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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

High-power diode lasers for optical communications applications

High-power, single-mode, double-heterojunction AlGaAs diode lasers are being developed to meet source requirements for both fiber optic local area network and free space communications systems. An individual device, based on the channeled-substrate-planar (CSP) structure, has yielded single spatial and longitudinal mode outputs of up to 90 mW CW, and has maintained a single spatial mode to 150 mW CW. Phase-locked arrays of closely spaced index-guided lasers have been designed and fabricated with the aim of multiplying the outputs of the individual devices to even higher power levels in a stable, single-lobe, anastigmatic beam. The optical modes of the lasers in such arrays can couple together in such a way that they appear to be emanating from a single source, and can therefore be efficiently coupled into optical communications systems. This paper will review the state of high-power laser technology and discuss the communication system implications of these devices.

Carlin, D. B.↗
NASA NTRS1986

Boundary layer control by acoustic excitation

An experimental program in which the effectiveness of a single large eddy break-up device (LEBU) blade is enhanced by proper acoustic excitation is described. An acoustic pulse, phase-locked to the incident large eddies and directed at the LEBU can enhance the large eddy cancellation process and can in effect lead to a decrease in the skin friction coefficient. Cross-correlation data and turbulence intensity measurements show that this acoustic excitation causes eddy cancellation at the trailing edge of the manipulator plate. It is concluded that both reduced turbulent boundary layer mixing and significant drag reduction can be achieved upon proper acoustic input to a LEBU.

Papathanasiou, A. G.↗
NASA NTRS1986

Coherent structures in a turbulent boundary layer. I - Generation of 'artificial' bursts

In an effort to better understand the physics and structure of coherent events in a turbulent boundary layer, an attempt is made to produce 'artificial' bursts. These are generated in a turbulent boundary layer developed on a flat plate towed in an 18 m water channel and thus with negligible freestream turbulence. The burstlike events are produced by either withdrawing near-wall fluid from two minute holes separated in the spanwise direction or by pitching a miniature delta wing that is flush-mounted to the wall. Either of these two actions generates a hairpinlike vortex and low-speed streak that resemble naturally occurring structures. The resulting sequence of events that occur at a given location can be uniquely controlled, thus allowing detailed examination via phase-locked measurements and flow visualization. In this paper, the proof of concept of artificial burst generation is established by comparing the artificial events with natural, random bursts using flow visualization and hot-film signals.

Gad-El-hak, M.↗
NASA NTRS1986

Observations on the structure of an edge-tone flowfield

The unsteady flowfield in the region in which a rectangular jet interacts with the leading edge of a 20-deg wedge is investigated experimentally. Edge-tone flowfields at Mach 0.8 in the blowdown facility at Stanford University are recorded by phase-locked Schlieren photography using the apparatus and techniques described by Krothapalli and Horne (1984); detailed surface-pressure and velocity measurements are obtained at Mach 0.23 in the NASA Ames anechoic-chamber facility and presented graphically. It is shown that the jet-edge interaction produces a secondary vortex at the leading edge when an established shear layer or primary vortex is present, and that the primary and secondary vortices then evolve from a counterrotating vortex pair to a large single vortex which migrates along the wedge surface (in agreement with the observations of Rockwell et al., 1986). From a comparison with data on airfoil-trailing-edge flows (Satyanarayana, 1977) it is inferred that an instantaneous zero pressure differential is present at the leading edge of the wedge.

Krothapalli, A.↗
NASA NTRS1986

Phase lock acquisition for sampled data PLLs using the sweep technique

Simulation results of the swept-acquisition performance of residual carrier phase-locked loops (PLLs) are reported. The loops investigated are sampled data counterparts of the continuous time type II and III loops currently in use in Deep Space Network receivers. It was found that sweep rates of 0.2 B(sub L)(2) to 0.4 B(sub L)(2) Hz/s can be used, depending on the loop parameters and loop signal-to-noise ratio (SNR), where B(sub L) is the one-sided loop noise bandwidth. Type III loops are shown to be not as reliable as type II loops for acquisition using this technique, especially at low SNRs.

