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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Near optimum digital phase locked loops.

Near optimum digital phase locked loops are derived utilizing nonlinear estimation theory. Nonlinear approximations are employed to yield realizable loop structures. Baseband equivalent loop gains are derived which under high signal to noise ratio conditions may be calculated off-line. Additional simplifications are made which permit the application of the Kalman filter algorithms to determine the optimum loop filter. Performance is evaluated by a theoretical analysis and by simulation. Theoretical and simulated results are discussed and a comparison to analog results is made.

Polk, D. R.↗
NASA NTRS1976

Performance analysis of digital FM demodulators for fading channels in the threshold region

The quasi-optimum digital FM demodulators for fading channels reported earlier (1975) reflected the system performance accurately under high signal to noise ratio conditions. In this paper, the prediction of the system performance for low SNR values is considered. A new set of error variance algorithms is developed from the filter algorithm assuming a Gaussian distribution for the state estimation errors and taking the effect of high frequency terms into consideration. Simulation analysis for an FM system with Rician fading channel shows that these algorithms predict the system performance accurately in the threshold region.

Dharamsi, M. T.↗
NASA NTRS1979

EUV observations of the equatorial aurora

Using the Apollo-Soyuz mission's extreme ultraviolet telescope, it proved possible to observe the equatorial aurora on four different occasions. The observations were made from 220 km in quiet geomagnetic conditions. In all cases, signals were well above ambient background in the 50 to 150 A, 114 to 150 A, 170 to 600 A, and 500 to 780 A bands. The existence of a strong feature in the 50 to 150 A region may be an indication that other elements besides helium - possibly oxygen and nitrogen - contribute to the enhancement.

Paresce, F.↗
NASA NTRS1979

Some implications of reciprocity for two-way clock synchronization

The difficulties related to propagation perturbances in one-way and two-way methods for the synchronization of remote clocks are defined, and a possible means of circumventing these problems in the two-way method is suggested. In the two-way method, if signals are launched from two sources, A and B, then the two signals arriving at A and B will be displaced in arrival time by an amount that is equal to the difference in launch times of the two signals. Thus, the only condition to comparing clocks is that the medium be isotropic. The practice implementation of this is explored theoretically, in some detail, with respect to the Loran-C navigation system.

Jespersen, J. L.↗
NASA NTRS1984

Saturation Phenomena in LIF Measurements, Appendix B

Optical saturation is the phenomenon in which the laser-induced rates of absorption and spontaneous emission between two levels, induced by a laser, become comparable to or greater than the spontaneous emission and collision rates connecting these levels. This results in the excited state population N(u) acquiring a value of similar magnitude to that of the ground state, N(e). Under these conditions the observed fluorescence signal, which is proportional to N(u), is no longer linearly dependent on laser intensity I, but increases at a slower rate, and in principle ultimately becomes independent of I. A conceptual picture of optical saturation using a two-level picture is described. This is, however, inadequate for the description of a real experiment involving a molecule, such as OH, for several reasons, which will be explained briefly; these are the multi-level nature of the electronic states and energy transfer among them and effects due to spatial, spectral, and temporal fluctuations in the laser pulse.

Source record↗
NASA NTRS1985

Data presentation techniques for rotating machinery malfunction diagnosis

Baseline steady state data is excellent for documentation of vibration signals at normal operating conditions. Assuming that a set of initial data was acquired with the machinery in a good state of repair, any future changes or deterioration in mechanical condition can be easily compared to the baseline information. Often this type of comparison will yield sufficient information for evaluation of the problem. However, many malfunctions require the analysis of transient data in order to identify the malfunction. Steady-state data formats consist of: Time Base Waveform, Orbit, Spectrum. Transient data formats consist of: Polar, Bode, Cascade. Our objective is to demonstrate the use of the above formats to diagnose a machine malfunction. A turbine-driven compressor train is chosen as an example. The machine train outline drawing is shown.

Spettel, T.↗
NASA NTRS1987

Improved VAS regression soundings of mesoscale temperature structure observed during the 1982 atmospheric variability experiment

An Atmospheric Variability Experiment (AVE) was conducted over the central U.S. in the spring of 1982, collecting radiosonde date to verify mesoscale soundings from the VISSR Atmospheric Sounder (VAS) on the GOES satellite. Previously published VAS/AVE comparisons for the 6 March 1982 case found that the satellite retrievals scarcely detected a low level temperature inversion or a mid-tropospheric cold pool over a special mesoscale radiosonde verification network in north central Texas. The previously published regression and physical retrieval algorithms did not fully utilize VAS' sensitivity to important subsynoptic thermal features. Therefore, the 6 March 1982 case was reprocessed adding two enhancements to the VAS regression retrieval algorithm: (1) the regression matrix was determined using AVE profile data obtained in the region at asynoptic times, and (2) more optimistic signal-to-noise statistical conditioning factors were applied to the VAS temperature sounding channels. The new VAS soundings resolve more of the low level temperature inversion and mid-level cold pool. Most of the improvements stems from the utilization of asynoptic radiosonde observations at NWS sites. This case suggests that VAS regression soundings may require a ground-based asynoptic profiler network to bridge the gap between the synoptic radiosonde network and the high resolution geosynchronous satellite observations during the day.

