Calibration Evaluation and Radiometric Testing of Field Radiometers with the SeaWiFS Quality Monitor (SQM)

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Abstract
One of the goals of calibration and validation programs supporting ocean color satellites is to produce water-leaving radiances with an uncertainty of 5% in clear-water regions. This objective requires field instruments with a calibration and measurement capability that is on the order of 1%. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project, in collaboration with the National Institute of Standards and Technology, has developed a portable illumination source with three temperature-stabilized internal monitors designed to provide a stable light field for checking the optical stability of radiometers used to measure the in situ optical properties of seawater. This device is called the SeaWiFS Quality Monitor (SQM). A recent field evaluation during an extensive research cruise indicates the SQM has the following capabilities: (a) the SQM can be used to track the stability of field radiometers at less than the 1% level in terms of the radiometric response of the instruments—on average 0.30% (±0.15%) for radiance sensors and 0.58% (±0.20%) for irradiance sensors; (b) the SQM light field is sufficiently stable to allow for a sensitive measure and, thus, modeling of changes in the radiometric detectors;(c) based on the radiometers used during the field evaluation, daily SQM measurements are needed to resolve the temporal changes in the response of the sensors; and (d) SQM performance, in terms of the generated light field and the SQM internal monitors, is very stable and decayed only by approximately 0.6% during the course of the 36-day deployment with most of the decay attributed to a change in the operating voltage of one of the lamps.

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