Active microwave systems, in the form of radar, have long been used in terminal approach areas and on board aircraft to warn of precipitation. Recent attention to the danger of aircraft icing has led to a concept for a passive microwave system capable of recognizing the presence of liquid water ahead of the aircraft and estimating (with suitable algorithms) the water content and temperature in advance of entering the cloud.
Our concept for a Microwave Icing Avoidance System (MIAS) incorporates two symmetrical forward-looking beams, viewing the atmosphere at small (~2 degrees) angles above and below the flight path, with disparate window frequencies above and below the 60 GHz oxygen absorption band. Mie theory predicts a strong emission of radiance at the higher frequency, where the size parameter (droplet circumference divided by electromagnetic wavelength) approaches unity. The lower frequency, where clouds and drizzle are less absorptive, provides greater visibility for seeking an exit from such a cloud if inadvertantly entered. Comparison of the two sensing channels permits an estimate of the water content along a given path. Operation at window frequencies away from the strong 60 GHz absorption is important in maximizing visibility through the atmosphere. Software simulations permit quantifying the accuracy to be expected from such a sensing implementation.
A web (www) page for discussion and analysis of such simulations is in preparation and is expected to be available to the aviation weather community by the time of the Conference
The 8th Conference on Aviation, Range, and Aerospace Meteorology