--Grey Valenti (grey@berfle.gsfc.nasa.gov), NASA Goddard Space Flight Center
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The Sea-viewing Wide Field-of-view Sensor (SeaWiFS), was launched on August 1, 1997, from Vandenberg Air Force Base, California. After a month of adjusting its orbit and a few weeks of testing data, SeaWiFS began providing the first ocean color data in over 10 years, and the first continuous, synoptic look at the global biosphere ever. |
SeaWiFS was designed to collect ocean color data which are critical for studying ocean primary production and global biogeochemistry, and assessing the ocean's role in the global carbon cycle, as well as fisheries research and coastal management.
As it turns out, SeaWiFS is giving the science community much more than ocean color data. "The images are more than we ever could have hoped for," said Gene Feldman, who heads SeaWiFS's data processing team. "Although originally designed to just study the oceans, we've also developed a way of using it to study the land as well, and as a result, can study the 'global biosphere' for the very first time."
SeaWiFS is a cost-sharing collaboration between the National Aeronautics and Space Administration (NASA) and Orbital Sciences Corporation (OSC) wherein NASA's Goddard Space Flight Center (GSFC) specified the data attributes and bought the research rights to these data. The SeaWiFS Project at GSFC is responsible for the calibration and validation of these data. OSC provided the spacecraft, instrument, and launch, and is responsible for spacecraft operations for five years at a fixed price, while retaining the operational and commercial rights to these data. In late December, the SeaWiFS Project completed its evaluation of the data and determined that the data met all of the contractual specifications.
Satellite ocean color research began with the proof-of-concept Nimbus-7/Coastal Zone Color Scanner (CZCS), which collected data on an irregular temporal and spatial basis from November 1978 to June 1986. Despite coverage limitations and severe degradation in the sensor's sensitivity, the CZCS mission produced a substantial volume of published scientific results on marine ecosystem dynamics, which prompted the U.S. to launch SeaWiFS as a component of NASA's Earth Science Enterprise. SeaWiFS collects global data (ocean and land) every two days.
Since operations began on September 18, 1997, in addition to providing global estimates of oceanic chlorophyll-a and other bio-optical quantities to the international research community, a number of improvements in the processing code and algorithms for navigation, calibration, quality control, and atmospheric correction were implemented. Based on the results of the initial validation, a reprocessing of all data was initiated in mid-January and completed by mid-February 1998. At this time, the data products can now be considered science quality.
Independent verification of the SeaWiFS products is possible using match-up data from a number of postlaunch bio-optical cruises. The Atlantic Meridional Transect (AMT) is a primary source of validation data. Twice a year, the Royal Research Ship James Clark Ross steams a meridional transect of the Atlantic Ocean between Grimsby (UK) and Stanley (Falkland Islands), as part of the annual research activities of the British Antarctic Survey. In September, the ship sails from the UK, and the following April it makes the return trip, sailing from Stanley. To date, there have been five AMT cruises. The U.S. participation is established through a collaboration between the SeaWiFS Project and the Plymouth Marine Laboratory. The SeaWiFS participation is directed by SeaWiFS Deputy Project Scientist Stanford Hooker. With the wide diversity of ecosystems encountered (two hemispheres and two seasons are sampled over a period of 30-35 days), an AMT cruise is like many cruises rolled into one, making it inevitable that new observations and new discoveries will emerge on every transect. Later this year, the SeaWiFS Project will also be conducting field experiments with the European Community Joint Research Center in the northern Adriatic Sea.
The initial validation results demonstrate that the SeaWiFS instrument has been stable over the first four months of operation, the vicarious calibration approach using MOBY data results in consistent global water-leaving radiances, and the products meet the accuracy goals over a limited, but diverse, set of validation sites.
In order to protect the commercial market, the OSC contract contains a two-week embargo on the release of SeaWiFS data by NASA. Because NASA has the contractual responsibility for calibration and validation of the data and derived products, however, the contract contains provisions to allow the distribution of near-real-time data to participants in field studies, and calibration and validation activities.
Five of these agreements have gone to U.S. coastal stations, including NASA GSFC, the Naval Research Laboratory at Stennis Space Center, the Monterey Bay Aquarium Research Institute, the University of Alaska at Fairbanks and, in the future, a National Oceanic and Atmospheric Administration/National Marine Fisheries Service facility in Hawaii. The SeaWiFS Project copies decrypted data from these stations in real time.
The other eight agreements can be assigned for times up to three months to other HRPT stations. To receive one of these temporary assignments, an investigator or program office must submit a different application, co-signed by the HRPT station manager, to the SeaWiFS Project. This form is also available from the SeaWiFS Home Page. Any of these 13 stations may release real-time data to investigators conducting field studies with the approval of the SeaWiFS Project.
Onboard collection of one-kilometer resolution local-area-coverage (LAC) data is scheduled for 10 to 12 minutes each day by the SeaWiFS Project and is submitted as a command sequence to OSC for transmission to the spacecraft. Figure 1 shows the distribution of LAC targets and the areas covered by the satellite on a specific day (Oct. 30, 1997). Several of the targets shown are fixed sites, such as MOBY; these targets are scheduled for every overpass. Onboard LAC data are not scheduled separately if a given site is within the area of a SeaWiFS real-time station. For example, the Bermuda Atlantic Time-Series site is covered by the Bermuda HRPT station and is thus not scheduled separately.
Because data from satellite-mounted ocean color sensors can be extremely useful in planning and carrying out field studies, the SeaWiFS Project will provide real-time images to field investigations who are assisting with calibration and validation. To receive this support, investigators must provide station locations at least seven days in advance. This is the amount of time needed for processing the onboard LAC data acquisition into the spacecraft command sequence. Investigators can tailor products to their needs, specifying the size of the area to be covered or which SeaWiFS geophysical product they want, for example, by filling out an order form available from the Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Home Page (http://simbios.gsfc.nasa. gov/) under "Scheduling." Once the SeaWiFS Project receives and processes the
data, the requested area is extracted, mapped for the specific project with latitude and longitude grids, and sent by electronic mail to a designated location.
The objective of the SeaWiFS Project is to make data as accessible and usable as possible so as to promote the scientific goals of the mission. To assist the data user community in understanding the various technical aspects of ocean color remote sensing and the SeaWiFS Project's development efforts, the Project has supported the publication of 43 technical memorandums and has several others near completion. The series is available from the SeaWiFS Home Page. Users are invited to suggest ways to improve the products generated from the data.