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At least 127 records · Page 7

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NASA NTRS1981

An overview of several rain and attenuation models

The key features of the Rice-Holmberg, Dutton-Dougherty, Global, Lin, and Piecewise Uniform Rain Rate models for estimation of cumulative attenuation statistics on Earth-space millimeter paths are summarized. The models provide either rain rate statistics or attenuation statistics, which can be related by use of the specific attenuation and the effective path length relation. The effective path length is defined. The advantages and disadvantages of each model for specific applications are discussed. The Global model is recommended as the design tool for predicting propagation effects in Earth-space communication systems.

Source record↗

NASA NTRS1982

A two-component rain model for the prediction of attenuation and diversity improvement

A new model was developed to predict attenuation statistics for a single Earth-satellite or terrestrial propagation path. The model was extended to provide predictions of the joint occurrences of specified or higher attenuation values on two closely spaced Earth-satellite paths. The joint statistics provide the information required to obtain diversity gain or diversity advantage estimates. The new model is meteorologically based. It was tested against available Earth-satellite beacon observations and terrestrial path measurements. The model employs the rain climate region descriptions of the Global rain model. The rms deviation between the predicted and observed attenuation values for the terrestrial path data was 35 percent, a result consistent with the expectations of the Global model when the rain rate distribution for the path is not used in the calculation. Within the United States the rms deviation between measurement and prediction was 36 percent but worldwide it was 79 percent.

Crane, R. K.↗

NASA NTRS1981

The physics of creep and attenuation in the mantle

The objective is to investigate the role of dislocations in both creep and attenuation. Attenuation in the mantle at seismic frequencies is thought to be caused by the glide of dislocations in the subgrains. Both kink and impurity drag can contribute to the glide time constant. The kink-formation, or Peierls barrier, model for dislocation glide is seen as a low-temperature, high-frequency mechanism most appropriate for pure systems. It is noted that a small amount of impurity drag brings the dislocation glide characteristic time into the seismic band at upper-mantle temperatures. The attenuation and creep behavior of the mantle are related by way of the dislocation structure. Analysis of the various possible mechanisms is facilitated by casting them and the geophysical data in terms of a pre-exponential characteristic time and an activation energy.

Anderson, D. L.↗

NASA NTRS1983

Range safety signal attenuation by the Space Shuttle main engine exhaust plumes

An analysis of attenuation of the range safety signal at 416.5 MHz observed after SRB separation and ending at hand over to Bermuda, during which transmission must pass through the LOX/H2 propelled main engine exhaust plumes, is summarized. Absorption by free electrons in the exhaust plume can account for the nearly constant magnitude of the observed attenuation during this period; it does not explain the short term transient increases that occur at one or more times during this portion of the flight. It is necessary to assume that a trace amount (about 0.5 ppm) of easily ionizable impurity must be present in the exhaust flow. Other mechanisms of attenuation, such as scattering by turbulent fluctuations of both free and bound electrons and absorption by water vapor, were examined but found to be inadequate to explain the observations.

Pearce, B. E.↗

NASA NTRS1980

An experimental investigation of noise attenuating techniques for space-shuttle canisters

A model of the space shuttle thermal canister was acoustically tested to determine the amount of noise attenuation which could be derived using a simple, single-wall canister construction having rectangular shape. Acoustic testing was performed on the basic canister and with noise-attenuating design modifications. The basic canister experienced noise amplifications at 56 and 80 Hz, which are attributed to the fundamental canister acoustic mode and local panel structural resonances, respectively. The standing wave response at 56 Hz was effectively suppressed by the incorporation of a cardboard baffle midway between the canister end-caps (an additional overall noise reduction of 4 dB). The canister was next tested with 14%, 22.5% and 31% sound absorptive coverages on the interior walls. The coverage was effective between 400-3000 Hz; the maximum benefit (9 dB) occurring at 1600 Hz. Viscoelastic damping strips bonded to the canister exterior provided an additional 4 to 5 dB attenuation over much of the frequency range and has an overall reduction of about 10 dB as compared to 4.4 dB without damping. A significant reduction of the resonant effect at 80 Hz was noted.

