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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Ka-band propagation studies using the ACTS propagation terminal and the CSU-CHILL multiparameter, Doppler radar

An increase in the demand for satellite communications has led to an overcrowding of the current spectrums being used - mainly at C and Ku bands. To alleviate this overcrowding, new technology is being developed to open up the Ka-band for communications use. One of the first experimental communications satellites using this technology is NASA's Advanced Communications Technology Satellite (ACTS). In Sept. 1993, ACTS was deployed into a geostationary orbit near 100 deg W longitude. The ACTS system employs two Ka-band beacons for propagation experiments, one at 20.185 GHz and another at 27.505 GHz. Attenuation due to rain and tropospheric scintillations will adversely affect new technologies proposed for this spectrum. Therefore, before being used commercially, propagation effects at Ka-band must be studied. Colorado State University is one of eight sites across the United States and Canada conducting propagations studies; each site is equipped with the ACTS propagation terminal (APT). With each site located in a different climatic zone, the main objective of the propagation experiment is to obtain monthly and yearly attenuation statistics. Each site also has secondary objectives that are site dependent. At CSU, the CSU-CHILL radar facility is being used to obtain polarimetric radar data along the ACTS propagation path. During the expected two to four year period of the project, it is hoped to study several significant weather events. The S-band radar will be used to obtain Ka-band attenuation estimates and to initialize propagation models that have been developed, to help classify propagation events measured by the APT. Preliminary attenuation estimates for two attenuation events will be shown here - a bright band case that occurred on 13 May 1994 and a convective case that occurred on 20 Jun. 1994. The computations used to obtain Ka-band attenuation estimates from S-band radar data are detailed. Results from the two events are shown.

Beaver, J.↗
NASA NTRS1973

ATS-F Comsat Millimeter Wave Propagation Experiment

The ATS-F Comsat Millimeter Wave Propagation Experiment has been designed to gather statistical data on the attenuation caused by rain at millimeter wave frequencies. These data will be used to determine system design parameters for future communications satellite systems operating at frequencies above 10 GHz. The experiment has 39 ground terminals transmitting at 13.2 or 17.8 GHz to a transponder on board the ATS-F satellite. The transponder retransmits these signals at 4 GHz to a central earth terminal which records their amplitudes once each second. The data will be analyzed to provide probabilities of attenuation as functions of parameters such as rainfall, location, and time. These probabilities can then be used to determine the required power margins of millimeter wave communications systems. Techniques of overcoming severe attenuation such as site diversity and the use of a spot beam to increase the power level at selected locations will also be evaluated.

Westerlund, L. H.↗
NASA NTRS1996

Analysis of the ACTS-Vancouver Path Propagation Data

The data described here cover the two-year period from 1 December 1993 to 30 November, 1995. The analysis is done for two sets of valid data: (a) observed or total attenuation data, which includes all atmospheric effects plus the additional attenuation resulting from wet antenna surfaces during rain events, and (b) adjusted attenuation data as in (a) minus estimated values of attenuation due to wetting the antenna surfaces. In both cases, average and worst month cumulative distribution functions (CDF's), fade-duration statistics, and fade-slope statistics for the 20 and 27 GHz beacons are presented.

Kharadly, M.↗
NASA NTRS1981

Calculation of the attenuation and phase displacement per unit of length due to rain composed of ellipsoidal drops

All of the phenomena which influence the propagation of radiowaves at frequencies above 10 GHz (attenuation, depolarization, scintillation) can by intensified by parameters directly derived from a solution of individual scatter, naturally in addition to be meteorological elements which characterize the physical medium. The diffusion caused by rainy precipitation was studied using Mie's algorithm for rain composed of spherical drops, and Oguchi's algorithm for rain composed of drops in an ellipsoidal form with axes of rotational symmetry arrange along the vertical line of a generic reference point. Specific phase displacement and attenuation along the principal planes, propagation of radiowaves in generic polarization, and propagation with inclined axes are also considered.

Maggiori, D.↗
NASA NTRS1975

The COMSAT 13 and 18 GHz Propagation Experiment

The ATS-6 COMSAT Propagation Experiment was designed to gather statistical data on attenuation caused by rain and snow at 13 and 18 GHz. These data will be used to determine system design parameters for future communications satellite systems operating at frequencies above 10 GHz. The experiment used 25, 18 GHz, and 15, 13 GHz unattended ground transmitters at 25 locations in the eastern U.S. to transmit to the ATS-6. The ATS-6 transponder converts these carriers to frequencies around 4150 MHz and transmits these signals to the COMSAT large horn antenna/data acquisition and receiving facility at Andover, Me. This facility calibrates and digitally records each carrier once per second. These data have been processed and analyzed for the period from July to Nov. 2, 1972. Plans are being made to conduct the experiment in Europe and India during the 35 deg E longitude site phase of ATS-6 operations.

