PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Mary A. Hardin
FOR IMMEDIATE RELEASEJuly 25, 1996
TOPEX/POSEIDON MEAN SEA LEVEL RISE OBSERVATIONS TO BE REVISED
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
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)
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
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.