Aguirre, S.↗
NASA NTRS1986

A comparison of methods for DPLL loop filter design

Four design methodologies for loop filters for a class of digital phase-locked loops (DPLLs) are presented. The first design maps an optimum analog filter into the digital domain; the second approach designs a filter that minimizes in discrete time weighted combination of the variance of the phase error due to noise and the sum square of the deterministic phase error component; the third method uses Kalman filter estimation theory to design a filter composed of a least squares fading memory estimator and a predictor. The last design relies on classical theory, including rules for the design of compensators. Linear analysis is used throughout the article to compare different designs, and includes stability, steady state performance and transient behavior of the loops. Design methodology is not critical when the loop update rate can be made high relative to loop bandwidth, as the performance approaches that of continuous time. For low update rates, however, the miminization method is significantly superior to the other methods.

Aguirre, S.↗
NASA NTRS1987

Fixed lag smoothers for carrier phase and frequency tracking

The application of fixed lag smoothing algorithms are presented for the problem of estimation of the phase and frequency of a sinusoidal carrier received in the presence of process noise and additive observation noise. A suboptimal structure consists of a phase-locked loop (PLL) followed by a post loop correction to the phase and frequency estimates. When the PLL is operating under a high signal-to-noise ratio, the phase detector is approximately linear, and the smoother equations then correspond to the optimal linear equations for an equivalent linear signal model. The performance of such a smoother can be predicted by the linear filtering theory. However, if the PLL is operating near the threshold region of the signal to noise ratio, the phase detector cannot be assumed to be linear. Then the actual performance of the smoother can significantly differ from that predicted by linear filtering theory. Both the theoretical and simulated performance of such smoothers derived on the basis of various models for the phase of frequency processes are presented.

Kumar, R.↗
NASA NTRS1986

Aspects of the 40-50 day oscillation during the northern summer as inferred from outgoing longwave radiation

The intraseasonal variability (ISV) of tropical convection during the northern summer and its connection with the monsoon over India and east Asia is studied from a planetary-scale perspective. A 40-50 day oscillation consisting primarily of an east-west oriented dipole with centers located over the Indian Ocean/Subcontinent and the western Pacific is shown to be an important component of the ISV of tropical convection during summer. Over these regions, the oscillation undergoes complex structural changes which are manifested in simultaneous cloudiness propagation eastward along the equator from the eastern Indian Ocean to the western Pacific, northward from the Indian Ocean into the Subcontinent, and northwestward from the equatorial western Pacific to the South China Sea. The oscillation is also found to be phase-locked to the monsoon onset of India, the Mei-yu regime over South China, and, in general, the seasonal variation of convection over the equatorial Indian-West Pacific Ocean.

Lau, K.-M.↗
NASA NTRS1986

Performance of PLL synchronized optical PPM communication systems

Receiver timing synchronization of an optical PPM communication system can be achieved using a phase-locked loop (PLL) if the photodetector output is properly processed. The synchronization performance is shown to improve with increasing signal power and decreasing loop bandwidth. The bit error rate performance of the PLL synchronized PPM system is analyzed and compared to that of the perfectly synchronized system. It is shown that the increase in signal power needed to compensate for the imperfect sychronization is small (less than 0.1 dB) for loop bandwidths less than 0.1 percent of the slot frequency.

Chen, C.-C.↗
NASA NTRS1986

Laser Doppler measurement techniques for spacecraft

Two techniques are proposed for using laser links to measure the relative radial velocity of two spacecraft. The first technique determines the relative radial velocity from a measurement of the two-way Doppler shift on a transponded radio-frequency subcarrier. The subcarrier intensity-modulates reciprocating laser beams. The second technique determines the relative radial velocity from a measurement of the two-way Doppler shift on an optical frequency carrier which is transponded between spacecraft using optical Costas loops. The first technique might be used in conjunction with noncoherent optical communications, while the second technique is compatible with coherent optical communications. The first technique simultaneously exploits the diffraction advantage of laser beams and the maturity of radio-frequency phase-locked loop technology. The second technique exploits both the diffraction advantage of laser beams and the large Doppler effect at optical frequencies. The second technique has the potential for greater accuracy; unfortunately, it is more difficult to implement since it involves optical Costas loops.

