A FIELD EXPERIMENT IN SAHARAN ALGERIA FOR THE CALIBRATION OF OPTICAL SATELLITE SENSORS

A field experiment took place in February-March 1993 to characterize t he reflectance properties of four Saharan desert sites, identified fro m satellite imagery as having remarkably stable optical properties. Th e objective of the experiment was to measure reflectances so that they can be used as references for the calibration of optical satellite se nsors. Bidirectional measurements of the surfaces in reflectance (and also in polarization) were collected in different planes, in the visib le, near infrared, and shortwave infrared. Particular attention was gi ven to instrumental calibration, with independent calibration experime nts in the laboratory and in the held. The surface reflectance measure ments were then adjusted against an empirical model of bidirectional r eflectance, and converted to reflectances at the top of the atmosphere (TOA) using an atmospheric radiative transfer model. The angular depe ndence of these TOA reflectances, named reference reflectances, was te sted against those seen by AVHRR on the four desert sites. This compar ison shows that multiangular calibration of optical sensors using refe rence reflectances can be achieved with an accuracy better than 1 per cent.