1. A method for real time determination of selected characteristics of atmospheric refractivity utilizing signals from a passive radiation emission measurement device comprising the steps of: sensing brightness temperature across at least one selected predictable atmospheric thermal radiation emission region at the measurement device and providing output signals indicative thereof; and applying refractivity characteristics retrieval coefficients to said signals, said retrieval coefficients provided from a retrieval system trained by refractivity characteristics calculated from a priori or modeled atmospheric states and correlative brightness temperature observables.