Kinman, Peter W.↗
NASA NTRS1987

Clock synchronization of a large multiprocessor system in the presence of malicious faults

An interconnection algorithm is presented for achieving clock synchronization in a multiprocessor system. The system is assumed to be maliciously faulty, i.e., some processors are out of synchronization and lie about their clock state to other intragroup or intergroup processors. A phase-locked clock network design is proposed which groups the clocks in the system into diverse clusters. The clusters are then treated as single clock units from the perspective of the network. The algorithm minimizes the number of interconnections while permitting synchronization of large multiprocessor systems controlling time-critical applications such as aircraft, nuclear reactors and industrial processes.

Shin, Kang G.↗
NASA NTRS1987

Trellis coded modulation for 4800-9600 bits/s transmission over a fading mobile satellite channel

The combination of trellis coding and multiple phase-shift-keyed (MPSK) signaling with the addition of asymmetry to the signal set is discussed with regard to its suitability as a modulation/coding scheme for the fading mobile satellite channel. For MPSK, introducing nonuniformity (asymmetry) into the spacing between signal points in the constellation buys a further improvement in performance over that achievable with trellis coded symmetric MPSK, all this without increasing average or peak power, or changing the bandwidth constraints imposed on the system. Whereas previous contributions have considered the performance of trellis coded modulation transmitted over an additive white Gaussian noise (AWGN) channel, the emphasis in the paper is on the performance of trellis coded MPSK in the fading environment. The results will be obtained by using a combination of analysis and simulation. It will be assumed that the effect of the fading on the phase of the received signal is fully compensated for either by tracking it with some form of phase-locked loop or with pilot tone calibration techniques. Thus, results will reflect only the degradation due to the effect of the fading on the amplitude of the received signal. Also, we shall consider only the case where interleaving/deinterleaving is employed to further combat the fading. This allows for considerable simplification of the analysis and is of great practical interest. Finally, the impact of the availability of channel state information on average bit error probability performance is assessed.

Divsalar, Dariush↗
NASA NTRS1986

Wall turbulence control

A variety of wall turbulence control devices which were experimentally investigated are discussed; these include devices for burst control, alteration of outer flow structures, large eddy substitution, increased heat transfer efficiency, and reduction of wall pressure fluctuations. Control of pre-burst flow was demonstrated with a single, traveling surface depression which is phase-locked to elements of the burst production process. Another approach to wall turbulence control is to interfere with the outer layer coherent structures. A device in the outer part of a boundary layer was shown to suppress turbulence and reduce drag by opposing both the mean and unsteady vorticity in the boundary layer. Large eddy substitution is a method in which streamline curvature is introduced into the boundary layer in the form of streamwise vortices. Riblets, which were already shown to reduce turbulent drag, were also shown to exhibit superior heat transfer characteristics. Heat transfer efficiency as measured by the Reynolds Analogy Factor was shown to be as much as 36 percent greater than a smooth flat plate in a turbulent boundary layer. Large Eddy Break-Up (LEBU) which are also known to reduce turbulent drag were shown to reduce turbulent wall pressure fluctuation.

Wilkinson, Stephen P.↗
NASA NTRS1987

Optimum filters and smoothers design for carrier phase and frequency tracking

The report presents the application of fixed lag smoothing algorithms to the problem of estimation of the phase and frequency of a sinusoidal carrier received in the presence of process noise and additive observation noise. A suboptimal structure consists of a phase-locked loop (PLL) followed by post-loop correction to the phase and frequency estimates. When the PLL is operating under high signal-to-noise ratio, the phase detector is approximately linear, and the smoother equations then correspond to the optimal linear equations for an equivalent linear signal model. The performance of such a smoother can be predicted by linear filtering theory. However, if the PLL is operating near the threshold region of the signal-to-noise ratio, the phase detector cannot be assumed to be linear. Then the actual performance of the smoother can significantly differ from that predicted by linear theory. In this report we present both the theoretical and simulated performance of such smoothers derived on the basis of various models for the phase and frequency processes.

