The pressure-modulated CO2 radiometer is a new kind of instrument capable of making temperature soundings in the 40–80-km region of the earth’s atmosphere. It is intended to be mounted on a polar-orbiting satellite, where it will give global coverage of the upper atmosphere in a region that is not well understood at present but that is, as rocket soundings show, clearly the seat of many interesting and vigorous phenomena. The new technique employs a cell containing carbon dioxide as a filter. The pressure and hence transmission of this cell is periodically modulated, resulting in the selection of thermal radiation from the strong lines in the spectrum of atmospheric CO2. This radiation originates at levels in the atmosphere where the pressure is low. The energy grasp of the device is large enough to give high sensitivity. Tests with a laboratory prototype and a balloon-borne instrument show that the device, if mounted outside the atmosphere, could detect changes of around 1 K in the temperature at 65-km altitude.
F. W. Taylor, J. T. Houghton, G. D. Peskett, C. D. Rodgers, and E. J. Williamson, "Radiometer for Remote Sounding of the Upper Atmosphere," Appl. Opt. 11, 135-141 (1972)