Search NASA⌕ Search

SEARCH · Search NASA

Results for “bandwidths”

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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Fokker-Planck electron diffusion caused by an obliquely propagating electromagnetic wave packet of narrow bandwidth

The relativistic motion of electrons in an intense electromagnetic wave packet propagating obliquely to a uniform magnetic field is analytically studied on the basis of the Fokker-Planck-Kolmogorov (FPK) approach. The wavepacket consists of circularly polarized electron-cyclotron waves. The dynamical system in question is shown to be reducible to one with three degrees of freedom. Within the framework of the Hamiltonian analysis the nonlinear diffusion tensor is derived, and it is shown that this tensor can be separated into zeroth-, first-, and second-order parts with respect to the relative bandwidth. The zeroth-order part describes diffusive acceleration along lines of constant unperturbed Hamiltonian. The second-order part, which corresponds to the longest time scale, describes diffusion across those lines. A possible transport theory is outlined on the basis of this separation of the time scales.

Hizanidis, Kyriakos↗
NASA NTRS1989

High rate concatenated coding systems using bandwidth efficient trellis inner codes

High-rate concatenated coding systems with bandwidth-efficient trellis inner codes and Reed-Solomon (RS) outer codes are investigated for application in high-speed satellite communication systems. Two concatenated coding schemes are proposed. In one the inner code is decoded with soft-decision Viterbi decoding, and the outer RS code performs error-correction-only decoding (decoding without side information). In the other, the inner code is decoded with a modified Viterbi algorithm, which produces reliability information along with the decoded output. In this algorithm, path metrics are used to estimate the entire information sequence, whereas branch metrics are used to provide reliability information on the decoded sequence. This information is used to erase unreliable bits in the decoded output. An errors-and-erasures RS decoder is then used for the outer code. The two schemes have been proposed for high-speed data communication on NASA satellite channels. The rates considered are at least double those used in current NASA systems, and the results indicate that high system reliability can still be achieved.

Deng, Robert H.↗
NASA NTRS1989

Bandwidth-limited robust nonlinear sliding control of pointing and tracking maneuvers

It is shown how multiaxial spacecraft tracking and pointing maneuvers with known control bandwidth and given tracking error bounds can be implemented by variable-structure control in the presence of uncertain vehicle and target dynamics. It is shown how to select a nonlinear sliding surface relating attitude and rate variables, as well as a Lyapunov function in the surface variables that absorbs multiplicative model undertainties, thereby simplifying the computation of control corrections. It is then shown how a boundary layer envelope can be designed, within which the components of the surface error dynamics can be modeled as the outputs of designer-selected decoupled low-pass filters. Closed-loop stability conditions accounting for the coupling between the attitude error dynamics and the surface error dynamics are then obtained.

Dwyer, Thomas A. W., III↗
NASA NTRS1989

Broadening the interface bandwidth in simulation based training

Currently most computer based simulations rely exclusively on computer generated graphics to create the simulation. When training is involved, the method almost exclusively used to display information to the learner is text displayed on the cathode ray tube. MICROEXPERT Systems is concentrating on broadening the communications bandwidth between the computer and user by employing a novel approach to video image storage combined with sound and voice output. An expert system is used to combine and control the presentation of analog video, sound, and voice output with computer based graphics and text. Researchers are currently involved in the development of several graphics based user interfaces for NASA, the U.S. Army, and the U.S. Navy. Here, the focus is on the human factors considerations, software modules, and hardware components being used to develop these interfaces.

Somers, Larry E.↗
NASA NTRS1991

High-contrast, large optical bandwidth field-induced guide/antiguide modulator

An electric field induced-guide/antiguide optical intensity modulator which has a very wide optical bandwidth from 1 to 1.55/micron which is the largest ever reported before for an intensity modulator. A TE mode ON/OFF ratio larger than 21 dB at 1.15 micron, and a propagation loss about 1 dB at 1.3 micron has been measured. The electrooptic effects, along with carrier effects have been exploited to increase the refractive index under the guide and adjacent antiguide electrodes by applying reverse biases to them.

Huang, T. C.↗
NASA NTRS1992

Bandwidth-Synthesizing FM Radar

Proposed frequency-modulation (FM) radar system uses sawtooth-wave-form linear frequency sweeps and return-signal-processing scheme in which samples of signals from N consecutive sweep periods are combined to multiply range resolution by factor of N. System "synthesizes" bandwith N times that of transmitted signal in sense that to increase resolution of ordinary radar system where consecutive samples are not combined, it is necessary to increase bandwidth of transmitted signal by same factor. Used in FM radar altimeters.

