Analysis of the effects of water on the ACTS propagation terminal antenna

The National Aeronautics and Space Administration (NASA) advanced communications technology satellite (ACTS) propagation experiment was designed to observe the attenuation produced by rain on earth-satellite paths operating in the Kalpha-band. Within the first month of operation, the unwanted effects of water on the antenna reflector surface were noted. After the reflector and feed surfaces aged, wet-antenna attenuation values as high 7 dB at high-rain rates could be attributed to the combined effect of a water layer on the reflector surface and water wetting the feed window surface. A model was developed to calculate the antenna reflector and feed surface water layer thickness values as a function of position on each surface. The thickness values were input to a layered dielectric scattering model to calculate the additional attenuation produced by the water layers as a function of rain rate on the antenna. The wet-antenna-attenuation prediction model was verified by sprayer tests. The goal of the ACTS propagation experiment was to obtain path attenuation statistics; statistics that represent the effects of rain on the earth-satellite path but not on the antenna itself. To satisfy that goal, the wet-antenna attenuation prediction model was used to remove the effects of water on the antenna from the combined antenna-plus-path attenuation statistics produced by the experiment. The overall efficacy of the model was demonstrated by comparing the corrected pathloss statistics from two of the ACTS propagation experiment sites with earlier COMSTAR path loss measurements made at or near those sites. The empirical distribution functions from both the ACTS and COMSTAR experiments were identical within the expected uncertainty of an empirical annual distribution of attenuation by rain.