Carl Wunsch of the Massachusetts Institute of Technology, one of the acknowledged pioneers of the altimeter, said the biggest contribution of the technology may be conceptual rather than scientific.

"The greatest achievement of the altimeter is that it has showed us that the ocean system changes rather dramatically everyday and has shifted the view of it from this almost geological phenomenon creeping along very slowly to something much more interesting in which fluid is moving in all directions at all times," Wunsch said.

The radar altimeter offers valuable information on the state of the ocean by providing measurements of the height of the ocean surface. This is done by sending 1.800 separate radar pulses down to Earth per second, then recording how long their echoes take to bounce back.

Knowing the height of the sea surface tells scientists a great deal about what is happening at lower depths. Before the advent of radar altimetry, oceanographers had no way of looking at the ocean as a whole, which is essential because changes in one part of the ocean will eventually affect the whole rest of the ocean.

"We are here to celebrate our success because we have managed to measure the height of sea surface with such extreme sensitivity that we are able to detect even two centimeters difference over 100 kilometers (64 miles)," said Lee-Lueng Fu of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

This ability permits oceanographers to measure changes in ocean currents and create a weather chart of ocean circulation for the first time - something Pierre-Yves Le Traon of the French Research Institute for Exploitation of the Sea said was the instruments' most outstanding achievement.

Ocean forecasting behaves much the same as weather forecasting: If there is high pressure (signified by higher sea levels), an anti-cyclonic ocean circulation takes place, which usually translates into good weather conditions, while low pressure (signified by lower sea levels) signifies that a cyclonic ocean current is present.

This type of forecasting has important potential consequences for society and the world's economies. For example, it allows scientists to forecast El Niņo events and the flooding of low-lying areas (such as Venice), as well as predict the trajectory of pollutants, which allows oil spills to be contained more quickly by placing barriers in their pathways.

The ability to measure the sea surface height, which varies across the ocean, with such accuracy allowed oceanographers to discover planetary waves, which Paolo Cipollini, of the U.K. National Oceanography Centre, calls the real success story of radar altimetry.

Planetary waves, also called Rossby waves, were thought to have existed in the ocean as far back as 1930, but it was impossible to know for sure, because they occur internally and are very small on the surface, about 10 centimeters (three inches) high, making them impossible to detect from onboard an oceanographic research vessel.

Cipollini said radar altimetry offered proof of these waves for the first time. As oceanographers started mapping the sea surface height, they began seeing the internal waves, which extend 500 or 1000 kilometers underneath the ocean, moving by following the measurements on the surface.

"It has been suggested that planetary waves are one mechanism which brings nutrients from the deep sea up to the surface, which would make them important for the carbon cycle," Cipollini said, adding that the waves are considered very important, because they may be responsible for setting the main circulation patterns in the ocean.

"So it could be that these mysterious waves that up until 20 years ago we weren't even able to see are also important for biologists and for people studying how the ocean is reacting to global warming," he added.

ESA has had radar altimeters in orbit since July 1991, when ERS-1 was launched, which was followed by ERS-2 in 1995 and Envisat in 2002.

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