Mirandy, L.↗

NASA NTRS1982

Microwave noise temperature and attenuation of clouds - Statistics of these effects at various sites in the United States, Alaska, and Hawaii

The microwave attenuation and noise temperature effects of clouds can result in serious degradation of telecommunications link performance, especially for low-noise systems presently used in deep-space communications. Although cloud effects are generally less than rain effects, the frequent presence of clouds will cause some amount of link degradation a large portion of the time. This paper presents a general review of cloud types and their water particle densities, attenuation and noise temperature calculations, and basic link signal-to-noise ratio calculations. Tabular results of calculations for 12 different cloud models are presented for frequencies in the range 10-50 GHz. Curves of average-year attenuation and noise temperature statistics at frequencies ranging from 10 to 90 GHz, calculated from actual surface and radiosonde observations, are given for 15 climatologically distinct regions in the contiguous United States, Alaska, and Hawaii. Nonuniform sky cover is considered in these calculations.

Slobin, S. D.↗

NASA NTRS1984

The role of the reflection coefficient in precision measurement of ultrasonic attenuation

Ultrasonic attenuation measurements using contact, pulse-echo techniques are sensitive to surface roughness and couplant thickness variations. This can reduce considerable inaccuracies in the measurement of the attenuation coefficient for broadband pulses. Inaccuracies arise from variations in the reflection coefficient at the buffer-couplant-sample interface. The reflection coefficient is examined as a function of the surface roughness and corresponding couplant thickness variations. Interrelations with ultrasonic frequency are illustrated. Reliable attenuation measurements are obtained only when the frequency dependence of the reflection coefficient is incorporated in signal analysis. Data are given for nickel 200 samples and a silicon nitride ceramic bar having surface roughness variations in the 0.3 to 3.0 microns range for signal bandwidths in the 50 to 100 MHz range.

Generazio, E. R.↗

NASA NTRS1985

The role of the reflection coefficient in precision measurement of ultrasonic attenuation

Generazio, E. R.↗

NASA NTRS1986

Remote sensing of tree attenuation at 870 MHz along simulated Earth-satellite paths

Tree attenuation at 870 MHz was studied using a helicopter as a source platform and a van with receiver and data acquisition instrumentation. Tree attenuation results were obtained with the van stationary and in motion to determine land mobile satellite systems link parameter requirements (expected fading due to roadside trees for mobile and stationary vehicles). Single tree attenuation results give worst case median fades as high as 15 dB although roadside tree values produce fades greater than 20 dB for small percentages of time. The cumulative fade distributions and their relative contributions as a function of path elevation angle, right side versus left side driving, and different road types are derived from the field measurements.

Vogel, W. J.↗

NASA NTRS1986

Tree attenuation at 869 MHz derived from remotely piloted aircraft measurements

Attenuation due to single trees is experimentally investigated using UHF transmissions at 869 MHz between a remotely piloted aircraft and a ground receiver system located in a stationary vehicle. Single trees of each tree type in full foliage were found to attenuate from 10-20 dB, with an average median value of about 12 dB. Attenuation coefficients associated with path lengths through the foliage may on average be about 1 dB/m, with maximum values closer to 2 dB/m.

Vogel, Wolfhard J.↗

NASA NTRS1988

Chopping-Wheel Optical Attenuator

Star-shaped rotating chopping wheel provides adjustable time-averaged attenuation of narrow beam of light without changing length of optical path or spectral distribution of light. Duty cycle or attenuation factor of chopped beam controlled by adjusting radius at which beam intersects wheel. Attenuation factor independent of wavelength. Useful in systems in which chopping frequency above frequency-response limits of photodetectors receiving chopped light. Used in systems using synchronous detection with lock-in amplifiers.

Leviton, Douglas B.↗

NASA NTRS1988

Measured and calculated acoustic attenuation rates of tuned resonator arrays for two surface impedance distribution models with flow

An experiment was performed to validate two analytical models for predicting low frequency attenuation of duct liner configurations built from an array of seven resonators that could be individually tuned via adjustable cavity depths. These analytical models had previously been developed for high frequency aero-engine inlet duct liner design. In the low frequency application, the liner surface impedance distribution is unavoidably spatially varying by virtue of available fabrication techniques. The characteristic length of this spatial variation may be a significant fraction of the acoustic wavelength. Comparison of measured and predicted attenuation rates and transmission losses for both modal decomposition and finite element propagation models were in good to excellent agreement for a test frequency range that included the first and second cavity resonance frequencies. This was true for either of two surface impedance distribution modeling procedures used to simplify the impedance boundary conditions. In the presence of mean flow, measurements revealed a fine scale structure of acoustic hot spots in the attenuation and phase profiles. These details were accurately predicted by the finite element model. Since no impedance changes due to mean flow were assumed, it is concluded that this fine scale structure was due to convective effects of the mean flow interacting with the surface impedance nonuniformities.