King, J. L.↗
NASA NTRS1979

Determining potential 30/20 GHz domestic satellite system concepts and establishment of a suitable experimental configuration

Issues and results in a NASA study of the potential concepts and markets for a multibeam 30/20 GHz domestic satellite system in the 1990s are presented. Issues considered include the reduction of signal attenuation due to rain, beam-beam interference isolation in the multibeam system, the method of access/modulation (FDMA, TDMA or hybrid) and the market for reduced reliability and wideband services. A hypothetical demonstration payload configuration which would attempt to resolve these issues is illustrated. The communications payload would employ a system of seven contiguous coverage spots in order to demonstrate a typical cell in a contiguous beam system having extensive frequency re-use, as in a direct-to-user system, and a single spot, typical of a trunking system, to determine signal isolation. The payload could be carried on several existing buses and is illustrated on an MMS bus.

Stevens, G. H.↗
NASA NTRS1981

Scattering of Pruppacher-Pitter raindrops at 30 GHz

Optimum design of modern ground-satellite communication systems requires the knowledge of rain-induced differential attenuation, differential phase shift, and cross polarization factors. Different available analytical techniques for raindrop scattering problems were assessed. These include: (1) geometrical theory of diffraction; (2) method of moment; (3) perturbation method; (4) point matching methods; (5) extended boundary condition method; and (6) global-local finite element method. The advantages and disadvantages of each are listed. The extended boundary condition method, which was determined to yield the most scattering results, is summarized. The scattered fields for Pruppacher-Pitter raindrops with sizes ranging from 0.5 mm to 3.5 mm at 20 C and at 30 GHz for several incidence angles are tabulated.

Yeh, C.↗
NASA NTRS1988

Propagation effects on spread-spectrum mobile satellite systems

In contrast to the situation at L-band, wide bandwidths of 500 MHz or more have been allocated for mobile satellite service at frequencies between 20 and 50 GHz. These broad bandwidths are well suited for the use of spread-spectrum. Certain system considerations about the use of such high frequencies for mobile satellite service are mentioned first, and attention is then given to propagation effects on high-frequency broad-band systems. Attenuation due to rain is a constant at 20 to 50 MHz, but would not be a serious problem if outages occurring for one to three percent of the time, depending on location, are considered to be acceptable. Clear air absorption becomes a significant factor above 40 GHz, but should not exceed 2 dB at a 10 degree elevation angle and frequencies below 40 GHz. Spread-spectrum provides a form of frequency diversity that helps to minimize the effects of multipath.

Flock, Warren L.↗
NASA NTRS1989

Experimental tests of range-profiling algorithms

The natural tendency in the design of a spaceborne weather radar is to select frequencies at 10 GHz or higher. A major disadvantage of this approach is that increased attenuation limits the maximum rain rate that can be accurately estimated. Here, a way to lessen this problem is suggested which uses estimates of path attenuation to reconstruct a profile of the rain rate.

Meneghini, R.↗
NASA NTRS1993

ACTS broadband aeronautical experiment

In the last decade, the demand for reliable data, voice, and video satellite communication links between aircraft and ground to improve air traffic control, airline management, and to meet the growing demand for passenger communications has increased significantly. It is expected that in the near future, the spectrum required for aeronautical communication services will grow significantly beyond that currently available at L-band. In anticipation of this, JPL is developing an experimental broadband aeronautical satellite communications system that will utilize NASA's Advanced Communications Technology Satellite (ACTS) as a satellite of opportunity and the technology developed under JPL's ACTS Mobile Terminal (AMT) Task to evaluate the feasibility of using K/Ka-band for these applications. The application of K/Ka-band for aeronautical satellite communications at cruise altitudes is particularly promising for several reasons: (1) the minimal amount of signal attenuation due to rain; (2) the reduced drag due to the smaller K/Ka-band antennas (as compared to the current L-band systems); and (3) the large amount of available bandwidth. The increased bandwidth available at these frequencies is expected to lead to significantly improved passenger communications - including full-duplex compressed video and multiple channel voice. A description of the proposed broadband experimental system will be presented including: (1) applications of K/Ka-band aeronautical satellite technology to U.S. industry; (2) the experiment objectives; (3) the experiment set-up; (4) experimental equipment description; and (5) industrial participation in the experiment and the benefits.

