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Masscon Mapping.



From this news release:
<http://science.nasa.gov/headlines/y2001/ast30oct_1.htm>
"Every month during GRACE's 5-year expected lifetime we will get a map of
Earth's gravitational field," says Michael Watkins, project scientist for
GRACE at NASA's Jet Propulsion Laboratory (JPL). "We'll be able to see
various phenomena that involve transporting mass around -- and how much
mass they're actually moving. These are things that aren't easy to see with
any other type of measurement."
How can GRACE measure these subtle variations in gravity from space? After
all, the satellites will be in free-fall around the Earth (like all objects
in orbit), so they can't measure gravity like gravimeters on the ground do:
by measuring how hard the ground pushes back against the weight of the
gravimeter.

To sense gravity in free-fall, GRACE will deploy a pair of identical
satellites in the same orbit -- one satellite 220 km (137 miles) ahead of
the other. As the pair circle Earth, regions of slightly stronger gravity
will affect the lead satellite first, pulling it slightly away from the
trailing satellite. By monitoring the distance between the two with
extraordinary precision (the satellites can sense a change of separation of
one micron -- about 1/50th the width of a human hair), GRACE will be able
to detect minute fluctuations in the gravitational field."