US, French Scientists Plan to Map Ocean From Satellite

THE new year brings a unique challenge to an international team of scientists who want to learn more about a poorly known climate factor - ocean circulation. They are preparing to make the first comprehensive survey of the topography of the ocean surface using a radar mapping satellite.

The year also presents a new challenge to the United States National Aeronautics and Space Administration. This will be the first NASA spacecraft to ride on a foreign rocket.

The Topex/Poseidon ocean topography experiment is a joint French/US mission. NASA is supplying the satellite and some of its instruments and paying about two-thirds of the mission cost. France's space agency, the Centre National d'Etudes Spatiales (CNES) is providing other instruments and, most important, an Ariane 42P rocket ready for launch in July from the Kourou space center in French Guiana.

For project scientist Lee-Lueng Fu at the California Institute of Technology's Jet Propulsion Laboratory in Pasadena, which manages Topex for NASA, this demanding mission breaks new professional ground in personal relations as well as scientific technique.

Asked if he had any reservations about launching the satellite on a foreign rocket, he said "certainly." But he quickly added that he has found the cooperation with French colleagues "very rewarding." He finds a greater challenge in mapping the sea.

Topex scientists hope to substantially increase understanding of ocean circulation. They cannot track sea currents directly. But sea-surface topography - the hills and valleys and broad slopes represented by small changes in sea level around the world - reflects the currents' flow.

To gain the desired knowledge, 38 investigator groups of Topex scientists in nine nations must turn their radar data into maps that show differences in sea level that are accurate to within a few centimeters over distances of hundreds or even thousands of kilometers.

This has never been done before. Dr. Fu says it requires controlling all possible errors to an unprecedented degree as Topex radar scans the planet from a circular orbit 1,336 kilometers (830 miles) high and inclined 66 degrees to the equator.

If this could be done, the scientific payoff would be immense. Fu explains that lack of understanding the role of the sea is the most glaring gap in knowledge of our Earth's climate processes.

He points out that computer climate models studying possible global warming "can have very wrong predictions" when they cannot take proper account of ocean heat transport.

Therefore, he says, "the biggest thrust of the Topex mission for society is to gain increased understanding of the ocean's role in global change."

He further explains that, for scientists who are trying to build that understanding, the most important thrust of the mission is to give them a better data base. The present picture of general ocean circulation is based on ship-based measurements taken at different times and places over decades. The result of such study is an average picture that assumes the ocean does not change. But, Fu notes, it does change.

Topex, on the other hand, will survey ocean topography on a planet-wide scale every 10 days for three to five years, according to the mission plan. The 2,620-kilogram (5,780-pound) satellite will have enough fuel to continue for 12 years if all goes well. Quality upgrades to ensure such a long life helped boost the satellite's cost 42 percent beyond the $121 million original estimate.

Fu says that US and French scientists already are discussing follow-up missions.

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