Possible measurement errors in calibrated AVHRR data

Visible and infrared meteorological satellite data are a primary source of global cloud observations. Such data are calibrated in order to provide its users with a method for converting from 'raw' measurements, called counts, to physically sensible measurements such as albedo or brightness temperature. This report describes the procedure for converting National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) raw counts to albedos and brightness temperatures. This procedure involves the use of 'calibration coefficients' that help define the relationship between the raw counts and the physical measurements they represent. Such relationships are referred to as 'look-up-tables'. In theory, calibration coefficients (and therefore look-up tables) do not change noticeably from one scanline of satellite data to the next. This makes feasible the generation of a constant look-up table that can be used over long periods of time for all data. In practice, calibration coefficients can and often do change from one scan to the next. These changes imply accuracy errors in measured brightness temperatures that are significant for some satellite data analysis algorithms; others are less severely affected. This report addresses the potential errors that can be expected when using constant look-up tables, i.e., by assuming that sensor calibration does not change from one scanline to the next.