The need for and development of MM-wave radiometer calibration targets with very low coherent backscatter

The need for radiometer calibration targets with very low coherent backscatter (S11) is not fully appreciated in the community which designs and uses mm-wave radiometers. We discuss why very low S11 is required and our attempts to design make and test calibration target with such desired low backscatter for calibrating both ground and space-based radiometers.