Search NASA⌕ Search

Engineering topics

Pratt, T.

Publications and source records attributed to Pratt, T..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Hysteresis effects in instantaneous frequency scaling of attenuation on 20 and 30 GHz satellite links

It has been observed with 20/30 GHz satellite beacon measurements that the ratio of 30 GHz to 20 GHz attenuation changes during some fade events. This ratio displays a hysteresis effect. This effect can be explained by a change in the drop size distribution (DSD) during the event. However, it appears only above approximately 6-8 dB of attenuation at 20 GHz. Instantaneous frequency scaling of attenuation is being proposed as part of an algorithm for uplink power control (ULPC) and the dynamic range of such an algorithm must be appropriately limited to avoid the hysteresis.

Sweeney, D. G.↗
NASA NTRS1990

Propagation experiments at Virginia Tech with the Olympus satellite

A major propagation experiment is in progress to measure atmospheric attenuation at 12, 20, and 30 GHz using the beacons of the Olympus satellite. The experiment uses a frequency-locked receiver system which locks to the 12-GHz beacon frequency and measures the signal strength of the coherent 20- and 30-GHz beacons in a 3-Hz bandwidth. The receiving system is augmented with radiometers in each beacon frequency band to set the clear-air attenuation levels of the beacon signals. Olympus propagation experiment objectives and the measurement system are described. Results from the measurement program will include fade statistics at all frequencies, attenuation scaling between frequencies, short baseline site diversity, and fade dynamics.

Pratt, T.↗
NASA NTRS1989

Research in LMSS propagation

The Virginia Tech Satellite Communications Group has participated in the Land Mobile Satellite System (LMSS) program through JPL sponsorship since 1985. Involvement has mainly been in modeling and simulation of propagation characteristics and effects. Models developed to predict cummulative fade distributions for fading LMSS signals include LMSSMOD and the Simple Models which approximate LMSSMOD. Models to predict the mean and standard deviation of signal attenuation through roadside vegetation, namely the Average Path Model, were developed. In the area of simulation, efforts have centered around the development of a software simulator that uses data bases derived from experimental data to generate simulated data with arbitrary statistical behavior. This work has progressed to the development of an integrated analysis and simulation package, LIPS. The basic theory and results for the models and simulator have been previously documented in reports and papers. All LMSS activities are summarized and details of this year's efforts are given.

Barts, R. M.↗
NASA NTRS1989

Olympus propagation and communication experiments in the US

The participation of the United States in Olympus experiments is outlined. These include, basic propagation measurement, the Cooperative Data Exchange Experiment (CODE), a rain scatter interference measurement, a small scale diversity experiment, the Personal Access Satellite System (PASS), and all systems that use adaptive fade compensation techniques similar to the Advanced Communications Technology Satellite (ACTS) of NASA.

Davarian, F.↗
NASA NTRS1986

Results of a three-year 11.6 GHz low-angle propagation experiment using the SIRIO satellite

The results of three years of attenuation and depolarization measurements at 11.6 GHz on a 10.7 deg elevation angle downlink from the SIRIO spacecraft are summarized. Cumulative distributions and worst-month statistics for rain rate, attenuation, and cross-polarization discrimination (XPD) are presented. Beacon attenuation and radiometric attenuation are compared, and the statistics of fade durations and of rapid XPD changes are discussed.

Bostian, C. W.↗
NASA NTRS1984

Ice depolarization on low-angle 2 GHz satellite downlinks

The impact of ice depolarization on the statistical performance of satellite downlinks were investigated. Propagation data recorded during 1979 and 1980 to see what impact of ice depolarization on link performance were analyzed. The effects on the cross polarization discrimination (XPD) statistics amounted to at most a 2 to 4 dB reduction in the XPD values which rain would have produced for a given percentage of time. Ice depolarization had no effect on the statistics of XPD values below the 0.01% level. Most of the severe ice depolarization events were associated with drops in barometric pressure and the passage of intense cold fronts through our area. Ice contents as the product of three individually undetermined quantities were defined: ice particle density, ice cloud thickness, and the average volume of the ice crystals. It is indicated that populations of ice particle with ice contents on the order of 0.002 m4/m3 are probably responsible for the lower values of measured XPD.

Stutzman, W. L.↗
NASA NTRS1984

A generalized analysis of dual-polarization radar measurements of rain

In the study of meteorological phenomena by radar, it has been shown that dual-polarization radar measurements can yield useful data about precipitation structures within the radar beam. The techniques available for dual-polarized radar measurement are differential reflectivity, or Z(DR), linear depolarization ratio (LDR), and circular depolarization ratio (CDR). This paper presents a theoretical analysis of backscattering from a volume filled with raindrops using Rayleigh scattering theory. A generalized radar equation is given in matrix form, and the dependence of backscatter measurements in terms of a rainstorm model including raindrop shape, size, and canting angle is examined. Calculated results which show the dependence of Z(DR) on beam elevation angle and polarization angle are presented. Some results for LDR and CDR are presented, showing how these quantities depend on raindrop parameters.

