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DMSP OLS - Operational Linescan System
Visible and infrared imagery from DMSP Operational Linescan System (OLS) instruments are used to monitor the global distribution of clouds and cloud top temperatures twice each day. The archive data set consists of low resolution global and high resolution regional, imagery recorded along a 3,000 km scan, satellite ephemeris and solar and lunar information. Infrared pixel values correspond to a temperature range of 190 to 310 Kelvins in 256 equally spaced steps. Onboard calibration is performed during each scan. Visible pixels are relative values ranging from 0 to 63 rather than absolute values in Watts per m^2. Instrumental gain levels are adjusted to maintain constant cloud reference values under varying conditions of solar and lunar illumination. Telescope pixel values are replaced by Photo Multiplier Tube (PMT) values at night. A telescope pixel is 0.55 km at high resolution (fine mode) and 2.7 km at low resolution(smooth mode). Low resolution values are the mean of the appropriate 25 high resolution values.