Stratospheric Aerosol and Gas Experiment (SAGE) I gathered data concerning the spatial distribution of stratospheric aerosols, ozone and nitrogen dioxide on a global scale. SAGE I used a Sun Photometer.
Scientific Objectives
The Stratospheric Aerosol and Gas Experiment I instrument, a Sun photometer, aboard the Applications Explorer Mission-2 (AEM-2) satellite began collecting data in October 1979. The scientific objective was to develop a global stratospheric aerosol, ozone, and nitrogen dioxide database that could be used for the investigation of the spatial and temporal variations of these species caused by seasonal and short-term meteorological variations, atmospheric chemistry, microphysics, and transient phenomena such as volcanic eruptions. The database could also be used for the study of trends, atmospheric dynamics and transport, and potential climatic effects.
The SAGE I sensor was designed to measure the attenuation of solar radiation at a spatial coverage of 80N to 80S, 180E to 180W, resulting from atmospheric aerosol, ozone, and nitrogen dioxide at four spectral regions through the Earth's atmosphere during each spacecraft sunrise and sunset.
Project Description
The AEM-2 satellite was placed in an orbit of approximately 600 kilometers at an inclination of 56 degrees to extend the latitudinal coverage for the solar occultation measurements from 79 degrees South to 79 degrees North.
The SAGE I instrument consists of four spectral channels centered at wavelengths of 1000, 600, 450, and 385 nanometers for measuring global data concerning aerosol vertical extinction profiles, ozone vertical concentrations profiles, and nitrogen dioxide vertical concentrations profiles during spacecraft sunrise and sunset. The SAGE I aerosol data were validated by comparison with correlative lidar and dustsonde in situ measurements, the ozone data were validated by comparison with balloon ECC ozonesonde and rocket measurements, and the nitrogen dioxide measurements were compared with climatology.
Instrument Operation
The operation of the instrument, during each sunrise and sunset measurement, was totally automatic. Prior to each sunrise or sunset, the instrument was rotated in azimuth to its predicted solar acquisition position. When the Sun entered the instrument's field of view, the instrument adjusted its azimuth position to lock onto the radiometric center of the Sun to within +/- 45 arcsec and then acquired the sun by rotating its scan mirror to the proper elevation angle. As the Sun traversed between the horizon and the tangent height of 150 kilometers, radiometric channel data were sampled at a rate of 64 samples per second per channel, digitized to 12-bit resolution, and recorded for later transmission back to Earth.
Additional SAGE I instrument information can be found in McCormick et al (1979). The SAGE I instrument collected data for almost three years until the AEM-2 satellite power subsystem failed.
Data Products: Data Used and Produced
The SAGE I science and engineering data, along with spacecraft time, position, and housekeeping data, were stored aboard the spacecraft and then down linked to NASA GSFC through a ground station. GSFC then forwarded these data to LaRC for processing and scientific analysis. GSFC also sent spacecraft and solar ephemeris data to NASA Langley Research Center (LaRC) on separate weekly tapes.
LaRC combines three data sources to produce the SAGE I MERDAT: (1) the SAGE I instrument data, (2) the spacecraft and solar ephemeris data, and (3) NOAA NMC temperature and density interpolations from the standard NMC spatial gridded analyses at the 18 standard pressure levels and at the tropopause for each tangent event location.
The MERDAT files are used as the data input to the inversion process. At the completion of the data processing, three Level 2 SAGE I products are produced: aerosol extinction profiles, ozone concentration profiles, and nitrogen dioxide concentration profiles.