THE UPPER-ATMOSPHERE RESEARCH SATELLITE MICROWAVE LIMB SOUNDER INSTRUMENT

The microwave limb sounder (MLS) on the Upper Atmosphere Research Sate llite (UARS) is the first satellite experiment using limb sounding tec hniques at microwave frequencies. Primary measurement objectives are s tratospheric ClO, O3, H2O, temperature, and pressure. Measurements are of thermal emission: all are performed simultaneously and continuousl y and are not degraded by ice clouds or volcanic aerosols. The instrum ent has a 1.6-m mechanically scanning antenna system and contains hete rodyne radiometers in spectral bands centered near 63, 183, and 205 GH z. The radiometers operate at ambient temperature and use Schottky-dio de mixers with local oscillators derived from phase-locked Gunn oscill ators. Frequency tripling by varactor multipliers generates the 183- a nd 205-GHz local oscillators, and quasi-optical techniques inject thes e into the mixers. Six 15-channel filter banks spectrally resolve stra tospheric thermal emission lines and produce an output spectrum every 2 s. Thermal stability is sufficient for ''total power'' measurements which do not require fast chopping. Radiometric calibration, consistin g of measurements of cold space and an internal target, is performed e very 65-s limb scan. Instrument in-orbit performance has been excellen t, and all objectives are being met.