The calibration and validation of SeaWiFS data

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) is the successor ocean color imaging system to the Nimbus-7 Coastal Zone Color Scanner (CZCS). The SeaWiFS calibration and validation effort includes spacecraft, atmospheric , sea surface, subsurface (or in situ), plus laboratory and data analysis c omponents which require pre- and postlaunch activities. The most important goals of this effort are to produce water-leaving radiances with an uncerta inty of 5% in clear-water regions and chlorophyll a concentrations within /-35% over the range of 0.05-50 mg m(-3). The first objective requires fiel d instruments with a calibration and measurement capability on the order of 1%; because these challenging in situ measurements will be acquired from a variety of field instruments over the five-year mission interval, a measur ement assurance program is required. This program consists of several activ ities: an accurate pre launch characterization and calibration of the SeaWi FS instrument; a Marine Optical Buoy (MOBY) rotation in clear water to prov ide a water-leaving radiance time series for postlaunch vicarious calibrati on: the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS) to hold th e relevant data; clearly defined SeaWiFS Ocean Optics Protocols (SOOP) for established data collection methodologies; annual SeaWiFS Intercalibration Round-Robin Experiments (SIRREXs) for intercomparing field and calibration equipment, and training scientific personnel; direct comparison to a nation al standard laboratory using the SeaWiFS Transfer Radiometer (SXR); a porta ble field source, called the SeaWiFS Quality Monitor (SQM), for monitoring the temporal stability of the calibration of field instruments; a highly ac curate atmospheric correction algorithm designed for the SeaWiFS instrument response functions; bio-optical algorithms that encompass a broad range of bio-optical provinces; and satellite data processing, duality control, and analysis procedures for monitoring the postlaunch performance of the senso r and the validity of the derived products. The culmination of many of thes e activities is the deployment of the instruments and methodologies on Atla ntic Meridional Transect (AMT) cruises between England and the Falkland Isl ands, a 13 000 km voyage spanning more than 100 degrees of latitude, with a calibration and measurement accuracy that is on the order of 1%. The AMT P rogram is the primary product validation activity supported by the SeaWiFS Project The AMT cruises also serve as a testbed for new technology developm ent and have demonstrated that high quality bio-optical data can be routine ly provided to the Project in near-real time. (C) 2000 Elsevier Science Ltd . All rights reserved.