Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) represents a new way of doing business for NASA. Rather than building, launching, and controlling a satellite to study an important aspect of the Earth’s environment, NASA will purchase commercially available data from a privately built satellite and use the data for environmental research.

The SeaWiFS instrument will observe the world’s oceans from space to measure "ocean color." The color of most of the world’s oceans varies with the concentration of microscopic marine plants, called "phytoplankton," which contain chlorophyll, a green pigment. Near coastlines, the color of the ocean is affected by chlorophyll, dissolved organic material and suspended sediments from rivers and lagoons. By observing the color of different parts of the oceans, scientists can measure the amount of these materials in the water.

Measuring phytoplankton concentrations is essential in understanding the role of the oceans in the global carbon cycle—the process by which carbon travels through the Earth’s atmosphere, oceans, land, and living organisms. Phytoplankton removes carbon dioxide from the atmosphere for internal use, and scientists want to understand this exchange of carbon dioxide and the role it plays in the global climate.

For example, the increase of carbon dioxide in the Earth’s atmosphere since the Industrial Revolution led some scientists to conclude the Earth’s temperature is rising. However, the exact "map" of the global carbon cycle is poorly understood, making it difficult to accurately predict the response of the global climate to changes such as increases in carbon dioxide. SeaWiFS data will help scientists understand one important portion of the carbon cycle.

Mission Structure

SeaWiFS scientists are developing, and will operate, a research data system that will process, calibrate, validate, archive, and distribute SeaWiFS data for research. All other aspects of the mission— satellite construction, launch, command and control, and tracking—are the responsibility of the Orbital Sciences Corporation (OSC), Fairfax, Va. NASA has contracted with OSC to provide the raw satellite data for five years.

OSC will integrate the SeaWiFS instrument, built by Hughes Electronics at Santa Barbara Remote Sensing, Goleta, Calif., into its SeaStar satellite and will market the data for commercial and operational use following launch.

Goals and Objectives

Ocean color data is identified as being critical by oceanographic scientists to study the oceans’ "primary production" and the interactions between the living and non-living components of the ocean. Primary production refers to the rate at which primary producers, such as algae and some bacteria, create organic material. These primary producers exist at the lowest levels of the food chain and absorb sunlight or chemical energy, rather than other organic material, as sources of energy.

SeaWiFS can view the world’s oceans every two days. Because oceans cover 70 percent of the Earth’s surface, SeaWiFS will provide information on a large part of the global biosphere. SeaWiFS also will provide important information for fisheries and coastal zone management. SeaWiFS data, which are also useful for viewing plants on land, can be combined with plant productivity data from other satellites, such as Landsat and some of NOAA’s -operational weather satellites, to measure the total global carbon exchange.

The primary objective of the SeaWiFS Project is the distribution of SeaWiFS data to scientists conducting research in this and related subjects. The goals of the SeaWiFS program include determining the size and types of primary production by marine phytoplankton on a global scale and measuring the oceans’ role in the global carbon cycle and other bio-geochemical cycles. Finally, in preparation for future Earth Observing System missions, the SeaWiFS scientific and technical experience will advance the applications of ocean color data and the technical capabilities required for the processing, management, analysis, and distribution of large amounts of data.

Instrument Operations

SeaWiFS is a successor to the Coastal Zone Color Scanner (CZCS), which operated aboard NASA’s Nimbus-7 satellite from 1978–1986 and proved that satellite sensors could detect ocean color from space. CZCS produced vivid (false color) images of phytoplankton concentrations in the oceans. Benefiting from the CZCS experience, SeaWiFS will measure more wavelengths of light (colors) than its predecessor, thereby improving the accuracy of the data and the ability to distinguish chlorophyll from other substances found in sea water. These improvements will result in a more accurate determination of phytoplankton concentration.

Spacecraft Description

SeaWiFS will be launched from an airplane off the U.S. West Coast. The launch vehicle will be an expendable Pegasus (stretch) rocket, released at an altitude of 39,000 feet (approximately 12 kilometers) from a modified Lockheed L-1011 jet. Following payload separation, an onboard hydrazine propul-sion system will then raise the spacecraft to its final 440-mile (705-kilometer) circular orbit within approximately 20 days after launch.

Calibration and Validation

NASA’s Goddard Space Flight Center (GSFC), Greenbelt, Md., is responsible for product assur-ance, calibration, and validation programs and to determine the degree to which the ocean color data fulfills the NASA requirements. Data collection will begin 30 days after launch. The four main goals of this program are: 1) to calibrate the SeaWiFS instru-ment; 2) to develop and validate methods of removing atmospheric effects that can degrade data quality; 3) to develop and validate derived data, such as chlorophyll concentrations and the fluctuation of the amount of light that penetrates the oceans; and 4) distribute the data to the scientific community.

Data Processing and Distribution

The SeaWiFS Data Processing System (SDPS) will receive approximately 381 megabytes of science data per day and generate science data products totaling up to three gigabytes (3,000 megabytes) per day. One gigabyte is equivalent to four sets of encyclopedias at 29 volumes per set, so SeaWiFS will produce the equivalent of 348 volumes of encyclopedias per day. The SDPS computers are sized to perform preliminary data processing within 24 hours of collection, as well as reprocessing the same data using additional information to increase the accuracy when required.

SeaWiFS will produce two types of science data: local area coverage (LAC) and global area coverage (GAC). The LAC provides coded data collected from the local area under the spacecraft’s flight path, within a 1,500-mile (2,414-kilometers) swath with three quarters of a mile (1.2 kilometers) as the smallestvisible area. The data are coded for security and transmitted continuously when the satellite is in daylight.

The GAC is designed to provide global coverage at 2.5 miles (4 kilometer) ground resolution. Every fourth LAC sample is stored on board the spacecraft to produce the GAC. These data are transmitted back to a ground station at NASA’s Wallops Flight Facility (WFF), Wallops Island, Va., once every 12 hours.

The permanent data archive will reside at GSFC’s Earth Observing System Data and Information Systems’ (EOSDIS) Distributed Active Archive Center (DAAC) and is planned to reach a storage capacity of six terabytes over the lifetime of the mission. A terabyte is equivalent to 1000 gigabytes, or 696,000 volumes of encyclopedias. Users also can access the archive electronically, and receive SeaWiFS data at reproduction costs.

Mission to Planet Earth

SeaWiFS is an essential component of NASA’s Mission to Planet Earth (MTPE), an ongoing effort to study how the global environment is changing. Using the unique perspective available from space, NASA will observe, monitor and assess large-scale environmental processes, such as the oceans’ productivity, focusing on climate change.

MTPE satellite data, complemented by aircraft and ground data, will allow scientists to better understand natural environmental changes and to distinguish natural changes from human-induced changes. MTPE data, which NASA will distribute to researchers worldwide, is essential to humans making informed decisions about their environment.

Management

The SeaWiFS Program, which includes the Project, and a broad basic- and applied-science support, is managed by NASA’s Office of Mission to Planet Earth, Washington, DC. This program provides an interface with other U.S. agencies and with international partners in the MTPE.

The SeaWiFS Project, an office of GSFC’s Laboratory for Hydrospheric Processes, manages the contract, is developing and will operate the research data system. The SeaWiFS Project manager is Dr. Mary L. Cleave. Dr. Charles R. McClain is the project scientist, the deputy project scientist is Dr. Stanford B. Hooker.