Stratospheric Aerosol and Gas Experiment I (SAGE I) Langley DAAC Project/Campaign Document
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Stratospheric Aerosol and Gas Experiment I (SAGE I) Langley DAAC Project/Campaign Document |
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The Stratospheric Aerosol and Gas Experiment I (SAGE I) instrument was launched February 18, 1979, aboard the Applications Explorer Mission-B (AEM-B) satellite (McCormick et al., 1979). The SAGE I instrument had four spectral channels centered at wavelengths of 1000, 600, 450, and 385 nanometers for nearly global measurement of aerosol extinction profiles and ozone and nitrogen dioxide concentration profiles. The AEM-B 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 collected data for almost three years until the AEM-B satellite power subsystem failed.
The SAGE I instrument was a sun photometer that measured the attenuation of solar radiation through the Earth's atmosphere during spacecraft sunrise and sunset in the four spectral regions mentioned above. The solar radiance data were combined with spacecraft ephemeris and NOAA meteorological data and then numerically inverted to yield altitude profiles of aerosol extinction at wavelengths of 1000 and 450 nanometers and altitude profiles of ozone and nitrogen dioxide concentration.
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.
Stratospheric Aerosol and Gas Experiment I, SAGE I
The platform for SAGE I was the Applications Explorer Mission-B (AEM-B). Nominal orbit parameters for AEM-B are listed below:
The scientific objective of SAGE I was to develop a global stratospheric aerosol, ozone, and nitrogen dioxide data base 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 and microphysics, and transient phenomena such as volcanic eruptions. The data base could also be used for the study of trends, atmospheric dynamics and transport, and potential climatic effects.
The measurement technique was solar occultation. The spectrometer was activated to take solar irradiance measurements during the periods when the line-of-sight from the SAGE I instrument to the Sun had tangent altitudes between sea level and 150 km. It was self-calibrating in that an exoatmospheric measurement was made before a sunset or after a sunrise measurement. Attenuation of sunlight by a species was measured and processed to produce profile data.
The latitude range for SAGE I varied with season. Overall coverage ranged from about 75 degrees North to 75 degrees South.
On a given day, the latitude coverage was from hundredths of a degree at the highest latitudes to 10 degrees near the equator. The longitudinal coverage was 360 degrees where the interval for consecutive sunrises or consecutive sunsets was about 24 degrees.
The SAGE I instrument was a four channel Sun photometer using a Cassegrainian-configured telescope, holographic grating, and four silicon photodiodes to define the four spectral channel bandpasses. Solar radiation was 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 the aperture on the focal plane was a 30 arc-second circle that produced a vertical resolution at the tangent point of about 0.5 km. Radiation passing through the aperture was transferred to the spectrometer section of the instrument containing the holographic grating and four separate detector systems. The holographic grating dispersed the incoming radiation into four spectral regions centered at 1000, 600, 450, and 385 nanometer wavelengths. Slits on the Rowland circle of the grating defined the spectral bandpass of the four spectral channels. The bandpasses were 50, 30, 20,and 10 nanometers, respectively, for the above mentioned wavelengths. The entire imaging and spectrometer system was inside the azimuth gimbal to allow the instrument to be pointed at the Sun without image rotation. The azimuth gimbal could be rotated over 360 degrees so that measurements could be made at any azimuth angle.
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 arc-seconds 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, the elevation mirror scanned vertically across the solar disc. The 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).
Atmospheric transmittance data at four wavelengths were obtained during solar occultation at each satellite sunrise and sunset. Solar irradiance measurements were made at 1000, 600, 450, and 385 nanometers. Supporting data included spacecraft location and velocity and conventionally collected meteorological data which included temperature, temperature error, geometric altitude, density, and density error at 18 pressure levels from 1000 millibars to 0.4 millibars. Model meteorological altitude and temperature at 0.1 and 0.04 millibars were supplied by the processing group at NASA LaRC. All data were time-tagged. Instrument housekeeping data were also provided.
Data are available by FTP or on media.