Abbe, Brian S.↗
NASA NTRS2010

Changes in the TRMM Version-5 and Version-6 Precipitation Radar Products Due to Orbit Boost

The performance of the version-5 and version-6 Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) products before and after the satellite orbit boost is assessed through a series of comparisons with Weather Surveillance Radar (WSR)-88D ground-based radar in Melbourne, Florida. Analysis of the comparisons of radar reflectivity near the storm top from the ground radar and both versions of the PR indicates that the PR bias relative to the WSR radar at Melbourne is on the order of 1dB for both pre- and post-boost periods, indicating that the PR products maintain accurate calibration after the orbit boost. Comparisons with the WSR-88D near-surface reflectivity factors indicate that both versions of the PR products accurately correct for attenuation in stratiform rain. However, in convective rain, both versions exhibit negative biases in the near-surface radar reflectivity with version-6 products having larger negative biases than version-5. Rain rate comparisons between the ground and space radars show similar characteristics

Liao, Liang↗
NASA NTRS1995

Effect of attenuation models on communication system design

The atmosphere has a significant impact on the design of a global communication system operating at 20 GHz. The system under consideration has a total atmospheric link attenuation budget that is less than 6 dB. For this relatively small link margin, rain, cloud, and molecular attenuation have to be taken into account. For an assessment of system performance on a global basis, attenuation models are utilized. There is concern whether current models can adequately describe the atmospheric effects such that a system planner can properly allocate his resources for superior overall system performance. The atmospheric attenuation as predicted by models will be examined.

Shimabukuro, Fred I.↗
NASA NTRS1973

The influence of polarization on millimeter wave propagation through rain

The measurement and analysis of the depolarization and attenuation that occur when millimeter wave radio signals propagate through rain are described. Progress was made in three major areas: the processing of recorded 1972 data, acquisition and processing of a large amount of 1973 data, and the development of a new theoretical model to predict rain cross polarization and attenuation. Each of these topics is described in detail along with radio frequency system design for cross polarization measurements.

Bostian, C. W.↗
NASA NTRS2000

Methods of Attenuation Correction for the TRMM Precipitation Radar Data

The surface reference technique (SRT) has been studied extensively both theoretically and experimentally over the last decade. It is only with the launch of the Tropical Rain Measuring Mission (TRMM) Precipitation Radar, however, that we can begin to test directly whether the technique provides a reliable means of attenuation correction for spaceborne weather radars. Preliminary results indicate that the method yields results that are qualitatively reasonable when the rain rate is moderate or high and when the surface provides a stable reference value. The structure of the normalized radar cross section of the surface (NRCS), however, is highly complex in the sense that the statistics change with background type, incidence angle, location, and season. As a consequence of this, the reliability of the path attenuation estimate is not proportional simply to the amount of rain attenuation but to the rain attenuation relative to the inherent variability of the reference target. To illustrate the behavior of the SRT, several overpasses of the Hurricane Bonnie are shown. We also show that the proper choice of reference data set (spatial, temporal and global) can be critical to the success of the method.

Meneghini, Robert↗
NASA NTRS1983

Propagation effects on satellite systems at frequencies below 10 GHz, a handbook for satellite systems design, 1st edition

Satellite communications below about 6 GHz may need to contend with ionospheric effects, including Faraday rotation and ionospheric scintillation, which become increasingly significant with decreasing frequency. Scintillation is most serious in equatorial, auroral, and polar latitudes; even the 4 to 6 GHz frequency range turns out to be subject to scintillation to a significant degree of equatorial latitudes. Faraday rotation, excess range or time delay, phase advance, Doppler frequency fluctuations, and dispersion are proportional to total electron content (TEC) or its variation along the path. Tropospheric refraction and fading affects low angle satellite transmissions as well as terrestrial paths. Attenuation and depolarization due to rain become less important with decreasing frequency but need consideration for frequencies of about 4 GHz and higher. Empirically derived relations are useful for estimating the attenuation expected due to rain for particular percentages of time. Aeronautical, maritime, and land mobile satellite services are subject to fading due to multipath propagation.

Flock, W. L.↗
NASA NTRS1978

Millimeter wave propagation modeling of inhomogeneous rain media for satellite communications systems

A theoretical propagation model that represents the scattering properties of an inhomogeneous rain often found on a satellite communications link is presented. The model includes the scattering effects of an arbitrary distribution of particle type (rain or ice), particle shape, particle size, and particle orientation within a given rain cell. An associated rain propagation prediction program predicts attenuation, isolation and phase shift as a function of ground rain rate. A frequency independent synthetic storm algorithm is presented that models nonuniform rain rates present on a satellite link. Antenna effects are included along with a discussion of rain reciprocity. The model is verified using the latest available multiple frequency data from the CTS and COMSTAR satellites. The data covers a wide range of frequencies, elevation angles, and ground site locations.

Persinger, R. R.↗