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       Engineers have given the TOPEX/Poseidon ocean topography satellite a new lease on life by successfully switching the principal instrument onboard the satellite to operate on its backup unit, extending the spacecraft's already unprecedented lifetime of monitoring ocean circulation patterns worldwide.

       With the switch to a fresh altimeter, the highly productive TOPEX/Poseidon mission, which produced the accurate prediction of the globally destructive El Niño phenomenon of 1997-98, could last for months or years to come. The satellite, launched in August 1992, was originally designed to last three to five years.

       Last month, commands were sent to the U.S.-French satellite to turn off its primary radar altimeter, which was showing signs of age, and to activate the backup altimeter. Preliminary data from the satellite analyzed by the TOPEX/Poseidon team at NASA's Jet Propulsion Laboratory, Pasadena, CA, indicated that the backup, or "side-B" instrument, is operating smoothly.

       "Barring any unforeseen problems with data acquisition, we will continue to use the spare altimeter to provide global ocean topography data," said David Hancock III, altimeter instrument scientist at the Wallops Flight Facility in Virginia, whose team is monitoring operational data from the joint NASA-Centre National d'Etudes Spatiales (CNES) satellite.

       Dr. Philip Callahan, head of the calibration team at JPL, said his team is calibrating data from the new altimeter to extend TOPEX/Poseidon's ability to record global ocean changes as subtle as 1 millimeter per year (0.04 inches per year) well into the new millennium. "This work is an excellent testing bed for cross calibration of TOPEX/Poseidon with its successor, JASON-1, which is scheduled for launch in May 2000," he said.

       From an orbital altitude of 1,336 kilometers (830 miles) above Earth, TOPEX/Poseidon has successfully acquired data on sea-surface heights, produced global maps of winds and waves, and detailed land and ice-sheet topography since 1992. It has recorded billions of time-specific measurements of ocean and topography to an accuracy of approximately 3 centimeters (1.2 inches). An international team of scientists has used the data to study global climate changes and such phenomena as the El Niño warming pattern in the Pacific Ocean, which occurs about every two to seven years, and a reverse trend, known as La Niña, which seems to follow El Niño winters and cools large expanses of ocean water.

       Although the primary altimeter, or "side A," is still operational, components have started to degrade from wear and tear on the satellite. The operations team expects to be able to use side B of the altimeter for the next several years, but will be able to switch back to side A if necessary, Hancock said.

       The Wallops Flight Facility Observational Science Branch, Wallops Island, VA, which is part of NASA's Goddard Space Flight Center Laboratory for Hydrospheric Processes, worked with the JPL science and engineering team to provide specifications for using the backup altimeter system. The team, in conjunction with members of Goddard's Applied Engineering and Technology Directorate, Greenbelt, MD, was responsible for designing the fully qualified, backup altimeter, built by the Johns Hopkins Applied Physics Laboratory, Baltimore, MD.

       The TOPEX/Poseidon mission is managed by the Jet Propulsion Laboratory for NASA's Office of Earth Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA.

3/8/99 DEA