avcal converts AVHRR sensor count values to values with engineering units. For channels 1 and 2 the units are per- cent albedo. For channels 3, 4, and 5 the units can be either percent albedo or brightness temperature. This pro- cess is often referred to as radiometrically calibrating the data. New calibration coefficients are computed for each 100 input lines.
The calibration algorithm is based on information contained in Techniques for Data Extraction and Calibration of TIROS- N/NOAA Series Satellite Radiometers for Direct Readout Users by Levin Lauritson and Gary Nelson (NOAA-NESS Publication #107, July 1979). Briefly, the visible and near infrared data (channels 1 and 2) are converted from raw counts to percent albedo using a linear relationship determined prior to launch. The thermal infrared data (channels 3, 4, and 5) are converted from raw counts to radiances with a linear relationship that is based on the raw count value associated with cold space (roughly 3 degrees Kelvin) and the raw count value associated with the temperature of an onboard target (approximately 300 degrees Kelvin). A slight nonlinearity in channels 4 and 5 is corrected using a quadratic function of radiance. Lastly, the infrared radiances are converted to temperature using the inverse Planck function.
To calibrate visible channels, avcal uses the ASCII data file avcal.coef the location of which is defined by the SAT- DATA environment variable. This is a time-dependent file, each record of which contains the calibration information on per-satellite basis together with the date label, defining the starting date when this information became valid. See this file for more information.
The output report provides the intercept (intcpt) and slope (slope) for each channel's linear calibration equation. It also provides the space (space) and target (target) counts. Lastly, it provides the temperatures (average) of the four platinum resistance thermistors (prt), which are embedded in the onboard target.