Since it is becoming increasingly difficult to obtain frequency assignments below 10 GHz for ground‐to‐aircraft wideband data links, frequencies in the window regions of the millimeter wave spectrum are now being considered for this application. In order to estimate system performance at these frequencies, the atmospheric attenuation along the path must be known. In this paper, 440 sets of slant‐path attenuation data at frequencies of 15 and 35 GHz are presented. The attenuations are measured with an 8.8 m paraboloidal antenna located at Prospect Hill, Waltham, Massachusetts, using the sun as a source. Meteorological data such as surface temperature, pressure and absolute humidity were recorded at the time of the measurements along with somewhat subjective descriptions of cloud conditions along the path. The systematic measurement and recording of a substantial amount of attenuation data at many elevation angles, under variable atmospheric conditions and at two frequencies, has provided a data base which permits an in‐depth statistical analysis of the relationships between attenuation, elevation angle, and atmospheric conditions. The mean attenuations at both 15 and 35 GHz decrease smoothly with increasing elevation angle and at 0°range from 5.44 to 8.80 dB and 15.47 to 24.12 dB respectively for clear to cloudy conditions. At an angle of 5° the mean attenuations at 15 and 35 GHz have a range of .69 to 1.08 dB and 2.45 to 9.02 dB respectively. For regions having a climatology comparable to that of the Boston area, the mean attenuations can be estimated from regression lines derived from the data.
Tropospheric refraction introduces errors into radar and radio communication systems by causing radio waves to travel along a curved path and at a speed which changes with position. Common error correction techniques, such as making estimates of refractive effects from surface refractivity, rely implicitly on the assumption that the troposphere is horizontally stratified. This study demonstrates that such an assumption may be unwarranted especially at elevation angles below 10°, and that such reliance can lead to substantial errors. The end product of this research is a system which employs atmospheric emission measurements in addition to surface refractivity to estimate refractive bending and range error. This approach relies on the similar dependence of refraction and emission on atmospheric parameters to account for nonhorizontal stratification. Performance results establish that such a system need employ no more than one radiometer channel to provide potential error reduction of more than 40% over conventional approaches, along with a potentially automated, real‐time operational capability.
The main objective of this paper is to examine the frequency dependence of slant path rain attenuation at 15 and 35 GHz in order to determine how well the attenuation at one frequency can be predicted from that at another; this has application in the design of earth-to-satellite communications. Also, some inferences on the drop size distribution of rain can be drawn from simultaneous attenuation measurements at two frequencies. The theory of rain attenuation is reviewed, and it is noted that the attenuation is a complex function of drop size, shape, orientation, index of refraction, and rain intensity along the path. Attenuation ratios are computed on the basis of both drop size and rain rate for frequencies at which there are measured data. Results obtained by other investigators are reviewed, and it is found that frequently, attenuation ratios that would not have been predicted on the basis of a surface rain model such as a Laws and Parsons model have been reported; indications are that attenuation is often produced from a widely dispersed distribution of large raindrops. During eight rainy days in 1975-1976, more than 10,000 simultaneous measurements of 15-and 35-GHz attenuations were recorded in the Boston area. On the basis of these results it is concluded that it is not possible to represent the drop size distribution of rain along a slant path in the Boston area by a Laws and Parsons model, since the ratios of the 35-GHz attenuations to the 15-GHz attenuations are significantly below those that would have been predicted by that model. The ability to predict slant path rain attenuation at one frequency on the basis of that at another is shown to be a function of frequency separation, climatology, and the type of statistics which are desired. in the f'mal section. 2. THEORETICAL CONSIDERATIONS It is known that high attenuations at frequencies of 15 and 35 GHz are due to a small extent to the atmospheric gases, oxygen, and water vapor and to a much larger extent to liquid water in the form of rain, wet snow, and water-laden clouds [Gunn and East, 1954]. Clouds and precipitation attenuate electromagnetic waves through both absorption and scattering processes. Equations for 781 782 ALTSHULER AND TELFORD SLANT PATH RAIN ATTENUATION 783
Refraction effects cause radio waves travelling through the atmosphere to be bent. Without knowledge of the atmospheric parameters along the ray path, the degree of bending cannot be calculated deterministically. Since atmospheric emission at microwave wavelengths is dependent on temperature, pressure, and relative humidity, radiometric data contain information about these parameters. This study will make use of a Monte Carlo simulation of the atmosphere to demonstrate that atmospheric emission measurements can be used with surface refractivity to increase greatly the accuracy with which refractive bending can be estimated.
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