An Evaluation of Oceanographic Radiometers and Deployment Methodologies

Abstract

The primary objective of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project is to produce water-leaving radiances within an uncertainty of 5% in clear-water regions, and chlorophyll a concentrations within 35% over the range of 0.05?50 mg m?3. Any global mission, like SeaWiFS, requires validation data from a wide variety of investigators. This places a significant challenge on quantifying the total uncertainty associated with the in situ measurements, because each investigator follows slightly different practices when it comes to implementing all of the steps associated with collecting field data, even those with a prescribed set of protocols. This study uses data from multiple cruises to quantify the uncertainties associated with implementing data collection procedures while using different in-water optical instruments and deployment methods. A comprehensive approach is undertaken and includes (a) the use of a portable light source and in-water intercomparisons to monitor the stability of the field radiometers, (b) alternative methods for acquiring reference measurements, and (c) different techniques for making in-water profiles. Three optical systems had quadrature sum uncertainties sufficiently small to ensure a combined uncertainty for the spaceborne and in situ measurements within a total 5% vicarious calibration budget. A free-fall profiler using (relatively inexpensive) modular components performed best (2.7% quadrature sum uncertainty), although a more sophisticated (and comparatively expensive) profiler using integral components was very close and only 0.5% higher. A relatively inexpensive system deployed with a winch and crane was also close, but ship shadow contamination increased the quadrature sum uncertainty to approximately 3.4%.