Calibration evaluation and radiometric testing of field radiometers with the SeaWiFS Quality Monitor (SQM)

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; (cf 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 0.60% 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.