Melting layer attenuation at 28.6 GHz from simultaneous comstar beacon and polarisation diversity radar data

Attenuation data at 28.6 GHz obtained from measurements of the Comstar beacon show that, for moderate rain, slant path attenuation may significantly exceed that calculated from simultaneous radar reflectivity measurements. Polarisation diversity radar data were used for positive identification of the rain and the melting layer, and for estimating the rain attenuation along the path. These results indicate that the melting layer attenuation is significant.     

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).     

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.     

Attenuation and space diversity statistics calculated from radar reflectivity data of rain

During the summer of 1973 the rain reflectivity environment in three-dimensional space was routinely recorded on digital tape at Wallops Island, Va. A mode of operation consisted of sampling periodically60degazimuth intervals over regions in which the rain activities were most intense and widespread. A series of plan-position indicator (PPI) sweeps over these intervals were implemented at a sequence of elevation angles starting from0.5degup io an angle above which the reflectivity values were below a designated threshold level. Approximately 500 such raster scans were acquired in which each scan was obtained in less than 4 min and covered a range interval of 10 to 140 km. Using the above data base, reflectivity profiles along representative earth-satellite paths were determined from which attenuation and space diversity statistics were calculated at the frequencies of 13 and 18 GHz. Specifically, the formk = aZ^{b}was used to deduce the total path attenuation, wherekis the attenuation coefficient (dB/km), andZis the reflectivity factor (mm⁶/m³). The constantsaandbwere calculated using the raindrop distribution for thunderstorm activity as proposed by Joss. Probabilities that the attenuations exceed given fade depths, diversity gain as a function of fade depths, and diversity gain as a function of site separation distances are characterized and compared at the various frequencies. For the space diversity mode, a site spacing of 15 km is shown to give rise to a near optimum condition for the diversity gain. Furthermore, the diversity gain has been demonstrated to be minimally influenced by the transmitter frequency.     

Derivation of parameters of Y-Z power-law relation from raindrop size distribution measurements and its application in the calculation of rain attenuation from radar reflectivity factor measurements

The work reported herein empirically derived the coefficients of the power-law relation between the specific attenuation Y and each of two other quantities: the radar reflectivity factor Z and the rainfall rate R. The derivation was accomplished using raindrop size distributions (DSDs) measured in Montreal and Toronto using the precipitation occurrence sensor system (POSS) and a Joss-Waldvogel disdrometer (JWD). The specific attenuation was calculated for both spherical and oblate spheroidal raindrops. Prior to doing so, the effects of inaccuracies in small drop detection by the two systems on the resulting Y-Z and Y-R relations were examined. In computing the relations, the influence of grouping the DSDs according to the corresponding values of Z was assessed. The results from the two sites were then combined in a regression analysis to determine the coefficients of the power-law expressions. The final expressions were used to calculate the probability distribution of rain attenuation over several path lengths at 30 GHz. Conclusions that can be drawn from this work include that (i) the uncertainties in the measurements of small drops do not appear to be critical to the derived relations for some frequencies considered in this work, (ii) in computing rain attenuation estimates from radar reflectivity factors, different criteria for grouping the reflectivity factor measurements can be adopted without changing the statistics of the corresponding attenuation estimates, (iii) differences, which are the smallest for frequencies around 20-30 GHz, are observed in the coefficients derived from the measured DSDs compared with those recommended by the ITU-R, and (iv) the attenuation statistics at 30 GHz computed by applying the Y-Z power-law expression are resistant to spatial averaging.     

Scintillation effects on total fade distributions forEarth-satellite links

Scintillation effects on the cumulative fade distributions for Earth-satellite links are considered. Two combination methods for scintillation and attenuation distributions are compared using filtered and unfiltered 11.2 GHz beacon measurements from Austin, TX, on a 5.8° elevation path. A combination method based on the assumption of independent scintillation and attenuation phenomena is proposed. Attenuation and total fade distributions at 19.8 and 29.7 GHz from Kirkkonummi, Finland, on a 12.7° elevation path, are presented for a summer period. The effects of attenuation and scintillation are also calculated using meteorological data from several radiosonde stations. The calculations show that the relative significance of the scintillation is smaller at frequencies from 20 to 50 GHz than at 12 GHz     

Prediction of slant path rain propagation statistics usingdual-polarized radar

The authors conducted a year-long experiment in which a dual-polarized S-band radar probed the volume surrounding two 11.45-GHz satellite downlink paths during rain. Accuracy was assessed by comparison to directly measured link attenuation with two 11-GHz beacon receivers 7.3-km apart at an 18.5° elevation angle, one colocated with the radar. Drop size distributions calculated from the radar horizontal reflectivity (Z_H) and differential reflectivity (ZDR) measurements were used to predict 11.45-GHz satellite beacon attenuations. The radar-predicted attenuations and those measured on the radio links agree, both on an event basis and in terms of annual cumulative distributions     

Correcting C-band radar reflectivity and differential reflectivitydata for rain attenuation: a self-consistent method withconstraints

