THE POLDER MISSION - INSTRUMENT CHARACTERISTICS AND SCIENTIFIC OBJECTIVES

This paper introduces the new Polarization and Directionality of the Earth's Reflectances (POLDER) instrument. The spaceborne POLDER sensor, which is selected to fly aboard the Japanese ADEOS satellite scheduled for launch in early 1996, has nearly complete phase C of its development at the Centre National d'Etudes Spatiales, the French space agency. To prepare for the 1996 space mission, airborne prototypes are being tested and evaluated in the framework of various measurement campaigns. The POLDER sensor is designed to collect global observations of polarized and directional solar radiation reflected by the Earth-atmosphere system for climate and global change studies. Aboard the ADEOS platform, the POLDER mission will provide near-daily coverage of the Earth at 6 x 7 km2 resolution. The POLDER system will offer unprecedented opportunities to observe biophysical parameters over the oceans and land surfaces. The sensor's unique features, when compared to current and planned spaceborne instruments, include its ability to: 1) measure polarized reflectance in the visible and near-infrared spectral regions; 2) observe Earth target reflectance from 12 directions during a single satellite pass; and 3) operate in two dynamic modes in order o achieve both high signal to noise ratio and wide dynamic range. Six of POLDER's eight channels are optimized for observing a atmospheric aerosols, clouds, ocean color, and land surfaces. The other two are centered on the H20 and 02 absorption bands for retrieving atmospheric water vapor amount and cloud top altitude, respectively. POLDER data will be subject to the high calibration standards defined by the POLDER mission team, with absolute calibration accuracies of 2% for the shorter wavelength channels (lambda less-than-or-equal-to 565 nm) and 3% for the longer 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 cycling, cloud-radiation interactions, the Earth radiation budget, ocean primary productivity, and continental biosphere dynamics. Using POLDER capability to measure bidirectional reflectance and polarization distribution in functions for estimating biophysical parameters, the POLDER mission's scientific objectives will be to: 1) map atmospheric aerosols, 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 amount; 4) improve Earth radiation budget estimates; 5) estimate chlorophyll-like pigment content in the ocean surface layer and its role in the carbon cycle; and 6) characterize land surface properties and vegetation cover. In meeting these objectives, POLDER will play an important role in advancing the measurement and research objectives of the World Climate Research Program (WCRP) and the International Geosphere and Biosphere Program (IGBP).