EXPERIMENTS using both infrared and microwave radiometers have been developed for the Nimbus series of satellites1,2. Selective chopper radiometers (SCR) for Nimbus 4 and Nimbus 5 incorporate cells containing carbon dioxide gas to accomplish the narrow-band filtering, thereby achieving a very high effective spectral resolution - of the order of the width of the absorption lines themselves. By this means emission from levels in the atmosphere as high as 45 km can be selected. The first SCR on Nimbus 4, which was six channels in the 15 µm CO2 band3,4, has worked successfully for more than 3 yr; data from it have provided the first global maps of stratospheric temperature at these high levels5-7, and stratospheric warmings have been identified8,9. © 1973 Nature Publishing Group.