Quantitative use of C-band radar measurements of reflectivity (Z _h) and differential reflectivity (Z_dr) demands the use of accurate attenuation-correction procedures, especially in convective rain events. With the availability of differential phase measurements (Φ_dp) with a dual-polarized radar, it is now possible to improve and stabilize attenuation-correction schemes over earlier schemes which did not use Φ_dp. The recent introduction of constraint-based correction schemes using Φ_dp constitute an important advance. In this paper, a self-consistent, constraint-based algorithm is proposed and evaluated which extends the previous approaches in several important respects. Radar data collected by the C-POL radar during the South China Sea Monsoon Experiment (SCSMEX) are used to illustrate the correction scheme. The corrected radar data are then compared against disdrometer-based scattering simulations, the disdrometer data being acquired during SCSMEX. A new algorithm is used to retrieve the median volume diameter from the corrected Z_h, corrected Z_dr , and K_dp radar measurements which is relatively immune to the precise drop axis ratio versus drop diameter relation. Histograms of the radar-retrieved D₀ compared against D₀ from disdrometer data are in remarkable good agreement lending further validity to the proposed attenuation-correction scheme, as well as to confidence in the use of C-band radar for the remote measurement of rain microphysics     

Prediction methods for rain attenuation statistics at variable path angles and carrier frequencies between 13 and 100 Ghz

Fade depth and space diversity statistics of propagation along earth-satellite paths have been calculated from radar reflectivity data of rain using modeling procedures. The reflectivity data base was obtained during the summer of 1973 at Wallops Island, Va., using a high resolutionSband radar interfaced with a computer and digital processing system. Fade statistics have been calculated for various path angles at several frequencies between 13 and 100 GHz. Subsequent analysis has demonstrated the ability to predict the following: 1) fade statistics at other path elevation angles given similar type statistics at a particular path angle, 2) space diversity statistics at other frequencies, given similar type statistics at a particular frequency, and 3) fade statistics at a third frequency given similar type statistics at two other frequencies. Although a specific data base was used pertaining to the climatology at Wallops Island, the techniques developed are general and may be applied to existing or future "fade measurements" at other climatological locations.     

Empirical models for rain fade time on Ku- and Ka-band satellitelinks

Rain attenuation data from the OLYMPUS satellite beacon measurements are used to investigate fade time in the Ku and Ka bands. Using statistical procedures, an empirical model is developed which predicts fade time as a function of attenuation level, frequency of operation, and fade duration interval. Total annual fade times are predicted over a frequency span of 12-30 GHz for attenuation levels in the 3-18 dB range and for fade duration intervals of 30-60 s, 60-120 s, 2-5 min, and 5-20 min. The predicted fade times are in good agreement with the measured values. An alternate model, described by two simple relationships in two different ranges of attenuation level, is also presented by simplifying the original single-equation model. The simplified model accounts for fades associated with stratiform rain and thunderstorms separately     

Scattering of radiowaves by a melting layer of precipitation inbackward and forward directions

A melting layer of precipitation is composed of melting snowflakes (snow particles); the assumption of spherical particles along with mass conservation is used. The melting layer is studied by deriving the size distribution of the melting snow particles, the thickness of a melting layer, the density of a dry snow particle, and the average dielectric constant of a melting snow particle. Vertical profiles of radar reflectivity and specific attenuation are computed at 1-100 GHz by using the Mie theory for five raindrop size distributions at rain rates below 12.5 mm/h. The radar bright band is explained with computed radar reflectivities at 3-10 GHz. It is shown that the radar bright band can be absent in the melting layer at frequencies above 20 GHz. This agrees with radar observations at 35 and 94 GHz. The specific attenuation, as well as the average specific attenuation of the melting layer, is divided into absorption part and scattering part. The latter is increasingly significant with the increase of frequency. The total zenith attenuation due to stratiform rain is divided into the rain zenith attenuation and the additional zenith attenuation, which is the difference between zenith attenuation, due to the melting layer, and attenuation, due to the same path length of the resulting rain. The additional zenith attenuation increases with the increase of rain rate even at frequencies above 20 GHz. This should be taken into account in radar remote sensing and satellite-Earth communications     

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.     

Results from the Virginia Tech propagation experiment using theOlympus satellite 12, 20 and 30 GHz beacons

A comprehensive set of propagation experiments was performed using the Olympus satellite 12, 20, and 30 GHz beacons. This set of experiments is unique in North America because of simultaneous reception of signals spanning the Ku- and Ka-bands from the same orbital slot, which permits direct inference of the frequency behavior of signal variations. The elevation angle from the receiving site in Blacksburg, VA, to the satellite was 14 degrees. Beacon, radiometric, and weather data for one year were analyzed. The statistical results for rain rate, beacon attenuation, attenuation ratios, radiometrically derived attenuation, fade duration and fade slope are presented. They are important to the design of Ku- and Ka-band satellite communication systems. The beacon attenuation results include cumulative statistics for attenuation with respect to free space and with respect to clear air. Attenuation ratio data are presented using attenuation with respect to clear air to focus on rain effects. Instantaneous attenuation ratios computed from instantaneous beacon levels were found to be nearly identical to statistical attenuation ratios obtained from cumulative attenuation statistics at each frequency     

