Re: The value of gravitational acceleration in the ocean

Tareq wrote:
These links predict the value of the surace gravity, based on the
Observed gravity data in the National Geodetic Survey's Integrated Data
Base which I can not find in the internet. The tool provided predicts
the gravitation in the USA only.

they don't meausre it. I can find no refrence for the density of the
earth material which is a vital quantity in calculating g.


background
http://en.wikipedia.org/wiki/Gravimetry


MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

Contact: JPL/Alan Buis (818) 354-0474
University of Texas Center for Space Research/Margaret Baguio (512) 471-6922
German Aerospace Center/Vanadis Weber 49 (0) 2203/601-3068

FOR IMMEDIATE RELEASE September 12, 2002

NEW GRAVITY MISSION ON TRACK TO MAP EARTH'S SHIFTY MASS

Six months into its mission to precisely measure Earth's shifting water masses
and map their effects on Earth's gravity field, the joint NASA-German Aerospace Center
Gravity Recovery and Climate Experiment, or Grace, is already producing results of
considerable interest.

Using just 14 days of data, a preliminary Grace gravity field map has already
been produced and is proving to be substantially more accurate than the combined results
of more than three decades of satellite and surface instrument gravity measurements
collected before Grace.

Grace Principal Investigator Dr. Byron Tapley of the University of Texas Center
for Space Research in Austin said preliminary data analyses provide strong validation for
the Grace mission concept and its on-orbit sensor and satellite performance. "The quality
of these preliminary Grace gravity fields tells us that the mission is on track to achieve
its
performance objectives," Tapley said. "In addition to improving our knowledge of
Earth's mean gravity field, the ability to measure time variations in gravity will be a new
and important Grace contribution."

"This first Grace gravity field data is about 10 times more accurate for large-scale
features than any pre-Grace gravity model of Earth," said Dr. Michael Watkins, Grace
project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "While these
first fields are already extremely promising, we expect to do up to 10 times better after
we perform additional calibrations of the instruments."

"The Grace mission will soon demonstrate its mission goal to help us better
understand how variations in Earth's gravity field reflect changes in climate," said Dr.
Christoph Reigber, Grace co-principal investigator at the German Geo-Research Center
and principal investigator for the German Challenging Minisatellite Payload, or Champ,
mission. Champ was launched two years ago as the first in a series of dedicated Earth
gravity field missions planned for this decade and is operating in parallel with Grace.

"Our joint U.S.-German science team continues to process and evaluate Grace's
diverse geophysical data in preparation for the release of our first science products in the
near future," Reigber added.

Launched March 17, 2002, Grace senses minute variations in Earth's surface
mass and corresponding variations in Earth's gravitational pull. The monthly gravity
maps generated by Grace will be up to 1,000 times more accurate than current maps,
substantially improving the accuracy of many techniques used by oceanographers,
hydrologists, glaciologists, geologists and other scientists to study phenomena that
influence climate.

Among the first and most important applications for Grace's data will be to
improve our understanding of global ocean circulation. The hills and valleys in the
ocean's surface are due to currents and variations in Earth's gravity field. Grace enables
separation of those two effects to better measure ocean currents and their effect on
climate.

"These first results from Grace look very promising from an oceanographic point
of view," said Dr. Lee-Lueng Fu, Topex/Poseidon and Jason-1 project scientist at JPL.
"Even with this preliminary gravity field, Grace has enabled us to use Topex/Poseidon
and Jason-1 data to determine ocean circulation features more accurately."

Grace measures Earth's gravity field by measuring the separation between the
twin satellites with an accuracy of one millionth of a meter (less than 1/10th the width of
a
human hair). Grace's instruments must all work together and be very accurately
calibrated and aligned, a process the Grace project is just now completing during the
mission's commissioning phase, which began in early April. Science instruments and
supporting systems such as ground data processing have been activated, evaluated and
calibrated.

Watkins says the twin satellites and the mission's science data system are
generally performing well. "We're basically observing the smallest details of how these
satellites behave on orbit, 'fine tuning' them, and adjusting our analyses to take those
details into account," he said.

Grace is a joint partnership between NASA and the German Aerospace Center
(Deutsches Zentrum fuer Luft und Raumfahrt, or DLR). The University of Texas' Center
for Space Research has overall mission responsibility. GeoForschungsZentrum Potsdam
is responsible for the German mission elements. JPL manages the U.S. portion of the
project for NASA's Office of Earth Science, Washington, D.C. Science data processing,
distribution, archiving and product verification are managed under a cooperative
arrangement between JPL, the University of Texas' Center for Space Research and the
Geo-Research Center in Germany. For more information on Grace, see
http://www.csr.utexas.edu/grace , or
http://www.gfz-potsdam.de/grace .

JPL is a division of the California Institute of Technology in Pasadena.

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9-12-02 AB
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