The Upper Atmosphere Research Satellite microwave limb sounder instrument

TLDR

The MLS on UARS pioneered limb sounding at microwave frequencies. The instrument aims to measure stratospheric ClO, O₃, H₂O, temperature, and pressure. The MLS employs a 1.6‑m mechanically scanning antenna with heterodyne radiometers at 63, 183, and 205 GHz, using Schottky‑diode mixers, Gunn‑oscillator‑derived local oscillators, varactor‑multiplied frequency tripling, quasi‑optical injection, and 15‑channel filter banks to produce a spectrum every 2 s, with radiometric calibration every 65 s. The MLS has performed excellently in orbit, meeting all measurement objectives.

Abstract

The microwave limb sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) is the first satellite experiment using limb sounding techniques at microwave frequencies. Primary measurement objectives are stratospheric ClO, O 3 , H 2 O, temperature, and pressure. Measurements are of thermal emission: all are performed simultaneously and continuously and are not degraded by ice clouds or volcanic aerosols. The instrument has a 1.6‐m mechanically scanning antenna system and contains heterodyne radiometers in spectral bands centered near 63, 183, and 205 GHz. The radiometers operate at ambient temperature and use Schottky‐diode mixers with local oscillators derived from phase‐locked Gunn oscillators. Frequency tripling by varactor multipliers generates the 183‐ and 205‐GHz local oscillators, and quasi‐optical techniques inject these into the mixers. Six 15‐channel filter banks spectrally resolve stratospheric 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, consisting of measurements of cold space and an internal target, is performed every 65‐s limb scan. Instrument in‐orbit performance has been excellent, and all objectives are being met.