Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “SIGNAL FADING”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Results of the Compensated Earth-Moon-Earth Retroreflector Laser Link (CEMERLL) Experiment

Adaptive optics techniques can be used to realize a robust low bit-error-rate link by mitigating the atmosphere-induced signal fades in optical communications links between ground-based transmitters and deep-space probes. Phase I of the Compensated Earth-Moon-Earth Retroreflector Laser Link (CEMERLL) experiment demonstrated the first propagation of an atmosphere-compensated laser beam to the lunar retroreflectors. A 1.06-micron Nd:YAG laser beam was propagated through the full aperture of the 1.5-m telescope at the Starfire Optical Range (SOR), Kirtland Air Force Base, New Mexico, to the Apollo 15 retroreflector array at Hadley Rille. Laser guide-star adaptive optics were used to compensate turbulence-induced aberrations across the transmitter's 1.5-m aperture. A 3.5-m telescope, also located at the SOR, was used as a receiver for detecting the return signals. JPL-supplied Chebyshev polynomials of the retroreflector locations were used to develop tracking algorithms for the telescopes. At times we observed in excess of 100 photons returned from a single pulse when the outgoing beam from the 1.5-m telescope was corrected by the adaptive optics system. No returns were detected when the outgoing beam was uncompensated. The experiment was conducted from March through September 1994, during the first or last quarter of the Moon.

Wilson, K. E.↗
NASA NTRS1971

Investigation of line-of-sight propagation in dense atmosphere, phase 3, part 1

The investigation of microwave absorption in the 1 to 10 GHz frequency band by the Jovian atmosphere has continued, and an estimate of the strength of signal fading at these frequencies due to layers of turbulence in Jupiter's atmosphere is given. The microwave absorption due to gaseous ammonia is estimated both in terms of a power loss in dB/km, and in total power loss in dB for slant-path communication with a probe at altitudes down to pressures of several tens of atmospheres. The graphs indicate a frequency-squared scaling of the absorption, and appreciable losses at altitudes where the pressure is several atmospheres. An estimate of turbulence strength is given. This may turn out to be quite crude considering the absence of any relevant data. A planetary scaling law which appears to hold reasonably well for Earth to Venus, is extrapolated to Jupiter. No reasonable modifications of the estimate can alter the conclusion that direct-path fading is negligible for pressure regimes up to 20 atm.

Dewolf, D. A.↗
NASA NTRS1974

Channel simulation for direct detection optical communication systems

A technique is described for simulating the random modulation imposed by atmospheric scintillation and transmitter pointing jitter on a direct detection optical communication system. The system is capable of providing signal fading statistics which obey log normal, beta, Rayleigh, Ricean or chi-squared density functions. Experimental tests of the performance of the Channel Simulator are presented.

Tycz, M.↗
NASA NTRS1974

Channel simulation for direct-detection optical communication systems

A technique is described for simulating the random modulation imposed by atmospheric scintillation and transmitter pointing jitter on a direct-detection optical communication system. The system is capable of providing signal fading statistics which obey log-normal, beta, Rayleigh, Ricean, or chi-square density functions. Experimental tests of the performance of the channel simulator are presented.

Tycz, M.↗
NASA NTRS1974

Channel simulation for optical communication systems

A technique is reported for simulating the signal fading that will be experienced by typical optical communication systems. The desired irradiance or amplitude fading statistics can be simulated by incorporating a linearized optical modulator subsystem between the transmitter and receiver. This technique has been implemented in the design and construction of a laboratory channel simulator. The design of the processing electronics is discussed along with the results of tests performed for each mode of operation.

Tycz, M.↗
NASA NTRS1977

Hardware simulation of KU-band spacecraft receiver and bit synchronizer, phase 2, volume 1

The acquisition behavior of the PN subsystem of an automatically acquiring spacecraft receiver was studied. A symbol synchronizer subsystem was constructed and integrated into the composite simulation of the receiver. The overall performance of the receiver when subjected to anomalies such as signal fades was evaluated. Potential problems associated with PN/carrier sweep interactions were investigated.

Lindsey, W. C.↗
NASA NTRS1982

Assessment of radar resolution requirements for soil moisture estimation from simulated satellite imagery

Radar simulations were performed at five-day intervals over a twenty-day period and used to estimate soil moisture from a generalized algorithm requiring only received power and the mean elevation of a test site near Lawrence, Kansas. The results demonstrate that the soil moisture of about 90% of the 20-m by 20-m pixel elements can be predicted with an accuracy of + or - 20% of field capacity within relatively flat agricultural portions of the test site. Radar resolutions of 93 m by 100 m with 23 looks or coarser gave the best results, largely because of the effects of signal fading. For the distribution of land cover categories, soils, and elevation in the test site, very coarse radar resolutions of 1 km by 1 km and 2.6 km by 3.1 km gave the best results for wet moisture conditions while a finer resolution of 93 m by 100 m was found to yield superior results for dry to moist soil conditions.

