NASA's Stratospheric Aerosol and Gas Experiment II (SAGE II) determines the spatial distributions of stratospheric aerosols, ozone, nitrogen dioxide, water vapor, and cloud occurrence by mapping vertical profiles and calculating monthly averages of each.  

The SAGE II instrument, a Sun Photometer, was launched into a 57-degree inclination orbit aboard the Earth Radiation Budget Satellite (ERBS) in October 1984 and operated through August 2005. SAGE II data includes Aerosols, NO₂, O_3, and water vapor profiles.

Scientific Objectives

From 1984 to 2005, the SAGE II instrument flew aboard the Earth Radiation Budget Satellite (ERBS) collecting data at a spatial coverage of 80N to 80S, 180E to 180W as well as being processed and archived at NASA LaRC to produce four Level 2 data products: aerosol extinction profiles at 1020, 525, 453, and 385 nanometers, and ozone, water vapor, and nitrogen dioxide mixing ratio profiles. These products are nearly global in coverage, with data spanning from 80 degrees North to 80 degrees South latitudes.

The accuracy of these data was verified by extensive validation efforts and the data sets are now archived and available for general scientific use. These data may be of use to investigators who are interested in:

• Spatial and temporal variations of ozone, aerosols, water vapor, and nitrogen dioxide caused by seasonal and short-term meteorological variations, atmospheric chemistry and microphysics, decadal trends in ozone, and transient phenomena, such as volcanic eruptions
• Upper tropospheric studies of aerosols
• Climatology studies of cirrus clouds

Project Description

The SAGE II instrument is a seven-channel Sun photometer using a Cassegrainian-configured telescope, holographic grating, and seven silicon photodiodes, some with interference filters, to define the seven spectral channel bandpasses. Solar radiation is reflected off a pitch mirror into the telescope with an image of the Sun formed at the focal plane. The instrument's instantaneous field-of-view, defined by an aperture in the focal plane, is a 1/2 X 2-1/2 arc-minute slit that produces a vertical resolution at the tangent point on the earth's horizon of about 0.5 kilometers.

Radiation passing through the aperture is transferred to the spectrometer section of the instrument containing the holographic grating and seven separate detector systems. The holographic grating disperses the incoming radiation into the various spectral regions centered at the 1020, 940, 600, 525, 453, 448, and 385 nanometer wavelengths. Slits on the Rowland circle of the grating define the spectral bandpass of the seven spectral channels. The spectrometer system is inside the azimuth gimbal to allow the instrument to be pointed at the Sun without image rotation. The azimuth gimbal can be rotated over 370 degrees so that measurements can be made at any azimuth angle.

Instrument Operation

The operation of the instrument during each sunrise and sunset measurement is totally automatic.

1. Prior to each encounter, the instrument is rotated in azimuth to its predicted solar acquisition position.
2. When the Sun's intensity reaches a level of one percent of maximum in the Sun sensor, the instrument adjusts its azimuth position to lock onto the radiometric center of the Sun to within +/-45 arc-seconds.
3. It then begins acquisition of the Sun by rotating its pitch mirror in a predetermined direction (sunrise or sunset).
4. When the Sun is acquired, the pitch mirror rotates back and forth across the Sun at a rate of about 15 arc-minutes per second.
5. The radiometric channel data are sampled at a rate of 64 samples per second per channel, digitized to 12-bit resolution, and recorded for later transmission back to Earth.