Radar and atmospheric electricity measurements associated with microwave attenuation and cross-polarization observed on a slant path

The present paper describes experiments performed near Paris using a C-band radar aimed at estimating rain attenuation and cross-polarization discrimination of the various ots signals received at Gometz-La-Ville. An empirical relationship between the measured reflectivity factor and attenuation is derived, allowing an experimental prediction. Correlation between radar reflectivity and cross-polarization is also investigated for a convective precipitation cell. In addition, electrostatic field measurements show a relation between fade depth and electric field intensity.     

Centimeter wave propagation experiments using the beacon signals of CS and BSE satellites

The wave propagation experiments using Japanese geostationary satellites CS (20/30 GHz) and BSE (12/14 GHz) satellites have been performed at the Kashima earth station of the Radio Research Laboratories (RRL). Cumulative rain attenuation and cross-polarization discrimination (XPD) statistics are given for the period of three years at 11.7 GHz (vertical polarization) and for the period of four years at 19.5 GHz (circular polarization). It is shown that the yearly rainfall rate and attenuation distributions are well approximated by log-normal distributions, and the XPD distribution is well approximated by a normal distribution. Monthly and time-of-day variation of the attenuation and XPD distributions are presented. Duration statistics of attenuation and XPD are presented and characterized. Other characteristics in the wave propagation, such as effective path length, frequency dependence of attenuation, and joint statistics of attenuation and XPD are derived and discussed. Rainfall events are classified into three rainfall types, "stratus," "cumulus," and "others" using measurements of the radar reflectivity factor along the satellite-to-earth path, and the dependence of XPD characteristics on the rainfall type is also presented and discussed. Some prediction methods of calculating attenuation and XPD statistics are applied to the data obtained in these experiments and the predicted results are compared with the measured ones. It is found that some corrections are needed when the XPD statistics are predicted from the attenuation statistics using the theoretical relation between XPD and attenuation.     

Backward and forward scattering by the melting layer composed ofspheroidal hydrometeors at 5-100 GHz

This paper addresses the behavior of the differential reflectivity, specific attenuation, and specific phase shift due to a melting layer composed of oblate-spheroidal hydrometeors. The results are based on a melting layer model and scattering computations derived from the point-matching technique with the truncation and recurrence adjusted. Computations at 5-100 GHz for five raindrop size distributions at rain rates below 12.5 mm/h are presented. In general, the reflectivity factor and differential reflectivity features with height at centimeter wavelengths agree with available radar measurements. At millimeter wavelengths, contributions to the radar backscatter from smaller hydrometeors become more and more important as the frequency increases and approaches 100 GHz. This should be instructive for utilizing millimeter wavelength radar techniques in radar remote sensing studies of the melting layer. Corresponding vertical profiles of the specific attenuation and phase shift are also presented at 5-100 GHz. The differential attenuation and phase shift indicate the particle shape effects. These attenuation and phase shift become more and more considerable as the frequency increases. Such forward scattering calculations should prove useful for studying propagation effects caused by the melting layer for satellite-earth communications, including depolarizations     

Path attenuation statistics influenced by orientation of rain cells

The influence of path azimuth on fade and space diversity statistics associated with propagation along earth-satellite paths at a frequency of 18 GHz is examined. The approach utilizes the methods employed by Goldhirsh and Robison [1], and Goldhirsh [2], in which a radar rain reflectivity data base obtained during the summer of 1973 is injected into a modeling program and the attenuation along parallel earth-satellite paths are obtained for a conglomeration of azimuths. In this work the statistics are separated into two groupings: one pertaining to earth-satellite paths oriented in the northwest-southeast and the other in the northeast-southwest quadrants using a fixed elevation angle of45/deg. The latter case shows fading to be greater with a degraded space diversity suggesting rain cells to be elongated along this direction. Great circle distance intervals along which the path attenuations are greater than a fixed threshold level (i.e., 2 dB) are defined here as "attenuation cell sizes." These cell dimensions are analyzed for both sets of quadrants and are found to have average values which are larger by 2 km in the northeast-southwest quadrants; a result consistent with the fade and space diversity results. Examination of the wind direction for the 14 rain days of data analyzed shows good correlation of the average or median wind directions with the directions of maximum fading and degraded space diversity.     

Comparison of radar derived slant path rain attenuations with the COMSTAR beacon fades at 28.56 GHz for summer and winter periods

New results are described from an experiment for testing and improving the accuracy of radar derived slant path attenuations of the 28.56-GHz COMSTAR beacon signal using an electromechanical disdrometer located at Wallops Island, VA. Continued overall good agreement between measured and radar derived fades were noted on a case-by-case basis and statistically for the new data base.