Chesters, Dennis↗
NASA NTRS1986

The role of modeling and flight testing in rotorcraft parameter identification

The importance of recognizing that each lower-order model used for rotorcraft parameter identification has a limited range of applicability is illustrated in some detail. Examples are given to illustrate the use of conditioning the test input signals and the potential of using multi-axis test inputs to enhance the parameter identifiability. The paper discusses the benefits and limitations of using frequency sweeps as flight-test input signals for identification of frequency response for rotorcraft and for the subsequent fitting of parametric transfer-function models. This paper demonstrates the major role played by analytical modeling and the understanding of the physics involved in the rotorcraft flight dynamics, particularly understanding the limit of lower-order models, in achieving successful rotorcraft parameter identification.

Chen, R. T. N.↗
NASA NTRS1988

Force-Balance Dynamic Display

Balance dynamic display unit (BDDU) is compact system conditioning six dynamic analog signals so they are monitored simultaneously in real time on single-trace oscilloscope. Typical BDDU oscilloscope display in scan mode shows each channel occupying one-sixth of total trace. System features two display modes usable with conventional, single-channel oscilloscope: multiplexed six-channel "bar-graph" format and single-channel display. Two-stage visual and audible limit alarm provided for each channel.

Ferris, Alice T.↗
NASA NTRS1985

Land mobile satellite transmission measurements at 869 MHz. A comparison of error probabilities for various message block durations and fade margins as a function of vegetation shadowing

In order to give vehicles travelling in rural areas of the U.S. access to the telephone network, a Land Mobile Satellite System is being planned. Previously presented data by the author were analyzed for the probability that the received signal level dropped below a threshold during short time blocks of various signal durations and the conditional probability that such an event would reoccur after a given delay. If it assumed that a fade threshold crossing of even 1 msec would produce an error in the transmission, then the derived quantities can be considered error probabilities. Extensive tables and graphs are presented with the results from the analysis. They can be used to devise communication strategies which will provide improved link availability in the presence of vegetation shadowing.

Vogel, Wolfhard J.↗
NASA NTRS1988

Efficient detection and signal parameter estimation with applications to high dynamic GPS receivers

A novel technique for simultaneously detecting data and estimating the parameters of a received carrier signal phase modulated by unknown data and experiencing very high Doppler, Doppler rate, etc. is discussed. Such a situation arises, for example, in the case of Global Positioning Systems (DPS) where the signal parameters are directly related to the position, velocity and acceleration of the GPS receiver. The proposed scheme is based upon first estimating the received signal local (data dependent) parameters over two consecutive bit periods, followed by the detection of a possible jump in these parameters. The presence of a detected jump signifies a data transition which is then removed from the received signal. This effectively demodulated signal is then processed to provide the estimates of global (data independent) parameters of the signal related to the position, velocity, etc. of the receiver. One of the key features of the proposed algorithm is the introduction of two different schemes which can provide an improvement of up to 3 dB over the conventional implementation of Kalman filter as applied to phase and frequency estimation, under low to medium signal-to-noise ratio conditions.

Kumar, R.↗
NASA NTRS1990

Study on the spectral efficiency of SFH-GMSK in land mobile telephone communication by direct simulation

A spread spectrum system, slow frequency hopping with GMSK (Gaussian minimum shift keying) modulation (SFH-GMSK), is proposed for mobile telephone communications. The system performance is evaluated using computer simulation and is compared with an unspread system. Results show that under multipath fading conditions, when the signal-to-noise ratio (SNR) is greater than 15 dB, slow frequency hopping gives some bit error rate improvement over the unspread system. Theoretical predictions indicate that a system efficiency of 20-65 users per cell can be achieved in the cellular configuration. Joint use of SFH-GMSK and FM is also investigated. It is shown that FM interference can cause serious degradation to the SFH-GMSK performance.

