Microwave radiometers have recently demonstrated a new capability by detecting internal waves in the atmospheric boundary layer and localizing their altitude. Varying the intersection height of a narrow antenna beam with that of a wide vertically pointing antenna beam allowed determination of the wave altitudes. The ground-based radiometers were located at San Diego where, in an experiment in May--June 1975, the Naval Electronics Laboratory Center provided ''atmospheric-truth'' for comparison to the radiometer data obtained by the Naval Research Laboratory. Preliminary results showed cases of correspondence between the signals of the passive radiometers and the active FM-CW radar and acoustic sounder systems. Examples included internal wave trains up to an hour in length.