Before there was SAGE, there was the Stratospheric Aerosol Measurement experiment, or SAM. SAM paved the way for SAGE. The experiment demonstrated that solar occultation measurements by photometer and camera could be used to determine the vertical distribution of stratospheric aerosols. Consisting of only a sun photometer, used to measure the sun’s intensity, and a camera, the experiment was flown on an Apollo spacecraft during the Apollo-Soyuz Test Project in July 1975. During the 9-day mission solar photographs were taken and measurements were recorded by the photometer.

The observations made by SAM were later verified by a balloon-borne aerosol counter and a ground-based laser system. Success in this experiment led to the launch of SAM II.

The SAM II experiment flew aboard the Nimbus-7 spacecraft from 1978 until 1993. Following the success of the SAM experiment and preceding the first SAGE instrument, SAM II provided vertical profiles of aerosol in both the Arctic and Antarctic polar regions. SAM II was designed to develop a stratospheric aerosol database for the polar regions, allowing studies of aerosol changes due to seasonal and short-term meteorological variations, atmospheric chemistry, cloud microphysics, volcanic activity and other disruptions. Like its successor, SAGE, SAM II was a spectrometer that used solar occultation as its measurement technique. SAM eventually led to the SAGE family of instruments, including SAGE III/ISS.

SAM II on the Nimbus-7 Observatory

The SAM II instrument was a single-channel Sun photometer employing a cassegrainian telescope and interference filter to define the spectral passband.

Solar radiation is reflected off a scan mirror into the telescope with an image of the Sun formed at the slot plate. The instrument’s instantaneous field of view, defined by the aperture on the slot plate, is a 20-arc-second circle which produces a vertical resolution on the horizon of approximately 0.5 km. Radiation passing through the aperture is collected with a field lends, passes through an interference filter, and is measured by a silicon photodiode detector.

The entire optical and detector system is contained in the azimuth gimbal to allow the instrument to be pointed at the Sun. Prior to spacecraft sunrise or sunset, the instrument is moved (i.e., pointed to the predicted solar acquisition angle). When the Sun enters the instrument field of view, the instrument locks onto the radiometric center of the Sun within +/- 1 arc minute in azimuth and then acquires the Sun in elevation by rotating the scan mirror.

Spatial / Temporal Coverage

  • Spatial Coverage: Polar Regions (Varies with season)
  • Spatial Resolution: The altitude profiles of aerosol extinction have a 1 km vertical resolution.
  • Temporal Coverage: 10/29/1978 – 12/18/1993
  • Temporal Resolution: There are approximately 28 SAM II measurements per day, 98 per week, and 5096 per year. The time between each successive sunrise or sunset is approximately 1 hour and 45 minutes.

Data Products

  • Aerosol Extinction Profiles

SAM II Aerosol Extinction Profile, December 27, 1979

SAM Science Team Members:

M. Pat McCormick, Hampton University (Previously NASA Langley Research Center)

Gerald W. Grams, Georgia Institute of Technology

Benjamin M. Hermann, University of Arizone

Thomas J. Pepin, University of Wyoming

Phillip B. Russell, SRI International Science


The SAM experiment lasted nine days in 1975. The instrument is now stored at NASA Langley Research Center in Hampton, Va.






The SAM II experiment flew aboard the Nimbus-7 spacecraft from 1978 until 1993. The platform is still in orbit today.