The RM-20A Radiometric Measurements package consisted of an optical system which included an internal dithering mirror, optical filters, a detector focal plane, data conditioning electronics packages, gimballing and servo control mechanisms, passive cooling and thermal control, and a logic unit for commanding. Radiometric measurements for both backgrounds and targets were planned to be performed by preprogramming the motion of the radiometer to retain the background of interest or the target within the field of view. The sensor consisted of an all reflective corner-corrector Schmidt optical system which focused incoming energy on a focal plane consisting of nearly 600 indium-antimonide photo-voltaic detectors and 20 silicon detectors for observation in the MWIR, SWIR, and visible spectral regions. It was to have been launched into a circular polar orbit onboard the P72-2 spacecraft in April 1975; however, the spacecraft did not achieve orbital flight because of a launch vehicle failure. Nevertheless, a brief description of the RM-20A experiment is expected to be of interest to the photo-optical instrumentation community.