This paper describes the Improved Stratospheric and Mesospheric Sounder (ISAMS) which is one of ten experiments due to be launched on the NASA Upper Atmosphere Research Satellite. ISAMS is an eight channel infra-red radiometer utilising pressure modulator radiometry which has been used successfully on Pioneer and Nimbus experiments. The detectors operate at a temperature of 80°K which is achieved by specially developed Stirling cycle coolers. The emphasis of the paper will be on the optical system which must meet the very tight requirements on field of view and pointing which come from the limb viewing approach. Also considered will be the structural integrity needed to overcome motional chopping, thermal problems associated with the optical design and the laboratory testing needed to verify the performance.