- Albert Schweitzer
History tends to look fondly upon trailblazers, even if they don't necessarily stick around. From musicians and actors to politicians and inventors, our lives are immeasurably enriched by the contributions of visionaries who left us.
So when NASA's Jet Propulsion Laboratory, Pasadena, Calif., launched an experimental satellite called Seasat to study Earth and its seas 35 years ago this week, only to see the mission end just 106 days later due to an unexpected malfunction, some at the time may have looked upon it as a failure. But this spunky satellite, which is still in orbit, shining in the night sky at magnitude 4.0, continues to live on through the many Earth and space observation missions it has spawned.
Seasat's tale began in 1969, when a group of engineers and scientists from multiple institutions convened at a conference in Williamstown, Mass., to study how satellites could be used to improve our understanding of the ocean. Three years later, NASA began planning for Seasat, the first multi-sensor spacecraft dedicated specifically to observing Earth's ocean. A broad user working group from many organizations defined its requirements. JPL was selected to manage the project, and numerous other NASA centers and government and industry partners participated. On the night of June 26, 1978, Seasat was launched from California's Vandenberg Air Force Base aboard an Atlas-Agena rocket, carrying with it three prototype radar instruments and two radiometers.
During its brief life, Seasat collected more information about ocean physics than had been acquired in the previous 100 years of shipboard research. It established satellite oceanography and proved the viability of several radar sensors, including an imaging radar, for studying our planet. Most importantly, it spawned many subsequent Earth remote-sensing satellites and instruments at JPL and elsewhere that track changes in Earth's ocean, land and ice, including many currently in orbit or in development. Its advances were also subsequently applied to missions to study other planets.
Post-Seasat NASA program manager Stan Wilson said Seasat demonstrated the potential usefulness of ocean microwave observations. "As a result, at least 50 satellites have been launched by more than a dozen space agencies to carry microwave instruments to observe the ocean. In addition, we have two continuing records of critical climate change in the ocean that are impacting society today: diminishing ice cover in the Arctic and rising global sea level. What greater legacy could a mission have?"
"Seasat flew long enough to fully demonstrate its groundbreaking remote sensing technologies, and its early death permitted the limited available resources to be marshaled toward processing and analyzing its approximately 100-day data set," said Bill Townsend, Seasat radar altimeter experiment manager. "This led to other systems, both nationally and internationally, that continued Seasat's legacy, enabling Seasat technologies to be used to better understand climate change."
Seasat's experimental instruments included a synthetic aperture radar (SAR), which provided the first-ever highly detailed radar images of ocean and land surfaces from space; a radar scatterometer, which measured near-surface wind speed and direction; a radar altimeter, which measured ocean surface height, wind speed and wave heights; and a scanning multichannel microwave radiometer that measured atmospheric and ocean data, including wind speeds, sea ice cover, atmospheric water vapor and precipitation, and sea surface temperatures in both clear and cloudy conditions.
On June 28, the Alaska Satellite Facility will release newly processed digital SAR imagery from Seasat. The imagery, available for download at http://www.asf.alaska.edu , will enable scientists to travel back in time to research the ocean, sea ice, volcanoes, forests, land cover, glaciers and more. Before now, only about 20 percent of Seasat SAR data had been processed digitally.
In oceanography, Seasat gave us our first global view of ocean circulation, waves and winds, providing new insights into the links between the ocean and atmosphere that drive our climate. For the first time, the state of an entire ocean could be seen all at once. Seasat's altimeter, which used pulses of microwave radiation to measure the distance from the satellite to the ocean surface precisely, mapped ocean surface topography, allowing scientists to demonstrate how sea surface conditions could be used to determine ocean circulation and heat storage. The data also revealed new information about Earth's gravity field and the topography of the ocean floor.
"The short 100-day Seasat mission provided a moment of epiphany to remind people that the vast ocean is best accessed from space," said Lee-Lueng Fu, JPL senior research scientist and project scientist for the NASA/French Space Agency Jason-1 satellite and NASA's planned Surface Water and Ocean Topography mission.
Seasat inspired a whole generation of scientists. "I decided to take a job offer at JPL fresh out of graduate school because I was told that the future of oceanography is in satellite oceanography and the future of satellite oceanography will begin with Seasat at JPL," said JPL oceanographer Tim Liu. "I did not plan to stay forever, but I have now been here more than three decades."
Since Seasat, advanced ocean altimeters on the NASA/European Topex/Poseidon and Jason missions have been making precise measurements of sea surface height used to study climate phenomena such as El Niño and La Niña. The newest Jason mission, Jason-3, is scheduled to launch in 2015 to continue the 20-plus-year climate data record. Satellite altimetry has been used to improve weather and climate models, ship routing, marine mammal studies, fisheries management and offshore operations. Seasat's scatterometer gave us our first real-time global map of the speed and direction of ocean winds, which drive waves and currents and are the major link between the ocean and atmosphere. A scatterometer is a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of Earth from an aircraft or a satellite. The technology was later used on JPL's NASA Scatterometer, Quikscat spacecraft, SeaWinds instrument on Japan's Midori 2 spacecraft and the OSCAT instrument on India's Oceansat-2. It will also be used on JPL's ISS-RapidScat instrument, launching to the International Space Station in the spring of 2014. Data from these scatterometers, including three scatterometers launched by the European Space Agency, help forecasters predict hurricanes, tropical storms and El Ninos.