Bao, H.↗
NASA NTRS1993

Detection of bearing failure in mechanical devices using neural networks

We present a novel time-domain method for the detection of faulty bearings that has direct applicability to monitoring the health of the turbo pumps on the Space Shuttle Main Engine. A feed-forward neural network was trained to detect modelled roller bearing faults on the basis of the periodicity of impact pulse trains. The network's performance was dependent upon the number of pulses in the network's input window and the signal-to-noise ratio of the input signal. To test the model's validity, we fit the model's parameters to an actual vibration signal generated by a faulty roller element bearing and applied the network trained on this model to detect faults in actual vibration data. When this network was tested on the actual vibration data, it correctly identified the vibration signal as a fault condition 76 percent of the time.

Burne, Richard A.↗
NASA NTRS1991

Signal processing considerations for low signal to noise ratio laser Doppler and phase Doppler signals

The relative performance of current methods used for estimating the phase and the frequency in LDV and phase Doppler applications in low signal to noise ratio conditions is analyzed. These methods include the Fourier analysis and the correlation techniques. Three methods that use the correlation function for frequency and phase estimations are evaluated in terms of accuracy and speed of processing. These methods include: (1) the frequency estimation using zero crossings counting of the auto-correlation function, (2) the Blackman-Tukey method, and (3) the AutoRegressive method (AR). The relative performance of these methods is evaluated and compared with the Fourier analysis method which provides the optimum performance in terms of the Maximum Likelihood (ML) criteria.

Ibrahim, K. M.↗
NASA NTRS1991

Radio science ground data system for the Voyager-Neptune encounter, part 1

The Voyager radio science experiments at Neptune required the creation of a ground data system array that includes a Deep Space Network complex, the Parkes Radio Observatory, and the Usuda deep space tracking station. The performance requirements were based on experience with the previous Voyager encounters, as well as the scientific goals at Neptune. The requirements were stricter than those of the Uranus encounter because of the need to avoid the phase-stability problems experienced during that encounter and because the spacecraft flyby was faster and closer to the planet than previous encounters. The primary requirement on the instrument was to recover the phase and amplitude of the S- and X-band (2.3 and 8.4 GHz) signals under the dynamic conditions encountered during the occultations. The primary receiver type for the measurements was open loop with high phase-noise and frequency stability performance. The receiver filter bandwidth was predetermined based on the spacecraft's trajectory and frequency uncertainties.

Kursinski, E. R.↗
NASA NTRS1999

Liquid Methane/Oxygen Injector Study for Mars Ascent Engines

As a part of the advancing technology of the cryogenic propulsion system for the Mars exploration mission, this effort aims at evaluating propellant injection concepts for liquid methane/liquid oxygen (LOX) rocket engines. Split-triplet and unlike impinging injectors were selected for this study. A total of four injector configurations were tested under combustion conditions in a modular combustor test article (MCTA), equipped with optically accessible windows, at MSFC. A series of forty hot-fire tests, which covered a wide range of engine operating conditions with the chamber pressure ranging from 320 to 510 and the mixture ratio from 1.5 to 3.5, were conducted. The test matrix also included a variation in the combustion chamber length for the purpose of investigating its effects on the combustion performance and stability. Initial assessments of the test results showed that the injectors provided stable combustion and there were no injector face overheating problems under all operating conditions. The Raman scattering signal measurement method was successfully demonstrated for the hydrocarbon/oxygen reactive flow field. The near-injector face flow field was visually observed through the use of an infrared camera. Chamber wall temperature, high frequency chamber pressure, and average throat section heat flux were also recorded throughout the test series. Assessments of the injector performance are underway.

Trinh, Huu Phuoc↗
NASA NTRS2000

Vasomotor tone does not affect perfusion heterogeneity and gas exchange in normal primate lungs during normoxia

To determine whether vasoregulation is an important cause of pulmonary perfusion heterogeneity, we measured regional blood flow and gas exchange before and after giving prostacyclin (PGI(2)) to baboons. Four animals were anesthetized with ketamine and mechanically ventilated. Fluorescent microspheres were used to mark regional perfusion before and after PGI(2) infusion. The lungs were subsequently excised, dried inflated, and diced into approximately 2-cm(3) pieces (n = 1,208-1,629 per animal) with the spatial coordinates recorded for each piece. Blood flow to each piece was determined for each condition from the fluorescent signals. Blood flow heterogeneity did not change with PGI(2) infusion. Two other measures of spatial blood flow distribution, the fractal dimension and the spatial correlation, did not change with PGI(2) infusion. Alveolar-arterial O(2) differences did not change with PGI(2) infusion. We conclude that, in normal primate lungs during normoxia, vasomotor tone is not a significant cause of perfusion heterogeneity. Despite the heterogeneous distribution of blood flow, active regulation of regional perfusion is not required for efficient gas exchange.

Non-NASA Center↗