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Instrument: SAGE II : Stratospheric Aerosol and Gas Experiment II
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The Stratospheric Aerosol and Gas Experiment II (SAGE II) on the Earth
Radiation Budget Satellite (ERBS) was a seven-channel sunphotometer
which measured stratospheric aerosols, ozone, water vapor, and
nitrogen dioxide during spacecraft sunrise and sunset. The SAGE II
mission objectives were to (1) develop a long-term global climatology
of stratospheric aerosol extinction, (2) monitor transient events
(e.g. volcanic eruptions), (3) develop a high-altitude cloud climatology,
and (4) provide stratospheric ozone, water vapor, and nitrogen dioxide
concentration profiles at 1 km vertical resolution. The SAGE II was a
two-axis gimbaled system capable of rotating in azimuth. The sun was
acquired when two azimuth sun sensors were activated and rotated in
azimuth until SAGE II was pointed at the radiometric centroid of the
sun. A scan mirror moved at fast rate of 3 deg/sec to acquire the sun.
The mirror scanned across the sun at a slow rate of 15 min/sec. When the
limb of the sun was reached, the servo mechanism reversed and scanned the
sun again. The sunlight was reflected into a Cassegranian telescope and
focused on an aperature that defined a 0.5-minute elevation angle by 2.5
minute azimuth angle instantaneous field-of-view (IFOV). The aperature was
the entrance slit to the spectrometer, which used a holographic grating and
filters to isolate the seven wavelength bands (0.385 to 1.02 micrometer).
Channels 1,2,3,4, and 7 used neutral density filters and channels 2,5, and 6
used bandpass filters. All seven channels were sampled within 1 ms at 64 Hz
and digitized to a 12-bit word. The SAGE II instrument measured the solar
irradiance during each spacecraft sunrise and sunset so that two profiles
from specific locations above the Earth were obtained during each spacecraft
revolution. SAGE II made about 15 sunrise and sunset measurements each
day, seperated by 24.5 degrees in longitude and slightly in latitude.
The maximum latitudes covered were 80 N to 80 S.
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