Kumar, Rajendra↗
NASA NTRS1987

Nonlinear dynamics, chaos and complex cardiac arrhythmias

Periodic stimulation of a nonlinear cardiac oscillator in vitro gives rise to complex dynamics that is well described by one-dimensional finite difference equations. As stimulation parameters are varied, a large number of different phase-locked and chaotic rhythms is observed. Similar rhythms can be observed in the intact human heart when there is interaction between two pacemaker sites. Simplified models are analyzed, which show some correspondence to clinical observations.

Glass, L.↗
NASA NTRS1986

An amplitude modulated laser system for distance and displacement measurement

A laser distance and displacement measurement system is being developed to monitor small displacements in large space structures for strain analysis and structural control. The reflected laser beam is focused on a detector and the detected signal is mixed with the reference. Small displacements are indicated by a change in modulation frequency which is adjusted to maintain quadrature between the received signal and the reference signal from the voltage-controlled oscillator in a phase-locked loop. Measurement of absolute distance is accomplished by sweeping the modulation frequency from a quadrature lock point to an adjacent lock point.

Rogowski, Robert S.↗
NASA NTRS1987

A model El Nino-Southern Oscillation

A coupled atmosphere-ocean model is developed and used to study the ENSO (El Nino/Southern Oscillation) phenomenon. With no anomalous external forcing, the coupled model reproduces certain key features of the observed phenomenon, including the recurrence of warm events at irregular intervals with a preference for three to four years. It is shown that the mean sea surface temperature, wind and ocean current fields determine the characteristic spatial structure of ENSO anomalies. The tendency for phase-locking of anomalies is explained in terms of a variation in coupling strength associated with the annual cycle in the mean fields. Sensitivity studies reveal that both the amplitude and the time scale of the oscillation are sensitive to several parameters that affect the strength of the atmosphere-ocean coupling. Stronger coupling implies larger oscillations with a longer timescale. A critical element of the model oscillation is the variability in the equatorial heat content of the upper ocean. Equatorial heat content increases prior to warm events and decreases sharply during the events. A theory for this variability and the associated transitions between non-El Nino and El Nino states is presented. Implications of the model results for the prediction of El Nino events are discussed.

Zebiak, Stephen E.↗
NASA NTRS1987

Space-qualified submillimeter radiometer

The purpose of this research was to develop a reliable submillimeter wave spectrometer for space-borne high frequency spectral line work. The emphasis was on improving the efficiency of frequency multipliers to limit the system components to rugged, low power consumption solid-state devices. This research has allowed Millitech to develop increased efficiency and performance in Millitech's existing line of submillimeter components and systems. Millitech has fabricated and tested a complete solid-state spectrometer front end for use at 560 GHz (the 1(sub 10) to 1(sub 01) transition of water vapor). The spectrometer was designed with the rigors of flight conditions in mind. The spectrometer uses a phase-locked, solid-state Gunn diode oscillator as the local oscillator, employing a tripler to produce about 3 mW of power at 285 GHz, and a low noise second harmonic waveguide mixer which requires less than 2 mW of LO power. The LO (and the signal) is injected into the mixer by means of a quasioptical diplexer. The measured system noise temperature is 2800 K (DSB) over 400 MHz. The whole spectrometer front end is compact (21 cm by 21 cm by 24 cm), light (7.4 kg), and has a power consumption of less than 8 W. Other topics explored in this work include compact frequency agile phase lock loops, optical filters, and InP Gunn oscillators for low noise applications. As a result of this research, the improvement in the design of multipliers and harmonic mixers will allow their use as the LO power for a variety of satellite-borne receivers operating in the 200 to 600 GHz frequency range.

Huguenin, G. R.↗