Stapor, D. P.↗
NASA NTRS1984

S band radar differential reflectivity measurements in multiple polarization planes along satellite slant paths

A novel approach to making differential reflectivity measurements in multiple polarization planes has been proposed. This paper describes a versatile technique using a mechanical polarization switch and the systematic errors in differential reflectivity measurements which result. Initial results of simultaneous X band slant path measurements of attenuation and depolarization at 11.6 GHz and differential reflectivity measurements at 2.8 GHz using a slow polarization switch are presented.

Marshall, R. E.↗
NASA NTRS1984

Instantaneous diversity gain in 10-30 GHz satellite systems

As usually defined, diversity gain is a statistical measure which conveys little or no information about the instantaneous behavior of site-diversity reception in a satellite communications system. A new quantity called instantaneous diversity gain is introduced and some measurements of it from an 11.6 GHz low-angle two-site downlink are presented. It is shown how instantaneous diversity gain is related to system reliability and some results are presented which indicate that designs based on statistical diversity gain will achieve their intended reliability levels.

Towner, G. C., III↗
NASA NTRS1983

The VPI&SU multiple polarization plane ZDR radar - The OCTOPOD radar

The 'OCTOPOD' radar extends the concept of differential reflectivity (ZDR) to multiple polarization planes, so that ZDR measurements can be made in any pair of orthogonal linear polarizations. This allows the mean canting angle of oblate raindrops or ice particles to be determined. Attention is given to the radar hardware employed, the rotating feed system that generates the multiple polarization planes, and the computer controlled data acquisition system which extracts both reflectivity and ZDR data.

Andrews, J. H.↗
NASA NTRS1983

Slant path attenuation and cross-polarization prediction

The accuracy with which slant-path attenuation and cross-polarization are predicted using differential reflectivity (ZDR) radar data is discussed. Consideration is given to the use of generalized relationships between radar reflectivity (Z), ZDR, and rainfall rate (R); and between R, attenuation, and cross-polarization (XPD) on a slant path. The forward-scatter matrix which is used to calculate the slant path attenuation and XPD is described. It is shown that the use of a double exponential drop size distribution and forward-scatter matrices improves the accuracy of the prediction of slant-path attenuation and XPD. Data obtained with the VPI & SU S-band radar, using slow switching between orthorzonal polarizations is presented. The results are illustrated in graphs.

Pratt, T.↗
NASA NTRS1983

Attenuation and depolarization of low angle and site diversity slant paths

This paper reviews some experimental data collected over a three year period at Blacksburg, VA. Attenuation and crosspolarization were measured on two 10.7 degree elevation angle paths to the SIRIO satellite, with a baseline separation between the two receiving sites of 7 km. Results for attenuation and crosspolarization at the two sites are presented, together with some limited data for joint performance of the two sites when used in a diversity mode.

Pratt, T.↗
NASA NTRS1983

System performance of dual polarization satellite communication links affected by attenuation and depolarization

New models have been developed to analyze the performance of dual polarization frequency re-use links when affected simultaneously by attenuation and depolarization. The models have been used to determine the baseband signal to noise ratio for analog modulation links using FM and bit error rate for links using digital modulation (PSK). The methods are applicable to links using slant paths to satellites and also to terrestrial links.

Pratt, T.↗
NASA NTRS1982

Results of the VPI&SU Comstar experiment

This paper summarizes annual and cumulative attenuation data, depolarization data, and associated local rain rate distributions obtained with the Comstar family of 19.04- and 28.56-GHz satellite beacons during the years 1977-1981. It discusses the relationships between attenuation and rain rate and between attenuation and depolarization, compares measured data on the joint distribution of attenuation and depolarization, and examines the limitations that propagation effects will impose on future 20/30-GHz satellite communications systems.

Andrews, J. H.↗
NASA NTRS1982

Initial results from the VPI&SU SIRIO diversity experiment

The first year of observations of the precipitation effects on the 11.6-GHz beacon signal from the SIRIO satellite are reported. This experiment is unique in that it uses dual-polarized receivers in a diversity configuration and a low path elevation angle. Rain rate, attenuation, and isolation statistics are presented for both sites. In addition, attenuation diversity gain is plotted and compared to current models. Isolation diversity gain is also discussed. It was found that little improvement in dual-channel isolation is obtained with site diversity at this low elevation angle. The influence of ice clouds was noted in the months of October and November.

Towner, G. C.↗