Wu, Chialin↗
NASA NTRS1992

Investigation of Bandwidth-Efficient Coding and Modulation Techniques

The necessary technology was studied to improve the bandwidth efficiency of the space-to-ground communications network using the current capabilities of that network as a baseline. The study was aimed at making space payloads, for example the Hubble Space Telescope, more capable without the need to completely redesign the link. Particular emphasis was placed on the following concepts: (1) what the requirements are which are necessary to convert an existing standard 4-ary phase shift keying communications link to one that can support, as a minimum, 8-ary phase shift keying with error corrections applied; and (2) to determine the feasibility of using the existing equipment configurations with additional signal processing equipment to realize the higher order modulation and coding schemes.

Osborne, William P.↗
NASA NTRS1992

Bandwidth efficient CCSDS coding standard proposals

The basic concatenated coding system for the space telemetry channel consists of a Reed-Solomon (RS) outer code, a symbol interleaver/deinterleaver, and a bandwidth efficient trellis inner code. A block diagram of this configuration is shown. The system may operate with or without the outer code and interleaver. In this recommendation, the outer code remains the (255,223) RS code over GF(2 exp 8) with an error correcting capability of t = 16 eight bit symbols. This code's excellent performance and the existence of fast, cost effective, decoders justify its continued use. The purpose of the interleaver/deinterleaver is to distribute burst errors out of the inner decoder over multiple codewords of the outer code. This utilizes the error correcting capability of the outer code more efficiently and reduces the probability of an RS decoder failure. Since the space telemetry channel is not considered bursty, the required interleaving depth is primarily a function of the inner decoding method. A diagram of an interleaver with depth 4 that is compatible with the (255,223) RS code is shown. Specific interleaver requirements are discussed after the inner code recommendations.

Costello, Daniel J., Jr.↗
NASA NTRS1992

Bandwidth compression of the digitized HDTV images for transmission via satellites

This paper investigates a subband coding scheme to reduce the transmission bandwidth of the digitized HDTV images. The HDTV signals are decomposed into seven bands. Each band is then independently encoded. The based band is DPCM encoded and the high bands are encoded by using nonuniform Laplacian quantizers with a dead zone. By selecting the dead zone on the basis of energy in the high bands an acceptable image quality is achieved at an average of 45 Mbits/sec (Mbps) rate. This rate is comparable to some very hardware intensive schemes of transform compression or vector quantization proposed in the literature. The subband coding scheme used in this study is considered to be of medium complexity. The 45 Mbps rate is suitable for transmission of HDTV signals via satellites.

Al-Asmari, A. KH.↗
NASA NTRS1991

Comparative study on the performance of power and bandwidth efficient modulations in LMSS under fading and interference

Aspects of error performance of various power and bandwidth efficient modulations for the land mobile satellite systems (LMSS) were investigated under multipath fading and interferences by using Monte-Carlo simulation. A differential detection for 16QAM (quadrature amplitude modulation) was proposed to cope with Ricean fading and Doppler shift. Computer simulation results show that the performance of 16QAM with differential detection is as good as that of 16PSK with coherent detection and 3 dB better than that of 16PSK with differential detection, although it degrades by about 4.5 dB as compared to 16QAM with coherent detection under an additive white Gaussian noise (AWGN) channel. For the nonlinear channels, 16QAM with modified signal constellations is introduced and analyzed. The simulation results show that the modified 16QAM exhibits a gain of 2.5 dB over 16PSK under traveling-wave tube nonlinearity, and about 4 dB gain over 16PSK at the bit error rate of 10 exp -5 under AWGN. Computer simulation results for modified 16 QAM under cochannel interference and adjacent-channel interference are also presented.

Liu, Jian↗
NASA NTRS1992

Bandwidth and SIMDUCE as simulator fidelity criteria

The potential application of two concepts from the new Handling Qualities Specification for Military Rotorcraft was discussed. The first concept is bandwidth, a measure of the dynamic response to control. The second is a qualitative technique developed for assessing the visual cue environment the pilot has in bad weather and at night. Simulated Day Usable Cue Environment (SIMDUCE) applies this concept to assessing the day cuing fidelity in the simulator.