Ulaby, F. T.↗
NASA NTRS1981

Crop classification using airborne radar and LANDSAT data

Airborne radar data acquired with a 13.3 GHz scatterometer over a test-site near Colby, Kansas were used to investigate the statistical properties of the scattering coefficient of three types of vegetation cover and of bare soil. A statistical model for radar data was developed that incorporates signal-fading and natural within-field variabilities. Estimates of the within-field and between-field coefficients of variation were obtained for each cover-type and compared with similar quantities derived from LANDSAT images of the same fields. The classification accuracy provided by LANDSAT alone, radar alone, and both sensors combined was investigated. The results indicate that the addition of radar to LANDSAT improves the classification accuracy by about 10; percentage-points when the classification is performed on a pixel basis and by about 15 points when performed on a field-average basis.

Ulaby, F. T.↗
NASA NTRS1983

On the extraction of atmospheric turbulence parameters from radar backscatter Doppler spectra. Part 1: Theory

A theory is developed for the extraction of r.m.s. velocities of scatterer motions from spectra measured with a Doppler backscatter radar. The effects of finite beam-widths, finite pulse lengths, beam broadening, shear broadening, and the other such spectral contaminants are considered. It is shown that these contaminants can play a major role in determining the measured spectral widths (and, equivalently, the signal fading time), and so must be properly considered if the spectral widths are to be used to extract the r.m.s. motions of the scatterers. It is also shown that these r.m.s. motions can be used to estimate turbulence intensities, in those cases where turbulence is the dominant cause of the scatterer motions.

Hocking, W. K.↗
NASA NTRS1984

Effects of line-of-sight velocity on spaced-antenna measurements, part 3.5A

Horizontal wind velocities in the upper atmosphere, particularly the mesosphere, have been measured using a multitude of different techniques. Most techniques are based on stated or unstated assumptions about the wind field that may or may not be true. Some problems with the spaced antenna drifts (SAD) technique that usually appear to be overlooked are investigated. These problems are not unique to the SAD technique; very similar considerations apply to measurement of horizontal wind using multiple-beam Doppler radars as well. Simply stated, the SAD technique relies on scattering from multiple scatterers within an antenna beam of fairly large beam width. The combination of signals with random phase gives rise to an interference pattern on the ground. This pattern will drift across the ground with a velocity twice that of the ionospheric irregularities from which the radar signals are scattered. By using spaced receivers and measuring time delays of the signal fading in different antennas, it is possible to estimate the horizontal drift velocities.

Royrvik, O.↗
NASA NTRS1988

Proceedings of the Twelfth NASA Propagation Experimenters Meeting (NAPEX 12)

The NASA Propagation Experimenters Meeting was convened on June 9 and 10, 1988. Pilot Field Experiments propagation studies, mobile communication systems, signal fading, communication satellites rain gauge network measurements, atmospheric attenuation studies, optical communication through the atmosphere, and digital beacon receivers were among the topics discussed.

Davarian, Faramaz↗
NASA NTRS1990

Mobile satellite system propagation measurements at L-band using Marecs-B2

A mobile satellite system (MSS) propagation experiment at 1.5 GHz was performed near the east coast of the United States in central Maryland during December 1987 using the Marecs-B2 satellite as a transmitter platform. A receiving system in a vehicle measured signal fades caused by shadowing and multipath from roadside trees and utility poles. The propagation degradations were characterized for a system of three roads previously examined using a helicopter as the transmitter platform. The objectives of the Marecs-B2 MSS tests were to: (1) establish cumulative fade distributions for the particular satellite geometry for both rural and suburban roads; (2) validate the consistency of previous roadside tree measurements which employed a helicopter as the transmitter platform for the same system of roads; (3) obtain an additional set of fade levels at a lower angle hitherto not measured in central Maryland; and (4) combine the satellite-acquired data set with previous helicopter results and establish an analytic, empirically derived function describing the cumulative fade distributions for a family of path angles. An analysis of the satellite data has demonstrated the successful achievement of these objectives.

Vogel, Wolfhard J.↗
NASA NTRS1991

A statistical rain attenuation prediction model with application to the advanced communication technology satellite project. 3: A stochastic rain fade control algorithm for satellite link power via non linear Markow filtering theory

The dynamic and composite nature of propagation impairments that are incurred on Earth-space communications links at frequencies in and above 30/20 GHz Ka band, i.e., rain attenuation, cloud and/or clear air scintillation, etc., combined with the need to counter such degradations after the small link margins have been exceeded, necessitate the use of dynamic statistical identification and prediction processing of the fading signal in order to optimally estimate and predict the levels of each of the deleterious attenuation components. Such requirements are being met in NASA's Advanced Communications Technology Satellite (ACTS) Project by the implementation of optimal processing schemes derived through the use of the Rain Attenuation Prediction Model and nonlinear Markov filtering theory.

Manning, Robert M.↗