ICID 8 => SUN_VOLUME_GLOBAL

Illumination:

  • Sun irradiance over the quartz volume reflection diffuser.

Purpose:

  • Global sun irradiance measurements over the quartz volume reflection diffuser.
  • Provides global sun irradiance level-1b data product for normalisation of Earth radiance spectra to obtain sun-normalised Earth radiances, or absolutely calibrated Earth reflectances.
  • Monitoring of absolute radiometric irradiance calibration.
  • Monitoring of potential in-flight optical and/or detector degradation.
  • Monitoring of irradiance goniometry calibration (azimuth and elevation angle dependence).
  • Monitoring of spectral calibration.
  • Monitoring of spectral and spatial diffuser features.

Location:

  • Near the north pole, first light measurements, then dark measurements.

Usage:

  • From launch onwards, once per day.
  • As a result of an anomaly in the Folding Mirror Mechanism (FMM) no ICID 8 measurements were performed from 28 February 2006 up to (including) 11 June 2006, with only one measurement at 6 June 2006. After this period the anomaly was resolved and the regular measurement schedule of one measurement per day was resumed.

Description:

Sun over quartz volume reflection diffuser global measurement. First, 77 light measurements are performed at changing elevation angles (roughly in the range +4.0 degrees to -4.0 degrees), then 77 dark measurements are performed. As a function of the day in the year (season) the azimuth angle on the diffuser changes. The individual sun irradiance measurements in the elevation angle range between -3.0 degrees and +3.0 degrees are averaged in the 0-1 data processor to obtain the level-1b irradiance data product. Averaging is performed to reduce the impact of spectral and spatial diffuser features and to improve the signal-to-noise. The dark (background) measurements of ICID 9 are eventually used to correct these light measurements in the 0-1 data processing.