Initiatives for millimetre/submillimetre-wave sounding from geostationary orbit

The concept of microwave sounding from geostationary orbit is over three decades old, but can now be facilitated using submillimetre-wave sounding technology. A "GEM" concept has been pursued in the U.S. in the past few years, and now a "GOMAS" proposal has been submitted to ESA for implementation. The principle is to extend the number of bands usually exploited for sounding (54 GHz for temperature and 183 GHz for humidity) to higher frequencies (118 and 425 GHz for temperature, 380 GHz for humidity), so as to obtain higher geometric resolution for a given antenna size. Measurements in absorption bands at frequencies differently affected by liquid and ice water, enable simultaneous retrieval of temperature and humidity profiles, cloud ice and liquid columnar amounts and gross profile and, most important, precipitation, appropriately sampled at time intervals of some 15 minutes. The paper will highlight the basic concepts and technical features of the GEM/GOMAS project and comment about feasibility and sizing elements.