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

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

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: or

Model images are at: and and