A new radiometer is described for remote sounding of the temperature of the stratosphere and mesosphere (40-90 km) from a satellite platform. In order to select radiation emitted from close to the centres of CO
2
lines in the 15 pm infrared band, included in the optical system is a cell containing CO
2
gas whose pressure can be modulated at a frequency of ca. 15 Hz. The parts of the emission lines selected can be altered by two methods known as pressure scanning and Doppler scanning. A two-channel instrument for the Nimbus F satellite is described in detail.
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