Stratospheric Aerosol And Gas Experiment II Instrument: A Functional Description

Abstract

Design and performance data are presented for the Strato-spheric Aerosol and Gas Experiment II (SAGE II) instrument, which has been developed for the Earth Radiation Budget Satellite (ERBS). SAGE II is designed to monitor globally the vertical distribution of stratospheric aerosols, ozone, water vapor, and nitrogen dioxide by measuring the ex-tinction of solar radiation through the earth's atmosphere during the ERBS observatory solar occultations. Solar radiation is reflected from a flat scanning mirror into a Cassegrain telescope, which forms a solar image on the entrance slit of a grating spectrometer. The SAGE II instan-taneous field of view is scanned along the vertical solar diameter by the elevation scan mirror. The entire optical system is contained within an azimuth gimbal that tracks the solar radiometric centroid during the data event. This spectrometer, with help from three interference filters, iso-lates seven spectral wavelengths ranging from 0.385 um to 1.02 um. All seven channels use silicon photodiode detectors operated in the photo-voltaic mode. Detector outputs are multiplexed into a serial data stream for readout by the ERBS telemetry system. Each output is sampled 64 times per second and digitized to 12 bit resolution. SAGE II is a third generation instrument, following the highly successful Stratospheric Aerosol Measurement II (SAM II) and SAGE programs.