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

Abstract

Site Test Interferometers (STIs) have been deployed at various sites throughout the world to characterize atmospheric degradation for radio communications and radio astronomy applications. Radio astronomers make use of such instruments to assess atmospheric conditions for scheduling astronomical observations (usually making use of seasonal statistics) (Morris, 2014).A STI measures the difference in signal delay from a celestial source to two points on the surface of the Earth. The variations in that delay difference are primarily due atmospheric turbulence. For a two-element interferometer, the turbulence induces variations in the mean refractive indices along the two signal paths. Short-term statistics are useful to aid in scheduling communication links. Passes that fall on days with the best weather conditions allow for links involving small margins (high data rates), and those that fall on days with adverse weather conditions would be used on links with larger margins (lower data rates). The statistical behavior of the fluctuations will vary among different sites due to climate and altitude, and at any one site diurnally, seasonally, and with passage of weather systems. These instruments have been deployed at the NASA Deep Space Network (DSN) tracking sites of Goldstone (California), Madrid (Spain), and Canberra (Australia) to assess the prospect of these sites for arraying applications to allow for increased performance. A STI was also deployed at the Kennedy Space Center (KSC) in Florida to provide atmospheric characterization originally for the KABOOM experiment (Geldzahler et al., 2014;Huddleston et al., 2014). Morabito (2019) summarizes the phase delay statistics extracted from all four STIs as well as providing a bibliography (up to the end of 2018) of studies that made use of these statistics. Here phase delay statistics extracted from full years of data were presented; 2011-2018 for the Goldstone STI