Radio Refractive Index of Wet Atmosphere Estimated From Site Test Interferometer Data

Abstract

AbstractThe atmosphere plays a significant role in degrading Ka‐band (32 GHz) radio and optical communications for single antenna communications as well as degrading interferometric measurements such as used for radio astronomy and multi‐antenna arrays. Higher frequencies such as deep space Ka‐band (32 GHz) and near‐Earth K‐band (26 GHz) are more susceptible to this atmospheric degradation than at the lower frequency bands of S‐band (2.3 GHz) and X‐band (8.4 GHz). The various sources of atmospheric degradation include rain attenuation, cloud attenuation, gaseous attenuation, atmospheric noise temperature increase, and scintillation. For this study, we have focused on the strength of the turbulence caused principally by the wet atmosphere represented by the refractive index structure parameter Cn2, which is a measure of the variance of the refractive index over small incremental distances along the signal path, and thus is a strong function of water vapor variations at microwave frequencies. Here we provide a comprehensive study of this parameter derived from years of Site Test Interferometer measurements acquired from a variety of climates. We found that the strongest diurnal variation and correlation of Cn2 measurements with its model occurred for Goldstone, California representing a high‐desert climate, while weaker correlations were found for Kennedy Space Center representing a sub‐tropical climate, and Canberra, Australia and Madrid, Spain representing temperate climates. We quantified periods where the strongest correlation was observed such as Goldstone summers (as high as 0.7 for ∼3‐day periods) where the days are typically hot and humid while nights tend to be colder and drier.