Seasat's microwave radiometer, which subsequently flew on NASA's Nimbus-7 satellite, led to numerous successful radiometer instruments and missions used for oceanography, weather and climate research. Radiometers measure particular wavelengths of microwave energy. The Seasat radiometer's heritage includes the Special Sensor Microwave Imager instruments launched on United States Air Force Defense Meteorological Satellite Program satellites, the joint NASA/Japanese Aerospace Exploration Agency (JAXA) Tropical Rainfall Measuring Mission microwave imager, the Advanced Microwave Scanning Radiometer (AMSR)-E that flew aboard NASA's Aqua spacecraft, JAXA's current AMSR-2 instrument, and numerous other radiometers launched by Europe, China and India. The radiometer, scatterometer and SAR for NASA's Soil Moisture Active Passive mission to measure global soil moisture, launching in 2014, also draw upon Seasat's heritage.
By simultaneously flying a radiometer with a radar altimeter, Seasat demonstrated the benefit of using radiometer measurements of water vapor to correct altimeter measurements of sea surface height. Water vapor affects the accuracy of altimeter measurements by delaying the time it takes for the altimeter's signals to make their round trip to the ocean surface and back. This technique has been used on all subsequent NASA/European satellite altimetry missions.
Seasat's oceanographic mission also studied sea ice and its role in controlling Earth's climate. Its SAR provided the first high-resolution images of sea ice, measuring its movement, deformation and age. Today, SAR and scatterometers are also used to monitor Earth's ice from space.
"It's hard to imagine where we would be without the radiometer pioneered on Seasat, but certainly much further behind in critical Earth observations than we are now," said Gary Lagerloef of Earth & Space Research, Seattle, principal investigator of NASA's Aquarius mission to map ocean surface salinity. The Aquarius radiometer and scatterometer also trace their heritage back to Seasat.
Seasat's SAR monitored the global surface wave field and revealed many oceanic- and atmospheric-related phenomena, from current boundaries to eddies and internal waves.
Beyond the ocean, Seasat's SAR provided spectacular images of Earth's land surfaces and geology. Seasat data were used to pioneer radar interferometry, which uses microwave energy pulses sent from sensors on satellites or aircraft to the ground to detect land surface changes such as those created by earthquakes, and measure land surface topography. Three JPL Shuttle Imaging Radar experiments flew on the Space Shuttle in the 1980s/1990s. In 2000, JPL's Shuttle Radar Topography Mission used the technology to create the world's most detailed topographic measurements of more than 80 percent of Earth's land surface. Today, the technology is being used on JPL's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) airborne imaging radar system for a wide variety of Earth studies. Among the international SAR missions with heritages tracing to Seasat are the Japanese Earth Resources Satellite 1 and Advanced Land Observing System 1, the Canadian/U.S. Radarsat 1 and the European Space Agency's Remote Sensing Satellites. The technology will also be used on NASA's planned Surface Water and Ocean Topography mission, planned for launch in 2020.
Paul Rosen, JPL project scientist for a future NASA L-band SAR spacecraft currently under study, said Seasat's demonstration of spaceborne repeat-pass radar interferometry to measure minute Earth surface motions has led to a new field of space geodetic imaging and forms the basis for his new mission.
"Together with international L-band SAR sensors, we have the opportunity in the next five years to create a 40-year observation record of land-use change where overlapping observations exist," Rosen said. "These time-lapse images of change will provide fascinating insights into urban growth, agricultural patterns and other signs of human-induced changes over decades and climate change in the polar regions."
Beyond Earth, Seasat technology was used on JPL's Magellan mission, which mapped 99 percent of the previously hidden surface of Venus, and the Titan radar onboard the JPL-built and -managed Cassini orbiter to Saturn.
Seasat was managed by JPL for NASA, with significant participation from NASA's Goddard Space Flight Center, Greenbelt, Md.; NASA's Wallops Flight Facility, Wallops Island, Va.; NASA's Langley Research Center, Hampton, Va.; NASA's Glenn Research Center, Cleveland, Ohio; Johns Hopkins University Applied Physics Laboratory, Laurel, Md.; Lockheed Missiles and Space Systems, Sunnyvale, Calif.; and NOAA, Washington, D.C.
For more on Seasat, visit: http://podaac.jpl.nasa.gov/SeaSAT and http://www.jpl.nasa.gov/multimedia/seasat/intro.html . JPL is a division of the California Institute of Technology, Pasadena.