April Flowers for redOrbit.com – Your Universe Online
Even if they don’t last long, history tends to look fondly upon trailblazers.
Thirty-five years ago this week, NASA’s Jet Propulsion Laboratory (JPL) launched an experimental satellite called Seasat, with the mission to study Earth and its seas. An unexpected malfunction ended the mission after just 106 days, leading some to look on the satellite as a failure. Seasat is still in orbit, however, shining in the night sky at magnitude 4.0 and continuing to live on through the many Earth and space observation missions it spawned.
In 1969, a group of engineers and scientists from many institutions came together at a conference in Williamstown, Mass., to study how satellites could be used to improve our knowledge of the oceans. NASA began planning for Seasat three years later as the first multi-sensor spacecraft dedicated to observing Earth’s ocean. A broad user group, numerous NASA centers and industry partners worked together, culminating in a launch on June 26, 1978.
Seasat collected more information about ocean physics in its brief life than had been collected in the previous 100 years of shipboard research. The spacecraft established satellite oceanography and proved the viability of several radar sensors, including imaging radar, for studying our planet. The Seasat mission spawned many subsequent Earth remote-sensing satellites that track changes in Earth’s ocean, land and ice, including many currently in orbit or in development. The advances gained through Seasat have been applied to missions studying other planets as well.
Stan Wilson, post-Seasat NASA program manager, said Seasat demonstrated the potential value 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.”
A RICH HERITAGE
The experimental instruments aboard Seasat 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, 2013, the Alaska Satellite Facility plans to release newly processed digital SAR imagery from Seasat, which will be available for download here. Scientists will be able to travel back in time to research the ocean, sea ice, volcanoes, forests, land cover, glaciers and more. Prior to this release, only about 20 percent of Seasat SAR data had been digitally processed.
Seasat blazed trails in other areas as well. In oceanography, it provided our first global view of ocean circulation, waves and winds. This created new insights into the links between the ocean and atmosphere that drive our climate. The state of the entire ocean could be seen all at once for the first time ever. Using pulses of microwave radiation to measure the distance from the satellite to the ocean surface precisely, Seasat’s altimeter mapped ocean surface topography. This allowed scientists to demonstrate how sea surface conditions could be used to determine ocean circulation and heat storage. New information about Earth’s gravity field and the topography of the ocean floor was also revealed.
“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.
An entire generation of scientists took inspiration from Seasat’s short mission. “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.”
Precise measurements of sea surface height used to study climate phenomena such as El Nino and La Nina have been made since Seasat using advanced ocean altimeters on the NASA/European Topex/Poseidon and Jason missions. Jason-3, the latest Jason mission, is scheduled to launch in 2015 to continue the 20-plus-year climate data record. weather and climate models, ship routing, marine mammal studies, fisheries management and offshore operations have all been improved using satellite altimetry.
The scatterometer instrument – 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 – onboard Seasat gave us our first real-time global map of the speed and direction of ocean winds that drive waves and currents and are the major link between the ocean and atmosphere. Other missions, such as JPL’s NASA Scatterometer, Quikscat spacecraft, SeaWinds instrument on Japan’s Midori 2 spacecraft and the OSCAT instrument on India’s Oceansat-2, also used the technology first tested on Seasat. Data from these instruments help to forecast hurricanes, tropical storms and El Nino events.
Another trailblazing instrument on Seasat was the microwave radiometer, which subsequently flew on NASA’s Nimbus-7 satellite. This instrument, which measures particular wavelengths of microwave energy, led to numerous successful radiometer instruments and missions used for oceanography, weather and climate research. The descendants of Seasat’s radiometer include 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. Seasat’s legacy continues in the radiometer, scatterometer and SAR for NASA’s Soil Moisture Active Passive mission to measure global soil moisture, launching in 2014.
Seasat demonstrated the benefit of using radiometer measurements of water vapor to correct altimeter measurements of sea surface height by simultaneously flying a radiometer with a radar altimeter. The accuracy of altimeter readings are affected by water vapor, which delays the time it takes for the altimeter’s signals to make their round trip to the ocean surface and back. All subsequent NASA/European satellite altimetry missions have used this technique.
Sea ice, and its role in controlling Earth’s climate, was also part of Seasat’s oceanographic mission. The SAR instrument provided the first high-resolution images of sea ice, measuring its movement, deformation and age. The SAR also monitored the global surface wave field and revealed many oceanic- and atmospheric-related phenomena, from current boundaries to eddies and internal waves. Currently, SAR and scatterometers are 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 instruments also trace their heritage back to Seasat.
BEYOND THE OCEAN
Seasat’s SAR also provided spectacular images of Earth’s land surfaces and geology. Datasets from Seasat 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. In the 1980s and 1990s, three JPL Shuttle Imaging Radar experiments flew on the Space Shuttle. 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 in 2000. Currently, JPL’s Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) airborne imaging radar system for a wide variety of Earth studies is using the technology. Many international SAR missions owe a debt to Seasat as well, including 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. Planned to launch in 2020, NASA’s Surface Water and Ocean Topography mission will also use the technology.
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 said Paul Rosen, JPL project scientist for a future NASA L-band SAR spacecraft currently under study, and it forms the basis for his 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.”
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 both used Seasat technology.
April Flowers for redOrbit.com – Your Universe Online