Key, David↗
NASA NTRS1991

Bandwidth requirements for direct detection optical communication receivers with PPM signaling

The performance of a direct detection optical communication receiver for pulse position modulation (PPM) signal formats which contains an avalanche photodiode photodetector is calculated as a function of the parameters which characterize a raised cosine filter used in the receiver. It is shown that the performance penalty of this receiver relative to that of an optimal receiver which contains an integrator and assumes a perfectly rectangular PPM pulse shape is less than 1 dB at a receiver bit error rate of 10 exp -6. The advantage of the raised cosine filter is that the electrical bandwidth required is no more than one-half that of the integrator, that is, 1/tau as opposed to 2/tau where tau is the PPM time slot duration.

Davidson, Frederic M.↗
NASA NTRS1993

Temperature dependence of nu3 and nu4 bandwidths and complex refractive indices for crystalline methane

Infrared spectra of thin films of pure CH4 have been measured for a range of temperatures from 22 to 68 K. The bandwidth for the fundamental nu3 near 3000/cm varies from 12.7 to 33.2/cm, while that for nu4 near 1300/cm varies from 6.2 to 16.0/cm. The real and imaginary parts of the index of refraction also exhibit significant dependence on temperature. These broadband measurements at low temperatures are useful for an understanding of clouds and hazes in the atmosphere of Uranus, clouds and ices in the atmosphere of Titan, and ices on the surfaces of Pluto and Triton.

Ngoh, M. A.↗
NASA NTRS1995

Towards optimum demodulation of bandwidth-limited and low SNR square-wave subcarrier signals

The optimum phase detector is presented for tracking square-wave subcarriers that have been bandwidth limited to a finite number of harmonics. The phase detector is optimum in the sense that the loop signal-to-noise ratio (SNR) is maximized and, hence, the rms phase tracking error is minimized. The optimum phase detector is easy to implement and achieves substantial improvement. Also presented are the optimum weights to combine the signals demodulated from each of the harmonics. The optimum weighting provides SNR improvement of 0.1 to 0.15 dB when the subcarrier loop SNR is low (15 dB) and the number of harmonics is high (8 to 16).

Feria, Y.↗
NASA NTRS1995

Digital modulation schemes for high speed transmission through low bandwidth lowpass analog links

Existing NTSC (National Television System Committee) standard will be phased out and replaced with HDTV (High Definition Television) standard within the next 10 years. Accordingly, the existing video network system operated by NASA will become obsolete and requires either replacement or modification to accommodate digital transmission. Network replacement is extremely expensive, hence, several digital modulation schemes are investigated in this report to accomplish digital transmission over existing analog links saving NASA from the cost of network replacement. There are two competing transmission systems available for HDTV transmission over limited bandwidth channels. The cost and performance of the two competing schemes are remarkably similar. However, the input data rate in such a case is limited to 40 Mbit/s. Transmission of higher data rates is possible using simple signal processing techniques. On the other hand, a third transmission system, multilevel pulse amplitude modulation (M-PAM) is proposed. M-PAM is the first stage of the well known M-VSB. This M-PAM scheme is much simpler and uses the channel more efficiently. The three schemes are compared and preliminary conclusions were made. Despite of several similarities, each modulation scheme has it unique merits. To determine the suitability of each scheme, more investigations and laboratory tests for all schemes are needed.

Andrawis, Alfred S.↗
NASA NTRS1996

A Wide Bandwidth Model for the Electrical Impedance of Magnetic BearingS

Magnetic bearings are often designed using magnetic circuit theory. When these bearings are built, however, effects not included in the usual circuit theory formulation have a significant influence on bearing performance. Two significant sources of error in the circuit theory approach are the neglect of leakage and fringing effects and the neglect of eddy current effects. This work formulates an augmented circuit model in which eddy current and flux leakage and fringing effects are included. Through the use of this model, eddy current power losses and actuator bandwidth can be derived. Electrical impedance predictions from the model are found to be in good agreement with experimental data from a typical magnetic bearing.

Meeker, David C.↗
NASA NTRS1996

Storage and network bandwidth requirements through the year 2000 for the NASA Center for Computational Sciences

The data storage and retrieval demands of space and Earth sciences researchers have made the NASA Center for Computational Sciences (NCCS) Mass Data Storage and Delivery System (MDSDS) one of the world's most active Convex UniTree systems. Science researchers formed the NCCS's Computer Environments and Research Requirements Committee (CERRC) to relate their projected supercomputing and mass storage requirements through the year 2000. Using the CERRC guidelines and observations of current usage, some detailed projections of requirements for MDSDS network bandwidth and mass storage capacity and performance are presented.

Salmon, Ellen↗