Beam filling loss adjustments for ASR-9 weather channel reflectivity estimates
October 22, 1990
MIT Lincoln Laboratory Report ATC-177
The FAA is deploying over 100 new ariport surveillance radars (ASR-9) across the country. In contrast to earlier ASRs, the ASR-9 utilized a separate digital weather processing channel to provide air traffic controllers with timely, calibrated displays of precipitation intensity. The ASR-9 utilizes dual selectable fan-shaped elevation beams designed to track aircraft over a large volume. As a consequence, weather echoes received from these fan-shaped beams represent vertically-averaged quantities. If the precipitation only partially or nonuniformly fills the beam, then the vertically integrated reflectivity may underestimate the actual intensity of the storm. The ASR-9 weather channel corrects for this by adjusting the range-dependent six-level relfectivity thresholds. The appropriateness of the currently implemented correction has not been carefully examined and may require modification to take into account regional and morphological variability in storm structure. This report discusses the method used to derive new beam filling loss adjustments. An extensive database of volumetric pencil-beam radar data were used in conjunction with our ASR-9 simulation facility to derive adjustments aimed at calibrating the precipitation intensity reports to the maximum perceived hazard. The new corrections yield substantially improved results over the current corrections in producing these reflectivity reports.