Rain-rate duration statistics over a five-year period: a tool forassessing slant path fade durations

A rain gauge network of 10 tipping bucket rain gauges on the Mid-Atlantic coast of the United States has been in continuous operation since June 1, 1986. Rain-rate distributions and estimated slant path fade distributions at 20 and 30 GHz covering the first five-year period have been derived from the gauge network measurements and published data of Goldhirsh, Krichevsky and Gebo (see ibid., vol.40, no.11, p.1408, 1992). In this article, we present rain-rate time duration statistics. The conversion of rain-rate duration statistics derived from in situ measurements to slant path fade duration statistics is complicated because of the vertical and lateral inhomogeneity of the rain. A benchmark set of fade duration statistics at 20 and 30 GHz for a vertical path is derived from the rain-rate duration statistics employing Crane's (1980) global model. These results may be used by investigators for comparison with and/or conversion to slant path fade duration statistics. Such statistics are important for better assessing optimal coding procedures over defined bandwidths     

Rain rate statistics and fade distributions at 20 and 30 GHzderived from a network of rain gauges in the mid-Atlantic coast over afive year period

The authors examine five years of rain rate and modeled slant path attenuation distributions at 20 and 30 GHz derived from a network of ten tipping bucket rain gauges located on the mid-Atlantic coast of the US in the vicinity of Wallops Island, VA. Distributions are derived from the variable integration time data from 1-min averages or rain rate time series. It is demonstrated that for realistic fade margins at 20 GHz and above, the variable integration time results are adequate to estimate slant path attenuations using models which require 1-min averages. An accurate empirical formula is developed to convert the variable integration time rain rates to 1-min averages. Rain rate and fade distributions corresponding to the overall network average, yearly network average, and site distributions averaged over the five-year period are derived. Significant differences in the distributions are noted for the year-to-year (temporal variability) and the worst year site-to-site (spatial variability) cases     

Mobile satellite system propagation measurements at L-bandusing 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     

Multipath fading at L band for low elevation angle, land mobilesatellite scenarios

We characterize multipath fading at low elevation angles for unshadowed line-of-sight propagation over a land-mobile satellite link. Equipment aboard a van recorded a CW signal from INMARSAT's geostationary satellite MARECS-B2 while driving in the states of Utah, Nevada, Washington, and Oregon. Elevation angles ranged from 7° to 14°. The van carried a tracking helix antenna with beamwidths in the principal planes of approximately ±18°. Multipath fading was observed in flat and hilly terrain and is compared with an example of fading due to tree shadowing. The measurements demonstrate that multipath effects at low elevation angles may result in fades exceeding 7 dB for approximately 1% of the driving distance for a worst case scenario corresponding to driving in rolling, hilly terrain where the satellite is in front or behind of the vehicle. Fading in such a case is typically dominated by a single multipath reflection from a terrain feature. On the other hand, tree shadowing at low elevation (7°) may result in fades which exceed 25 dB at the 1% level     

Harmonic Analysis of an Array Comprised of Two Coupled Directive Antennas

A harmonic analysis is performed and the radiation patterns are deduced for an antenna system comprised of two parallel half-wave dipoles with parasitic directors above a reflecting plane where the mutual interactions of all the elements are considered. This work represents an extension of a previous analysis in which only a single dipole with a director above a reflecting plane was considered [1]. The results illustrate the importance of the radiated interference at the harmonic frequencies that may be introduced when sufficient harmonic power is available at the antenna input. As a criterion to determine the ?worst case? situation, the antenna is assumed to be matched and fed with equal power at each of the harmonics. The resultant field patterns show the following characteristics. 1) The presence of the director introduces side lobes at the fundamental frequency in both the meridional and azimuthal planes. These lobes peak at levels of 9 and 14 dB down, respectively, when compared with the density in the forward direction. 2) The higher the harmonic, the smaller the role played by the presence of the directors. At the seventh harmonic a maximum 1.5-dB difference in value exists when compared with the same antenna configuration when the directors are absent. 3) In the meridional plane at the higher harmonics, side lobe densities exist (e.g., at the fifth and seventh harmonics) that are as intense as the power density in the forward direction for the fundamental frequency.     

Cumulative slant path rain attenuation statistics associated with the Comstar beacon at 28.56 GHz for Wallops Island, VA

Cumulative slant path rain attenuation statistics at 28.56 GHz are given for the year period April 1, 1977 through March 31, 1978 for Wallops Island, VA. These results were arrived at using the direct measurements of a beacon signal emanating from the Comstar geosynchronous satellite. Yearly, monthly, and time-of-day fade statistics are presented and characterized. In addition, a 19.04-GHz yearly fade distribution, corresponding to a second Comstar beacon frequency, is predicted using the concept of effective path length, disdrometer, and rain-rate results. Specifically, it is shown that the yearly attenuation and rain-rate distributions follow with good approximation lognormal variations for most fade and rain-rate levels, respectively. Attenuations were exceeded for the longest and shortest periods of times for all fades in August and February, respectively. These months thus represented the "worst" and "best" months at all attenuation levels. The eight-hour time period showing the maximum and minimum number of minutes over the year for which fades exceeded 12 dB were approximately between 1600-2400, and 0400-1200 h (local time), respectively. In employing the predictive method for obtaining the 19.04-GHz fade distribution, it is demonstrated theoretically that the ratio of attenuations at two frequencies is minimally dependent on raindrop-size distribution, providing these frequencies are not widely separated (such as 28 and 19 GHz).     

