Calibration monitoring of the visible and near-infrared channels of the Along-Track Scanning Radiometer-2 by use of stable terrestrial sites

The Along-Track Scanning Radiometer-2 (ATSR-2) is equipped with visible and near-infrared channels at 1.6, 0.87, 0.66, and 0.56 microm. An in-flight visible calibration (VISCAL) system used to convert the raw signal to top-of-the-atmosphere reflectances is described. To monitor the long-term stability of the VISCAL, a number of large-area stable terrestrial sites have been employed. We describe the methods used to determine the long-term drifts in the ATSR-2 onboard calibration device and evaluate the suitability of the sites for calibration monitoring.