Monitoring the AVHRR calibration

Over the years growing consideration has been given to the use of remote sensing data to absolute calibration problems and gain drift monitoring. Although most of the satellites have on-board calibration devices, their use, especially in then shortwave bands, remains difficult because of their uncontrolled ageing. In order to solve such a problem, some experiments are periodically held to quantify the decay of the sensors and to get a realistic gain value. These experiments suffer from low repeatability (presently one is held approximately every 6 months), atmospheric contamination, directional effects and from the very few sites being used. In the study the authors present a method able to monitor the variation of the gain of the sensors during intermediate periods using large homogeneous areas as targets over the world. Several sites have been selected and observed over the last 6 years. Their bidirectional behavior has been studied as well as their temporal stability. Some of the targets seem appropriate and experiments with several satellites were held. An attempt to complete an intercalibration of the different sensors used was also made