Rain Cell Size Statistics Derived from Radar Observations at Wallops Island, Virginia

Results of an investigation are given describing two-di-two-dimensional rain cell size statistics employing radar and disdrometer data bases spanning more than 5 years and all seasons. Radar data were obtained employing the SPANDAR radar located at the NASA Wallops Flight Facility, Wallops Island, Virginia. These data corresponded to approximately 100 low-elevation azimuthal antenna scans encompassing 17 rain days. For each of these rain days, disdrometer data were also obtained. Analysis of the regression relations relating the rain rates to the disdrometer data enabled the determination of least square fit radar reflectivity factors. The individual disdrometer results along with the radar data were employed with a contouring program. More than 22 000 contours were generated, where each isopleth belonged to predefined rain rate intervals. Computing the areas of each of these contours, statistics were generated relating the equi-circle contour diameters and rain rate categories. Two types of contour have been analyzed. One pertained to the above-described isopleth regardless of the rain rate levels interior to it. Another type corresponded to those isopleths in which the rain rates interior to it were equal or greater than the isopleth values. These isopleths were referred to as "total" and "cell" contours, respectively. An abundance of total and cell contours were observed belonging to all rain rate categories. In particular, a dominant number of smallarea contours were observed belonging to the lower rain rate levels. The results showed that the average and median "equi-circle" cell contour diameters were 2.4 and 1.9 km, respectively.     

Attenuation of propagation through rain for an earth--Satellite path correlated with predicted values using radar

During the summer of 1974 and spring of 1975, measurements of attenuation of propagation through rain were made at Wallops Island, VA, using 13 and 18 GHz transmitters operating in the uplink mode toward the ATS-6 satellite. Simultaneously, rain reflectivity levels were measured along the earth-satellite path using a high resolution (0.4degbeamwidth)S-band radar having a scanning antenna. Four raingages and two disdrometers were also located in the vicinity of the transmitters. The radar and disdrometer data were used in a modeling program to predict attentuation levels which were subsequently compared to the directly measured fades over nearly simultaneous time intervals. Predicted attentuation levels were obtained for three drop size distributions; namely, those of Joss et al. for thunderstorm activity, Marshall-Palmer, and the average distribution measured in the vicinity of the transmitter (APL distribution). Comparisons between predicted and measured attenuation levels showed the APL dropsize distribution gave the smallest rms difference of 1.3 dB at 13 and 18 GHz although the rms difference corresponding to Marshall-Palmer was close to this value. Although the sample sizes were relatively small, the good agreement suggests the validity of using radar to model path attenuation to obtain attenuation statistics.     

Radar prediction of absolute rain fade distributions for Earth-satellite paths and general methods for extrapolation of fade statistics to other locations

Two distinct prediction methods are described. The first deals with a technique for establishing absolute fade statistics at a given site using a sampled radar data base. The second is a method for extrapolating absolute fade statistics from one location to another given simultaneously measured fade and rain-rate statistics at the former. Both methods employ similar conditional fade statistic concepts and use long term rain-rate distributious. The radar-predicted levels showed good agreement in probabilities associated with the cumulative fade distribution when compared to directly measured levels of the COMSTAR beacon at 28.56 GHz for the Wallops Island, VA, radar facility, Probability deviations ranging from 2 to 19 percent with an average of 11 percent were obtained upon comparing measured and predicted levels at given attenuations. The extrapolation method was tested employing the Wallops Island measured rain-rate statistics and fade distributious at 28.56 GHz. The method was also tested at 19.04 GHz using measured results for Austin, TX. The extrapolation of fade distributions to other locations at 28 GHz showed very good agreement with measured data at three sites located in the continental temperate region (average probability deviations of 11.5, 9, and 17 percent) and respectable agreement atone site in a wet subtropical region (30 percent deviation). At 19.04GHz,extrapolated levels were generally not as good as at 28 GHz (41, 28, 7, and 27 percent), although they were still quite respectable.     

Multiyear slant-path rain fade statistics at 28.56 GHz for Wallops Island, VA

Multiyear rain fade statistics at 28.56 GHz have been compiled for the region of Wallops Island, VA, covering the time periods April 1, 1977-March 31, 1978, and September 1, 1978-August 31, 1979. The 28.56-GHz attenuations were derived by monitoring the beacon signals from the Comstar geosynchronous satellite,D_{2}, during the first year, and satellite,D_{3}, during the second year. Comparisons are made of yearly, monthly, and time of day fade statistics for the first, second, and combined years. Although considerable year to year variations in exceedance times exist for the monthly and time of day fade statistics, the overall fade distributions for the individual years showed relatively small differences. For example, comparing the second year fades relative to those of the first year at equal percentages of time, less than 20 percent rms deviation was found. The year to year variations of rain rate distributions are also examined and show consistently small differences. The resultant fade distribution at 28.56 GHz for Wallops Island, VA, are compared with that arrived at using a prediction method which is a recent refinement of the International Radio Consultative Committee (CCIR) global model, and an rms decibel deviation of less than 14 percent was noted.