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Dr. M. Patrick McCormick, Principal Investigator and Science Team Leader
Dr. M. Patrick McCormick, Principal Investigator and Science Team Leader
Chu, W. P. and M. P. McCormick, May 1, 1979. Inversion of Stratospheric Aerosol and Gaseous Constituents from Spacecraft Solar Extinction Data in the 0.38 - 1.0 Micrometer Wavelength Region, Applied Optics, Vol. 18, No. 9, 1404-1413.
Chu, W. P. and M. P. McCormick, April 30, 1986. SAGE Observations of Stratospheric Nitrogen Dioxide, Jour. of Geophys. Res., Vol. 91, No. D5, 5465-5476.
Kent, G. S., November 18-19, 1980. SAGE Measurements of Mount St. Helens Volcanic Aerosols, NASA Conference Publication 2240, Atmospheric Effects and Potential Climatic Impact of the 1980 Eruptions of Mount St. Helens, 109-115.
Kent, G. S., U. O. Farrukh, P.-H. Wang, and A. Deepak, March 1988. SAGE I and SAM II Measurements of 1 micrometer Aerosol Extinction in the Free Troposphere, Jour. of Applied Met., Vol. 27.
McCormick, M. P., 1983. Aerosol Measurements from Earth Orbiting Spacecraft, Adv. Space Res., Vol. 2, No. 5, 73-86.
McCormick, M. P., April 1983. Global Distribution of Stratospheric Aerosols by Satellite Measurements, AIAA Journal, Vol. 21, No. 4, 633-635.
McCormick, M. P., G. S. Kent, G. K. Yue, and D. M. Cunnold, November 1981. SAGE Measurements of the Stratospheric Aerosol Dispersion and Loading from the Soufriere Volcano, NASA Technical Paper 1922.
McCormick, M. P., G. S. Kent, G. K. Yue, and D. M. Cunnold, June 4, 1982. Stratospheric Aerosol Effects from Soufriere Volcano as Measured by the SAGE Satellite System, Science, Vol. 216, 1115-1118.
McCormick, M. P., T. J. Swissler, E. Hilsenrath, A. J. Krueger, and M. T. Osborn, June 30, 1984. Satellite and Correlative Measurements of Stratospheric Ozone: Comparison of Measurements Made by SAGE, ECC Balloons, Chemiluminescent and Optical Rocketsondes, Jour. of Geophys. Res., Vol. 89, No. D4, 5315-5320.
McCormick, M. P., R. E. Veiga, and J. M. Zawodny, 1988. Comparison of SAGE I and SAGE II Stratospheric Ozone Measurements, Proceedings of the International Ozone Symposium.
McCormick, M. P., J. M. Zawodny, R. E. Veiga, J. C. Larsen, and P.-H. Wang, 1989. An Overview of SAGE I and II Ozone Measurements, Planet. Space Sci., Vol. 37, No. 12, 1567-1586.
Reiter, R. and M. P. McCormick, November 25, 1982. SAGE--European Ozonesonde Comparison, Nature, Vol. 300, No. 5890, 337-339.
Russell, P. B., M. P. McCormick, and T. J. Swissler, 1983. Validation of Aerosol Measurements by the Satellite Sensors SAM II and SAGE, Adv. Space Res., Vol. 2, No. 5, 123-126.
Russell, P. B., M. P. McCormick, T. J. Swissler, J. M. Rosen, D. J. Hofmann, and L. R. McMaster, June 1, 1984. Satellite and Correlative Measurements of the Stratospheric Aerosol. III: Comparison of Measurements by SAM II, SAGE, Dustsondes, Filters, Impactors and Lidar, Journal of the Atmosph. Sciences, Vol. 41, No. 11, 1791-1800.
Yue, G. K., M. P. McCormick, and W. P. Chu, June 30, 1984. A Comparative Study of Aerosol Extinction Measurements Made by the SAM II and SAGE Satellite Experiments, Journal of Geophys. Res., Vol. 89, No. D4, 5321-5327.
