Artist concept of Jason

Measurements of global sea-level rise from a U.S. instrument in space likely will be revised downward because of a recently discovered error in the data-processing software, mission scientists said.

Initial indications are that sea-level measurements from the U.S. altimeter aboard the U.S.-French TOPEX/Poseidon satellite likely will agree more closely with Earth-based tide gauges, as well as with the French altimeter on the satellite.

Preliminary findings from TOPEX/Poseidon data, first announced in December 1994, indicated the Earth's sea surface was rising about 3 millimeters (0.12 inches) annually for two years. Later, as more data were collected, the published estimate of sea level rise was increased to more than 5 millimeters (0.20 inches) per year.

Data collected from December 1992 to April 1996 have been updated and suggest that the new sea level rise estimate will be revised to 1 to 3 millimeters (0.04 to 0.12 inches) per year. Scientists have more confidence in the revised sea level rise estimate because Earth-based tide gauge measurements now closely agree with the TOPEX/Poseidon altimeters, according to Dr. Gary Mitchum, a mission science team member at the University of South Florida in Tampa.

"It is important to note that this processing error does not affect the success of the primary purpose of TOPEX/Poseidon, which is to accurately describe global ocean circulation," said William Townsend, acting associate administrator for NASA's Office of Mission to Planet Earth. "TOPEX/Poseidon has accomplished this task with significantly better-than-expected precision, and the satellite is now operating well beyond its three-year prime mission."

Precise understanding of global ocean circulation is a key element of climate change research. TOPEX/Poseidon has mapped this circulation with an absolute accuracy of 4 centimeters (1.8 inches), versus its design goal of 13 centimeters (5.9 inches).

The ability of TOPEX/Poseidon data to enable researchers to track and monitor El Nino, a warming of ocean waters in the Pacific Ocean that helps cause extreme weather events in the Americas, also is unaffected, Townsend added.

The altimeter data-processing error, which incorrectly adjusted for the drift of the onboard oscillator that controls the satellite clock, became noticeable only recently because of the extreme precision of the TOPEX/Poseidon measurements. The small error accumulated with time.

"Re-analysis of the TOPEX/Poseidon altimeter data is ongoing," according to Dr. Steve Nerem, a mission science team member now at the University of Texas at Austin. "In any case, it should be emphasized that the TOPEX/Poseidon data record is not nearly long enough to unequivocally detect global sea level trends caused by climate change."

Nevertheless, Nerem adds, "TOPEX/Poseidon is providing us with the most precise measurements ever made of global mean sea level change." The continuation of these measurements by a planned cooperative U.S.-French follow-on mission, called Jason- 1, is essential to measure the long-term trends in sea level and its relationship to climate change.

The TOPEX/Poseidon spaceborne altimeter bounces radar signals off the ocean's surface to get precise measurements of the distance between the satellite and the sea surface. These data are combined with measurements from other instruments that pinpoint the satellite's exact location in space. Every 10 days, scientists are able to produce a complete map of global ocean topography, the barely perceptible hills and valleys found on the sea surface. With detailed knowledge of ocean topography, scientist can then calculate the speed and direction of worldwide ocean currents.

TOPEX/Poseidon was launched in August 1992 aboard a French Ariane 4 vehicle from Kourou, French Guiana. The Jet Propulsion Laboratory manages the NASA portion of the mission for NASA's Office of Mission to Planet Earth, Washington, DC.

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