Keith Koehler
Wallops Flight Facility						Aug. 29, 1995
(Phone: 804-824-1579)
keith.koehler@ccmail.gsfc.nasa.gov

RELEASE NO:  95-17

Wallops Ocean Instrument Providing Top(ography) Data

	After three years in space on the TOPEX/Poseidon oceanographic satellite, a NASA
 Goddard Space Flight Center Wallops Flight Facility, Wallops Island, Va., instrument continues to
 provide the most accurate measurements of sea surface topography ever achieved by an orbiting radar
 altimeter.

	TOPEX/Poseidon, a joint mission between NASA and the Center Nationale d'Etudes
Spatiales (CNES), the French space agency, uses a radar altimeter to precisely measure sea-surface
heights. A team of engineers and scientists at Wallops was responsible for the development of the
altimeter and continues to support altimeter mission operations.


	Scientists use the TOPEX/Poseidon data to produce global maps of ocean circulation.  The
satellite information is enabling oceanographers to study the way oceans transport heat and nutrients
and how the oceans interact with weather patterns.

	The TOPEX altimeter has been in operation in space for more than 23,000 hours, transmitting
back to Earth measurements of ocean topography at the rate of twenty-per-second.  Wallops engineers
monitor the altimeter's performance and provide command sequences and measurement calibration
parameters to the project.  The altimeter was designed to last a minimum of three years, a period designated as its prime mission. 

	Laurence Rossi, the Wallops' TOPEX altimeter project manager said, "Even though the prime
mission has now been accomplished, the TOPEX measurements continue to be of such high quality and
usefulness for climate studies that the mission is being extended for another three years.  We fully expect
that the altimeter will continue to operate throughout the extended period."  

	"The extended mission will help us further achieve TOPEX/Poseidon's primary science goa
l which is to improve our knowledge of how oceans circulate," said Dr. Lee-Lueng Fu, TOPEX/Poseidon
project scientist at the Jet Propulsion Laboratory (JPL), Pasadena, Ca.


	The spacecraft's nominal altitude is 830 miles above the Earth's surface.  The Wallops' 
developed TOPEX altimeter uses dual-frequency radar pulses to precisely measure the distance to 
the ocean surface directly beneath the spacecraft as its orbital trace on the ocean surface moves at the
rate of 13,420 miles per hour.  All previous satellite-borne radar altimeters have used a single frequency.
The purpose of TOPEX's second frequency is to determine the correction for the effects of the Earth's
ionosphere on the radar measurements.

	Earth-based lasers track the spacecraft, providing measurements from which the location of 
the altimeter in space is known at all times.  Knowing the location of the altimeter, its distance to the ocean and atmospheric variables, sea surface elevations are being determined with an accuracy of one to two
inches. The orbital pattern is such that the ground trace is repeated every 10 days, to monitor ocean
topography changes during that interval.

	In its more than 14,000 orbits of Earth since launch on August 10, 1992, the satellite has
obtained the following scientific results:

* In early 1995, the TOPEX/Poseidon detected a new El Nino condition in the equatorial Pacific Ocean. 
This condition has been linked to the unusually rainy weather in California and the unseasonably warm
winter in the Northeast U.S.

* Preliminary results using data acquired from December 1992 to September 1994 indicate a global rise
in sea level of 0.12 inches per year.  By tracking sea level over the extended mission.  TOPEX/Poseidon
 will help scientists understand whether this rise is a short-term variation or part of a long-term trend.


* In the Gulf of Mexico, data are helping scientists and a U.S. oil company study ocean-circulation
phenomena, called eddies, that can disrupt off-shore oil drilling.

* In another application, the precision of the satellite's ocean measurements has enabled scientists to
calculate global tides across all the open oceans, an important step toward monitoring global ocean
circulation from space and understanding the complexities of global climate change.

	"The data set from the prime mission significantly exceeds all pre-launch expectations and
has provided oceanographers with their first global data set on the Earth's oceans," said Charles
Yamarone, the JPL TOPEX/Poseidon project manager.

	The TOPEX altimeter was built for Wallops by the Applied Physics Laboratory of the Johns
Hopkins University,  Laurel,  Md. The altimeter was integrated into the TOPEX/Poseidon spacecraft by
Fairchild Space of Germantown, Md. 

	
TOPEX/Poseidon is part of NASA's Mission to Planet Earth, a coordinated long-term research program
to study the Earth as single global environment.  JPL manages the U.S. portion of the mission for NASA's
Office of Mission to Planet Earth.
 
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