.9 K and bandwidth in excess of 100 MHz have been supplied to the Deep Space Network. These TWMs are used on the 64-meter antennas at Deep Space Stations 14 and 43 to meet the requirements of the Voyager Uranus encounter. The TWMs have improved isolator assemblies and new interstage matching configurations to reduce gain/bandwidth ripple. They are equipped with followup Field Effect Transistor Amplifiers as part of the design to meet the 100-MHz bandwidth requirements of very long baseline interferometry.

Trowbridge, D. L.↗
NASA NTRS1987

Investigation of a GaAlAs Mach-Zehnder electro-optic modulator

A GaAs modulator operating at 0.78 to 0.88 micron wavelength has the potential to be integrated with a GaAs/GaAlAs laser diode for an integrated fiber-optic transmitter. A travelling-wave Mach-Zehnder modulator using the electro-optic effect of GaAs and operating at a wavelength of 0.82 microns has been investigated for the first time. A four layer Strip-loaded ridge optical waveguide has been analyzed using the effective index method and single mode waveguides have been designed. The electro-optic effect of GaAs has also been analyzed and a modulator using the geometry producing the maximum phase shift has been designed. A coplanar transmission line structure is used in an effort to tap the potentially higher bandwidth of travelling-wave electrodes. The modulator bandwidth has been calculated at 11.95 GHz with a required drive power of 2.335 Watts for full intensity modulation. Finally, some preliminary experiments were performed to characterize a fabrication process for the modulator.

Materna, David M.↗
NASA NTRS1986

The 20 GHz spacecraft IMPATT solid state transmitter

The engineering development of a solid-state transmitter amplifier operating in the 20-GHz frequency range is described. This effort involved a multitude of disciplines including IMPATT device development, circulator design, multiple-diode circuit design, and amplifier integration and test. The objective was to develop a transmitter amplifier demonstrating the feasibility of providing an efficient, reliable, lightweight solid-state transmitter to be flown on a 30 to 20 GHz communication demonstration satellite. The work was done under contract from NASA/Lewis Research Center for a period of three years. The result was the development of a GaAs IMPACT diode amplifier capable of an 11-W CW output power and a 2-dB bandwidth of 300 MHz. GaAs IMPATT diodes incorporating diamond heatsink and double-Read doping profile capable of 5.3-W CW oscillator output power and 15.5% efficiency were developed. Up to 19% efficiency was also observed for an output power level of 4.4 W. High performance circulators with a 0.2 dB inserting loss and bandwidth of 5 GHz have also been developed. These represent a significant advance in both device and power combiner circuit technologies in K-band frequencies.

Best, T.↗
NASA NTRS1987

Frequency selective reflection and transmission at a layer composed of a periodic dielectric

The feasibility of using a periodic dielectric layer, composed of alternating bars having dielectric constants epsilon sub 1 and epsilon sub 2, as a frequency selective subreflector in order to permit feed separation in large aperture reflecting antenna systems was examined. For oblique incidence, it is found that total transmission and total reflection can be obtained at different frequencies for proper choices of epsilon sub 1, epsilon 2, and the geometric parameters. The frequencies of total reflection and transmission can be estimated from wave phenomena occurring in a layer of uniform dielectric constant equal to the average for the periodic layers. About some of the frequencies of total transmission, the bandwidth for 90% transmission is found to be 40%. However, the bandwidth for 90% reflection is always found to be much narrower; the greatest value found being 2.5%.

Bertoni, Henry L.↗
NASA NTRS1986

The microwave spectrum of solar millisecond spikes

The microwave radiation from solar flares sometimes shows short and intensive spikes which are superimposed on the burst continuum. In order to determine the upper frequency limit of their occurrence and the circular polarization, a statistical analysis was performed on digital microwave observations from 3.2 to 92.5 GHz. Additionally, fine structures were investigated with a fast 32-channel spectrometer at 3.47 GHz. It was found that about 10 percent of the bursts show fine structures at 3.2 and 5.2 GHz, whereas none occurred above 8.4 GHz. Most of the observed spikes were very short and their bandwidth varied from below 0.5 MHz to more than 200 MHz. Simultaneous observations at two further frequencies showed no coincident spikes at the second and third harmonic. The observations can be explained by the theory of electron cyclotron masering if the observed bandwidths are determined by magnetic field inhomogeneities or if the rise times are independent of the source diameters. The latter would imply source sizes between 50 and 100 km.

