The Sea-viewing Wide Field-of-view Sensor (SeaWiFS)—which was launched on August 1, 1997, onboard the Orbital Sciences Corporation (OSC; Dulles, VA) SeaStar spacecraft—took its first picture of the world`s oceans on September 16, 1997. The swirling composite image below represents the first 24 hours of data taken from an altitude of 440 miles (705 kilometers).
The colors indicate varying concentrations of chlorophyll in the oceans. Reds show high concentrations of chlorophyll, yellows and greens indicate intermediate concentrations, and blues and purples represent low concentrations. The black swaths indicate gaps between the orbits, where no data were collected. SeaWiFS observes the Earth from a noontime sun-synchronous orbit, which means that the sensor is always viewing the Earth at local noon for maximum solar illumination.
This orbit is most desirable for detecting concentrations of phytoplankton, microscopic green plants that live just beneath the ocean surface. Phytoplankton are critical in the global carbon cycle, and knowing their distribution will help scientists visualize the dynamics of ocean and coastal currents, the physics of mixing, and the relationships between ocean physics and large-scale patterns of ocean productivity.
The SeaWiFS consists of an electronic module and an optical scanner sensitive to eight central wavelengths ranging from 412 to 865 nm. The instrument was modified to produce a bilinear response. The original sensitivity is maintained up to 80% of the digital output range and then changed discontinuously to extend the dynamic range substantially. The net result is no expected saturation over clouds.
Scanning mechanisms drive an off-axis folded telescope and a rotating half-angle mirror that is phase-synchronized with, and rotating at half the speed of, the folded telescope. The rotating scanning telescope, coupled with the half-angle scan mirror arrangement, permits a minimum level of polarization without field-of-view rotation, over the maximum scan angle requirement of 58.3°.
The project is part of NASA`s Mission to Planet Earth Enterprise, a long-term, coordinated research effort to study the Earth as a global system. The SeaWiFS is the follow-up ocean color instrument to the Coastal Zone Color Scanner (CZCS), which was taken out of commission in 1986 after an eight-year mission. Using the vantage point of space, NASA is observing, monitoring, and assessing large-scale environmental processes such as ocean productivity, focusing on climate change.
Studying the biosphere
The global ocean color images from SeaWiFS will also be used by researchers to study how human-induced and natural events affect the global biosphere (see paired images). These images of the Chesapeake Bay offer two renditions of the SeaWiFS high-resolution data over the Mid-Atlantic coast of the USA on Sept. 19, 1997. The top panel, a combination of 412-, 555-, and 670-nm wavelengths, highlights vegetation associated with land morphology, such as the coastal plain and mountain ridges and valleys. White areas are clouds and dense aerosols. Ocean features are not noticeable.
The lower panel is a composite of ocean radiances at 412, 510 and 555 nm after atmospheric correction of the data. Particularly evident are plumes of material discharging from eastern Long Island Sound (upper right corner). Red and yellow areas in Chesapeake Bay indicate turbid waters while the blue hues offshore represent clear ocean water. Black areas (bottom right corner) are locations where processing could not be completed because of cloud cover and other quality control tests conducted during the processing.
For up-to-date images, visit the SeaWiFS Web site: seawifs.gsfc.nasa.gov/SEAWIFS.html.