Recent scientific assessments by the Intergovernmental Panel on Climate Change (IPCC), the National Research Council (NRC), and the report of a recent external advisory panel to NASA all come to the same conclusion: the largest uncertainties in our ability to predict future climate change are associated with the radiative effects of aerosols and clouds. Focused global measurements of aerosols and clouds are required to improve predictive capability. These necessary measurements are not provided by present spaceborne missions.

The CALIPSO mission provides these crucial measurements in a timely and cost-effective manner. The payload combines a 3-channel lidar with two carefully selected passive sensors-Wide Field Camera (WFC), and Imaging Infrared Radiometer (IIR)-to obtain unique data on aerosol and cloud vertical structure and optical properties. The CALIPSO satellite will fly in formation with Aqua to produce a coincident, 3-year, global data set that is essential for accurate quantification of aerosol and cloud radiative effects. CloudSat, another ESSP mission, will fly for two years in formation with CALIPSO to produce complementary science data for cloud structure, physical properties, and their effects on climate. PARASOL, the microsatellite mission recently approved by the CNES Scientific Advisory Committee, will fly in formation with CALIPSO to produce a complementary data set to CALIPSO, Aqua, and CloudSat. These four satellites together with the NASA Aura Satellite are referred to as the "afternoon constellation".