Staehli, M.↗
NASA NTRS1987

Error control techniques for satellite and space communications

The performance of bandwidth efficient trellis inner codes using two-dimensional MPSK signal constellations in a NASA concatenated coding is summarized. Work was also continued on trellis coded modulation using multi-dimensional signal sets. Achievable lower bounds on free distance trellis codes were proved and the existence of good trellis coded modulation (TCM) schemes were established for a variety of signal constellations. The performance of TCM schemes on fading channels is being investigated. Preliminary results indicate that bandwidth efficient trellis coding is feasible on such channels, but that the important design parameter is no longer the minimum free Euclidean distance.

Costello, Daniel J., Jr.↗
NASA NTRS1987

Digital control of highly augmented combat rotorcraft

Proposed concepts for the next generation of combat helicopters are to be embodied in a complex, highly maneuverable, multiroled vehicle with avionics systems. Single pilot and nap-of-the-Earth operations require handling qualities which minimize the involvement of the pilot in basic stabilization tasks. To meet these requirements will demand a full authority, high-gain, multimode, multiply-redundant, digital flight-control system. The gap between these requirements and current low-authority, low-bandwidth operational rotorcraft flight-control technology is considerable. This research aims at smoothing the transition between current technology and advanced concept requirements. The state of the art of high-bandwidth digital flight-control systems are reviewed; areas of specific concern for flight-control systems of modern combat are exposed; and the important concepts are illustrated in design and analysis of high-gain, digital systems with a detailed case study involving a current rotorcraft system. Approximate and exact methods are explained and illustrated for treating the important concerns which are unique to digital systems.

Tischler, Mark B.↗
NASA NTRS1987

Programmable electronic synthesized capacitance

A predetermined and variable synthesized capacitance which may be incorporated into the resonant portion of an electronic oscillator for the purpose of tuning the oscillator comprises a programmable operational amplifier circuit. The operational amplifier circuit has its output connected to its inverting input, in a follower configuration, by a network which is low impedance at the operational frequency of the circuit. The output of the operational amplifier is also connected to the noninverting input by a capacitor. The noninverting input appears as a synthesized capacitance which may be varied with a variation in gain-bandwidth product of the operational amplifier circuit. The gain-bandwidth product may, in turn, be varied with a variation in input set current with a digital to analog converter whose output is varied with a command word. The output impedance of the circuit may also be varied by the output set current. This circuit may provide very small ranges in oscillator frequency with relatively large control voltages unaffected by noise.

Kleinberg, Leonard L.↗
NASA NTRS1987

Development of a P-I-N HgCdTe photomixer for laser heterodyne spectrometry

An improved HgCdTe photomixer technology was demonstrated employing a p-i-n photodiode structure. The i-region was near intrinsic n-type HgCdTe; the n-region was formed by B+ ion implantation; and the p-region was formed either by a shallow Au diffusion or by a Pt Schottky barrier. Experimental devices in a back-side illuminated mesa diode configuration were fabricated, tested, and delivered. The best photomixer was packaged in a 24-hour LN2 dewar along with a cooled GaAs FET preamplifier. Testing was performed by mixing black-body radiation with a CO2 laser beam and measuring the IF signal, noise, and signal-to-noise ratio in the GHz frequency range. Signal bandwidth for this photomixer was 1.3 GHz. The heterodyne NEP was 4.4 x 10 to the -20 W/Hz out to 1 GHz increasing to 8.6 x 10 to the -10 W/Hz at 2 GHz. Other photomixers delivered on this program had heterodyne NEPs at 1 GHz ranging from 8 x 10 to the -20 to 4.4 x 10 to the -19 W/Hz and NEP bandwidths from 2 to 4 GHz.

Bratt, Peter R.↗