In addition to determining the calibration coefficients of the various thermometers monitoring the thermal condition of the instrument and measuring the spectral response function of the individual channels, the pre-flight calibration includes subjecting the instrument in its normal operating mode to conditions it will encounter after launch. A "thermal/vacuum" chamber simulates the thermal regime expected to be experienced in space.
During these tests, the characteristics of the instrument are determined, including the non-linearities of the mercury cadmium telluride infrared detectors
(see below). The AVHRR infrared detectors view two calibrated laboratory blackbody targets. The temperature of one is stepped between 175 and 315K in increments
of 10K up to 290K and in increments of 5K above 290K, while that of the other is held at 77K to represent the in-orbit space radiance measurement (Weinreb
et al. 1990). This procedure is repeated at three set temperatures of the AVHRR internal blackbody calibration target: 10
, 15, and 20. The thermometers
monitoring the external calibration targets are traceable to National Bureau of Standards references. The infrared emissivity of the external targets is better
than 0.996, while that of the internal target is calculated to be at least 0.994 (Weinreb et al. 1990).
The pre-flight calibration of the visible channels is accomplished by a large aperture integrating sphere equipped with 12 calibrated quartz iodide lamps (Kidwell 1988). The sphere is calibrated against a National Bureau of Standards reference of spectral irradiance. A calibration of the AVHRR Channels 1 and 2 is achieved by varying the number of lamps illuminated in the integrating sphere from 0 to 12. The calibrated spectral irradiance of the integrating sphere is modulated by the spectral response of each channel to give an effective radiance, which is divided by a calculated channel radiance due to sunlight, to derive effective albedo values. These values are used to derive a simple linear relationship between the albedo and sensor output, which is disseminated by NOAA to be applied to the in-orbit measurements.
