partial aperture solar calibrator PAC or PASC

One of two on-board external solar calibrators on Landsat 7 ETM+. The PASC consists of structure in front of the ETM+ aperture that has four independent pinholes for reflecting solar irradiance on a presumably parallel path to the optical axis of ETM+ as it comes over the North Pole and into sunlight. As compared to FASC measurements, PASC irradiance from a single pinhole at a time only fills a small part of the defining aperture. PASC measurements were made approximately once a day prior to Failure of the Scan-Line-Corrector (SLC), and once every other day subsequently. The SLC-failure also necessitated rewriting the Software for PASC processing by the image Assessment System (IAS) at EROS Data Center in Sioux Falls, SD. Unfortunately, there are two major problems with PASC data: 1) the elliptical PASC images are saturated in several bands, and 2) an as-yet unknown cyclic variation has made them unusable in an absolute sense. The relative Gain or Responsivity of the detectors can be calculated from factors that include, known solar spectral irradiance, measured solar zenith and azimuth angles. Also see full aperture solar calibrator and internal calibrator. (Source: Dr. John Barker)

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