The scientific objectives of the improved stratospheric and mesospheric sounder (ISAMS) experiment involve the measurement of global temperature and composition profiles from an instrument on the Upper Atmosphere Research Satellite (UARS). This paper describes the instrument concept, its design, and its performance as calculated and as measured in the laboratory. The data retrieval technique, operating modes, observing strategy, and the error budget are briefly discussed.

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The stratospheric and mesospheric sounder (s.a.m.s.) instrument was launched on the Nimbus G satellite on 24 October 1978. It is designed to measure temperature and concentration profiles of various gases in the height range 20-100 km by detecting either their thermal emission or, in some cases, resonant scattering of sunlight. The gases selected, CO 2 , CO, CH 4 , NO, N 2 O and H 2 O, significantly affect the upper atmosphere energy budget by their influence on the concentration of the primary sunlight absorber, ozone. This influence is disproportionate to their own concentration because of the existence of 'catalytic cycles’ which destroy ozone while regenerating the catalyst. A description of the instrument, its principles of operation and some of the methods of retrieval used is presented, together with some preliminary results from the first 3 months of operations.

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This paper describes the development, from a previously proven design approach, of a robust and simple Stirling cycle cooler with long life potential. The need for a closed cycle refrigerator for use in a spacecraft borne infra-red radiometer is explained. The refrigerator is to supply 1 watt of cooling at 80 K for less than 80 watts of input power, be able to survive the launch environment and subsequently run for 26000 hours. Clearance seals achieved with a spring suspension developed from earlier space proven mechanisms have led to the production of a linear split Stirling cycle machine with no apparent life limiting features. A servo control system, in conjunction with moving coil motors and LVDT position sensors, permits running of balanced pairs of mechanisms. The working fluid, helium at a pressure of 1.2 MPa, is contained within titanium bodies having gold O-ring seals. A vacuum bakeout procedure, based upon experience and outgassing trials, reduces residual contaminant release to acceptable levels. A prototype refrigerator has been subjected to a vibration test and has subsequently run for 6000 hours with no detectable change in performance.

The pressure-modulated CO(2) 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 CO(2). 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.

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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