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Overview The color of the ocean, which can be measured from space, reveals information on the presence and concentration of phytoplankton, sediments, and dissolved organic chemicals. By studying the color of the light scattered from the oceans, optical sensors can quantify the amount of chlorophyll and other constituents. This is important to the carbon cycle and hence global warming. The optical sensors can be deployed in or on the ocean, from aircraft, or from satellites. Correct interpretation of the data involves accounting for light scattered or absorbed by the atmosphere and reflected off the ocean's surface. Because this is difficult, and because optical instruments in space tend to degrade, a coordinated measurement program is the only approach (termed a calibration and validation program). The goal is to determine the water-leaving radiance with field instruments with an accuracy of 1%. For satellite instruments, the goal is 5%. The prime ocean color satellite instrument for NASA is the Sea-viewing, Wide Field-of-View Sensor (SeaWiFS), which was launched in August 1997. Next year, NASA will launch a satellite for the Earth Observing System (EOS) that will carry a second ocean color satellite, the Moderate Resolution Imaging Spectrometer (MODIS). Data acquired for several months from a recent Japanese instrument, the Ocean Color Temperature Scanner (OCTS), are also being studied by scientists. NOAA is acquiring daily measurements of ocean color using spectroradiometers on a tethered buoy. This system, named the Marine Optical Buoy (MOBY), is used to validate the data acquired with SeaWiFS. Statement of Work The work to be performed by the National Institute of Standards and Technology (NIST) in support of the SeaWiFS Project Office (SPO) are the following:
Goals and Accomplishments The radiometric measurements used to study ocean color are generally traceable to NIST through the lamp standards of spectral irradiance, which are calibrated on FASCAL. However, the link may be indirect. To ensure the accuracy of the measurements, the SeaWiFS Project and NIST developed a program that allowed for direct comparison of spectral radiance, provided for training, and established a means to track the performance of commercial sensors used for in situ measurements. NIST built and characterized the SeaWiFS Transfer Radiometer (SXR). The SXR is a six channel filter radiometer that is used to measure or verify the spectral radiance of radiometric sources. The SXR has been used to measure the radiance of the sphere sources used to calibrate OCTS, SeaWiFS, MOBY, and MODIS. Later, NIST built a field-deployable stable light source, the SeaWiFS Quality Monitor (SQM), which has been used on several Atlantic Meridional Transect cruises. The SQM has quantified, for the first time, the radiometric stability of optical sensors used for the in-water and at-surface radiometric measurements. In addition, NIST has played a key role in five SeaWiFS Intercalibration Round-Robin Experiments (SIRREXs). These activities allow for a direct comparison of sources and detectors, as well as training in proper measurement technique. For the support of NOAA's MOBY program, NIST designed field-deployable filter radiometers that have interchangeable optics, for spectral radiance and spectral irradiance. This allows the MOBY team to monitor the stability of both types of standard sources at the field site in Snug Harbor, Honolulu. Publications Johnson, B.C., Shaw, P.-S., Hooker, S.B., and Lynch, D., "Radiometric and engineering performance of the SeaWiFS quality monitor (SQM): A portable light source for field radiometers," J. Atmos. Oceanic Tech. 15, 1008-1022 (1998).
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