THE POLDER MISSION - INSTRUMENT CHARACTERISTICS AND SCIENTIFIC OBJECTIVES

This paper introduces the new Polarization and Directionality of the E arth's Reflectances (POLDER) instrument. The spaceborne POLDER sensor, which is selected to fly aboard the Japanese ADEOS satellite schedule d for launch in early 1996, has nearly complete phase C of its develop ment at the Centre National d'Etudes Spatiales, the French space agenc y. To prepare for the 1996 space mission, airborne prototypes are bein g tested and evaluated in the framework of various measurement campaig ns. The POLDER sensor is designed to collect global observations of po larized and directional solar radiation reflected by the Earth-atmosph ere system for climate and global change studies. Aboard the ADEOS pla tform, the POLDER mission will provide near-daily coverage of the Eart h at 6 x 7 km2 resolution. The POLDER system will offer unprecedented opportunities to observe biophysical parameters over the oceans and la nd surfaces. The sensor's unique features, when compared to current an d planned spaceborne instruments, include its ability to: 1) measure p olarized reflectance in the visible and near-infrared spectral regions ; 2) observe Earth target reflectance from 12 directions during a sing le satellite pass; and 3) operate in two dynamic modes in order o achi eve both high signal to noise ratio and wide dynamic range. Six of POL DER's eight channels are optimized for observing a atmospheric aerosol s, clouds, ocean color, and land surfaces. The other two are centered on the H20 and 02 absorption bands for retrieving atmospheric water va por amount and cloud top altitude, respectively. POLDER data will be s ubject to the high calibration standards defined by the POLDER mission team, with absolute calibration accuracies of 2% for the shorter wave length channels (lambda less-than-or-equal-to 565 nm) and 3% for the l onger wavelengths. A 1% accuracy is the goal for the intercalibration between the spectral channels. The POLDER instrument aboard ADEOS will contribute significantly to climate-related research on aerosol cycli ng, cloud-radiation interactions, the Earth radiation budget, ocean pr imary productivity, and continental biosphere dynamics. Using POLDER c apability to measure bidirectional reflectance and polarization distri bution in functions for estimating biophysical parameters, the POLDER mission's scientific objectives will be to: 1) map atmospheric aerosol s, including their sources and transport, and study their influence on the Earth radiation budget; 2) assess cloud properties, namely their height, phase and type; 3) estimate total integrated water vapor amoun t; 4) improve Earth radiation budget estimates; 5) estimate chlorophyl l-like pigment content in the ocean surface layer and its role in the carbon cycle; and 6) characterize land surface properties and vegetati on cover. In meeting these objectives, POLDER will play an important r ole in advancing the measurement and research objectives of the World Climate Research Program (WCRP) and the International Geosphere and Bi osphere Program (IGBP).