April 01, 2009
Recent orbit maneuvers put the Jason-1 satellite, a joint mission of NASA and the French space agency, on the opposite side of Earth from its successor, the Ocean Surface Topography Mission on the Jason-2 satellite, operated by the U.S. and French weather agencies. Jason-1 now flies over the same region of the ocean that Jason-2 flew over five days earlier. Its ground tracks fall mid-way between those of Jason-2, which are about 315 kilometers (195 miles) apart at the equator.
This interleaved tandem mission provides twice the number of measurements of the ocean's surface, bringing smaller features such as ocean eddies into view. The tandem mission also helps pave the way for a future ocean altimeter mission that would collect much more detailed data with its single instrument than the two Jason satellites now do together.
Scientists estimate that small currents and ocean eddies, the swirling bodies of water that spin off in different directions from bigger currents, contain more than 90 percent of the ocean's kinetic energy. They're responsible for transporting heat and carbon dioxide around the globe as well as nutrients. Eddies also often serve as nurseries and feeding grounds for marine life.
Mapping these mid-sized, or mesoscale, features is impossible with a single satellite altimeter. "When there is only one altimeter with narrow, widely separated tracks," says Ted Strub of Oregon State University in Corvallis, "a small feature such as an eddy can move into the space between the satellite tracks and be entirely missed--basically hiding--only to reappear when it moves under another track further offshore. The eddies appear to "wink" in and out of existence, making it difficult to track them."
For the best view of mesoscale features such as eddies, researchers combine data from multiple altimeters. In the past, those included the TOPEX/Poseidon and the successive European Space Agency's ERS altimeters. Today, only the two Jason satellites and ENVISAT, another European Space Agency mission, are currently in operation. ENVISAT has a different orbit from the Topex and Jason satellites and collects global data every 35 days. The Jason satellites cover the globe once every 10 days.
"The old version of our eddy dataset based on data from one altimeter in a 10-day repeat and the other in a 35-day repeat was capable of resolving features about 4 degrees of longitude, or about 250 miles," says Dudley Chelton, also of Oregon State University. He and his colleagues have developed new statistical methods that have increased that resolution down to about 3 degrees of longitude, or 180 miles. With its well-coordinated sampling pattern, the Jason-1/Jason-2 tandem mission should be able to improve the resolution by another 50 percent says Chelton.
Extracting precise sea surface height measurements from altimeter data is a particular challenge where the ocean meets land. Researchers hope to find a solution to this problem with the additional information provided by the new tandem mission. "We blank out about 50 kilometers as each altimeter track approaches the coast," says Strub. He is one of a group of researchers working to improve the quality of coastal altimeter data. "Since some of the problems in retrieving the data are due to atmospheric conditions and these change with the angle of the land-water interface," says Strub, "the greater number of the altimeter tracks crossing land is a benefit. It gives us a lot more data to work with."
As scientists begin to incorporate the increased amount of altimeter data from the new tandem mission into their research and ocean models, they're also preparing for the future. NASA and the French space agency CNES are working to develop a "next generation" altimeter mission that would provide high-resolution data over both the ocean and Earth's fresh water bodies.
The Surface Water Topography Mission, called SWOT, would make more precise measurements and have much wider coverage than current altimeter missions. "Flying together in this interleaved tandem mission, Jason-1 and Jason-2 give us a sharper picture of the ocean surface and will help us prepare for SWOT, which will increase the resolution by a factor of ten," says Lee-Leung Fu, JPL project scientist for the two Jason missions.
SWOT is one of the missions recommended for development by the National Research Council in its decadal survey. The mission's wide swath and high resolution would significantly improve measurements near the coast and make it possible to measure the water levels of lakes and rivers. Over the ocean, SWOT would have a resolution of about 10 kilometers (0.6 miles), making it possible to observe and map all the ocean's currents and eddies in detail. Mission studies are currently underway at NASA and CNES.
For now, however, the Jason-1/Jason-2 Tandem Mission is the only space mission providing information on the global ocean eddy field, says Fu, key observations for both science research and applications.
NASA’s Newest Ocean Altimetry Tandem Mission