Parrott, Tony L.↗

NASA NTRS1988

Modal attenuation in multilayered coated waveguides

Propagation and attenuation constants of low-order normal modes in a circular waveguide lined with lossy coating layers are calculated using a generalized dispersion equation. It is found that the use of multilayered coating can significantly enhance modal attenuations over a broader frequency range compared to that for a single-layer coated structure. For a cylinder with radius a = 2 lambda, the attenuation constants for the dominant modes are shown to increase by 20 dB per a by adding a lossless padding layer to a lossy magnetic coating. Application of this result in radar cross-section (RCS) reduction is discussed.

Chou, Ri-Chee↗

NASA NTRS1988

Ground-based radiometric observations of emission and attenuation at 20.6, 31.65, and 90.0 GHz

During 1987, ground-based zenith-viewing observations of atmospheric thermal emission were made at frequencies of 20.6, 31.65, and 90.0 GHz. At the locations of the experiments, San Nicolas Island, California, and Denver, Colorado, radiosonde observations of temperature and humidity were also available. The data, after conversion to attenuation using the mean radiating temperature approximation, are processed to derive attenuation statistics. In addition, both clear and cloudy attenuation characteristics are examined and compared with the most recent theories. Finally, the predictability and interdependence of the three separate channels are examined.

Westwater, Ed R.↗

NASA NTRS1988

Ultrasonic attenuation measurements determine onset, degree, and completion of recrystallization

Ultrasonic attenuation was measured for cold worked Nickel 200 samples annealed at increasing temperatures. Localized dislocation density variations, crystalline order and volume percent of recrystallized phase were determined over the anneal temperature range using transmission electron microscopy, X-ray diffraction, and metallurgy. The exponent of the frequency dependence of the attenuation was found to be a key variable relating ultrasonic attenuation to the thermal kinetics of the recrystallization process. Identification of this key variable allows for the ultrasonic determination of onset, degree, and completion of recrystallization.

Generazio, E. R.↗

NASA NTRS1989

Lateral noise attenuation of the advanced propeller of the propfan test assessment aircraft

Lateral noise attenuation characteristics of the advanced propeller are determined using the flight test results of the testbed aircraft, Propfan Test Assessment (PTA), with a single, large-scale propfan. The acoustic data were obtained with an array of ground-mounted microphones positioned at distances up to 2.47 km (8100 feet) to the side of the flight path. The aircraft was flown at a Mach number of 0.31 for a variety of operating conditions. The lateral noise attenuation in a frequency range containing the blade passage frequency of the propeller was found to have positive magnitudes on the propfan side and negative magnitudes on the opposite side. The measured attenuation exhibits a strong dependence upon the elevation angle. The results also display a clear dependence upon the angle at which the propeller and nacelle are mounted on the wing (inflow angle).

Chambers, F. W.↗

NASA NTRS1989

Relaxation behavior of ultrasonic attenuation in YBa2Cu3O7

It is shown that temperature-dependent ultrasonic attenuation data of YBa2Cu3O7 at various frequencies exhibit anomalies at temperatures close to Tc. These attenuation maxima are found to be the result of a relaxation mechanism added on top of an unusual attenuation background. It is proposed that the grain boundary motions induced by the structural distortion and the propagation of sound waves enhances the energy dissipation around Tc. Whether this structural distortion is the consequence of the onset of a superconducting state remains undetermined. It is also possible that either a tunneling effect or the acoustoelectric effect contributes to sound energy dissipation. The temperature dependence of ultrasonic velocity shows a softening around Tc which may be an intrinsic property of high-temperature superconductors.

Sun, K. J.↗

NASA NTRS1990

A Unified Statistical Rain-Attenuation Model for Communication Link Fade Predictions and Optimal Stochastic Fade Control Design Using a Location-Dependent Rain-Statistic Database

A static and dynamic rain-attenuation model is presented which describes the statistics of attenuation on an arbitrarily specified satellite link for any location for which there are long-term rainfall statistics. The model may be used in the design of the optimal stochastic control algorithms to mitigate the effects of attenuation and maintain link reliability. A rain-statistics data base is compiled, which makes it possible to apply the model to any location in the continental U.S. with a resolution of 0-5 degrees in latitude and longitude. The model predictions are compared with experimental observations, showing good agreement.

Manning, Robert M.↗