From: baalke@zagami.jpl.nasa.gov   
      
   David E. Steitz   
   Headquarters, Washington   
   (Phone: 202/358-1730)                           July 21, 2003   
      
   Alan Buis                        Margaret Baguio   
   JPL Pasadena, Calif.             Univ. of Texas (UT), Austin   
   (Phone: 818/354-0474)            (Phone: 512/471-6922)   
      
   Vanadis Weber                    Franz Ossing   
   German Aerospace Center          GeoForschungsZentrum Potsdam   
   (Phone: 49 (0) 2203/601-3068)    Phone: 49 (331) 288-1040)   
      
   RELEASE: 03-244   
      
   OCEANOGRAPHERS CATCH FIRST WAVE OF GRAVITY MISSION'S SUCCESS   
      
        The joint NASA-German Aerospace Center Gravity Recovery   
   and Climate Experiment (Grace) mission has released its first   
   science product, the most accurate map yet of Earth's gravity   
   field. Grace is the newest tool for scientists working to   
   unlock secrets of ocean circulation and its effects on   
   climate.   
      
   Created from 111 days of selected Grace data, to help   
   calibrate and validate the mission's instruments, this   
   preliminary model improves knowledge of the gravity field so   
   much it is being released to oceanographers now, months in   
   advance of the scheduled start of routine Grace science   
   operations. The data are expected to significantly improve   
   our ability to understand ocean circulation, which strongly   
   influences weather and climate.   
      
   Dr. Byron Tapley, Grace principal investigator at UT's Center   
   for Space Research, called the new model a feast for   
   oceanographers. "This initial model represents a major   
   advancement in our knowledge of Earth's gravity field. "Pre-   
   Grace models contained such large errors many important   
   features were obscured. Grace brings the true state of the   
   oceans into much sharper focus, so we can better see ocean   
   phenomena that have a strong impact on atmospheric weather   
   patterns, fisheries and global climate change."   
      
   Grace is accomplishing that goal by providing a more precise   
   definition of Earth's geoid, an imaginary surface defined   
   only by Earth's gravity field, upon which Earth's ocean   
   surfaces would lie if not disturbed by other forces such as   
   ocean currents, winds and tides. The geoid height varies   
   around the world by up to 200 meters (650 feet).   
      
   "I like to think of the geoid as science's equivalent of a   
   carpenter's level, it tells us where horizontal is," Tapley   
   said. "Grace will tell us the geoid with centimeter-level   
   precision."   
      
   So why is knowing the geoid height so important?  JPL's Dr.   
   Lee-Lueng Fu, scientist on Topex/Poseidon and Jason project   
   said, "The ocean's surface, while appearing flat, is actually   
   covered with hills and valleys caused by currents, winds and   
   tides, and also by variations in Earth's gravity field.   
   "Scientists want to separate out these gravitational effects,   
   so they can improve the accuracy of satellite altimeters like   
   Jason and Topex/Poseidon, which measure sea surface height,   
   ocean heat storage and global ocean circulation. This will   
   give us a better understanding of ocean circulation and how   
   it affects climate."   
      
   Dr. Michael Watkins, Grace project scientist at JPL, put   
   improvements to Earth's gravity model into perspective.   
   "Scientists have studied Earth's gravity for more than 30   
   years, using both satellite and ground measurements that were   
   of uneven quality. "Using just a few months of our globally   
   uniform quality Grace data, we've already improved the   
   accuracy of Earth's gravity model by a factor of between 10   
   and nearly 100, depending on the size of the gravity feature.   
   In some locations, errors in geoid height based upon previous   
   data were as much as 1 meter (3.3 feet). Now, we can reduce   
   these errors to a centimeter (0.4 inches) in some instances.   
   That's progress."   
      
   Dr. Christoph Reigber, Grace co-principal investigator at   
   GeoForschungsZentrum Potsdam, said, "As we continue to assess   
   and refine Grace's instruments and subsystems, we're   
   confident future monthly gravity solutions will be even   
   better than the map we're releasing now. "Those solutions   
   will allow us to investigate processes associated with slow   
   redistribution of mass inside Earth and on its land, ocean   
   and ice surfaces. Our initial attempts to identify such small   
   gravity signals with Grace look very promising."   
      
   Grace senses minute variations in gravitational pull from   
   local changes in Earth's mass by precisely measuring, to a   
   tenth of the width of a human hair, changes in the separation   
   of two identical spacecraft following the same orbit   
   approximately 220 kilometers (137 miles) apart. Grace will   
   map the variations from month to month, following changes   
   imposed by the seasons, weather patterns and short-term   
   climate change.   
      
   Grace is a joint partnership between NASA and the German   
   Aerospace Center. The UT Center for Space Research has   
   overall mission responsibility.  GeoForschungsZentrum Potsdam   
   is responsible for German mission elements. Science data   
   processing, distribution, archiving and product verification   
   are managed under a cooperative arrangement between JPL, UT,   
   and GeoForschungsZentrum Potsdam.  For more information,   
   visit:   
      
   http://www.csr.utexas.edu/grace or http://www.gfz-potsdam.de/grace   
      
   Model images are at:   
      
   http://photojournal.jpl.nasa.gov/catalog/PIA04652   
      
   and   
      
   http://www.csr.utexas.edu/grace and http://www.gfz-potsdam.de/grace   
      
   -end-   
      
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