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Ocean Surface Topography from Space
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Following the water with the Surface Water Ocean Topography mission
September 01, 2008

This animated gif has an image of a beautiful mountain lake, an ocean shore at sunset, and a picture of rivers.

"Follow the water" has been a strategy behind the latest Mars explorations. Scientists from NASA and CNES, the French space agency, want to take that concept in a different direction and apply it to Earth--tracking water around the globe both in the ocean and on land.

Unlike Mars, Earth has 'water, water everywhere.' But no one knows exactly how much there is, or if, in the future, there will be enough for us to drink. Oceans, lakes, rivers, reservoirs are constantly changing, interacting in ways that are not entirely clear. Add a warming planet, melting ice and more frequent floods, and the uncertainties about Earth's water resources and climate change mount.

U.S. and French oceanographers and hydrologists have joined forces to develop a new space mission to make the first global survey of Earth's surface water, observe the fine details of the ocean's surface topography, and measure how water bodies change over time. Still in an early concept stage, the new mission is called Surface Water Ocean Topography, or SWOT. SWOT was one of 15 missions listed in last year's National Research Council's decadal survey of Earth science as missions that NASA should implement in the coming decade.

"This mission goes from the watershed to the global oceans," says JPL oceanographer Lee-Leung Fu. Fu and Doug Alsdorf of Ohio State University, Columbus, are two of the leaders of the international group of researchers working to define the mission's goals and capabilities.

For oceanographers, SWOT will be part of the next generation of ocean-observing space satellites, providing data 10 times more detailed than current missions. "SWOT will take satellite oceanography well beyond what we are able to do now," says Fu. He is the project scientist for the current Jason-1 mission and the new Ocean Surface Topography Mission/Jason-2, which use radar altimetry to make precise measurements of the height of the sea surface.

Ocean currents are one of the most important means by which heat moves around the globe, so changes in circulation have a major effect on climate. SWOT will take advantage of new radar technology, he explains, that will allow it to survey the ocean surface with a resolution of one kilometer by one kilometer, much greater than the 200-300 kilometer (124-186-mile) resolution of today's satellites. "While the Jason missions see large-scale ocean circulation features, 90 percent of the ocean's energy is in ocean currents that occur on scales of 100 kilometers or smaller. We need to know about these smaller features if we are going to be able to compute the velocity and energy of ocean circulation," Fu says.

The increase in resolution of the new mission will let scientists map virtually all the ocean's currents, including those close to the coastlines, which are particularly important for the millions of people who live and work in coastal areas.

For hydrologists, scientists who study water, the new mission will provide their very first comprehensive view of Earth's freshwater bodies from space. Most measurements of Earth's surface water now are made by gauges on site, and data are unavailable for many regions.

The same technology that will enable SWOT to measure the ocean surface with such accuracy will allow it to determine the height and area of fresh water all over the globe at a resolution of about 30 meters by 30 meters, possibly even smaller.

"Water is our number one natural resource," says Alsdorf of the Ohio State University School of Earth Sciences. "In 2004, the President's Office of Science and Technology Policy's report on U.S. water resources asked the question, 'Does the U.S. have enough water?' The answer was 'We don't know.' We need to use modern science and technology to find out. SWOT is one of the solutions."

"If the U.S., one of the most technologically advanced countries in the world, doesn't know if we have enough water, the problem is even worse in most of the rest of the world, especially in developing nations," he adds.

While rainfall and groundwater supply some water needs, most of the water we use comes from surface water in lakes, reservoirs and rivers. What makes the mission especially useful for studying surface water, Alsdorf says, is that it makes its measurements over a wide swath, so it can measure surface area at the same time it measures height. "This means we can compute the storage changes of lakes and reservoirs," says Alsdorf. "There are a couple hundred million lakes around the world, and we'll know how much was gained or lost on a bi-weekly basis."

Flowing rivers present a different issue, Alsdorf explains, but the mission will make major improvements in determining how much water they hold. "SWOT can measure the slope of the water," he says, "and the steeper the slope, the faster the water is flowing." With this observation, hydrologists will be able to calculate a river's discharge. "This is key for knowing water availability for users or how much water might flood a downstream location," says Alsdorf.

A new type of radar called Ka-band radar interferometery makes the mission possible. The satellite will fly two radar antennae at either end of a 10-meter (33-foot) mast, allowing it to measure the elevation of the surface along a 120- kilometer (75-mile)-wide swath below. The new radar system is smaller but similar to the one that flew on NASA's Shuttle Radar Topography Mission, which made high-resolution measurements of Earth's land surface in 2000.

Today's ocean surface topography missions use a single, downward looking radar that measures the surface directly below the spacecraft with several hundred kilometers between satellite ground tracks. Researchers use sophisticated computer models to merge the data into a global view of sea surface height, but they lack the information needed to verify the accuracy of important details.

With its wide swath, SWOT will cover all Earth's lakes, rivers, reservoirs and oceans at least twice every 21 days. It will collect the same data everywhere, but since variations in the ocean's surface are larger in scale than those in freshwater bodies, the ocean data will be averaged over a slightly larger area than the land data to make processing more efficient. As a result, the resolution over freshwater will be higher than that over the ocean.

The mission is being developed as a collaboration between NASA and CNES. It builds on the very successful 25-year partnership between the two agencies to use radar altimetry to measure the surface of the ocean.

The scientists and engineers working on the project plan to finalize the mission's science goals and mission requirements by the end of the year, paving the way for a mission concept review next spring. "We hope to have a start by 2010 with a launch no earlier than 2015," says Fu. "We're ready to take on the next challenge."

More information about SWOT is available at:
http://bprc.osu.edu/water/index.php

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