The GRACE mission

The Gravity and Climate Experiment (GRACE) mission was finally launched March 16. 2002 with a ROCKOT-rocket from Plesetsk, Russia.
Flying in an altitude of 500 km above earth, and a distance of 220 km, the two satellites will be able to determine very small variations in the gravity field. The mission is expected to improve the current knowledge of the earth's gravity field by 1000 times. GRACE is being implemented as a collaboration between NASA and the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR). The GRACE mission uses the satellite-to-satellite tracking (SST) technique. The twin GRACE satellites are the instruments that measure the nonuniformities in the Earth's gravity field. Nonuniformities in the gravity field cause the relative distance between the centers of mass of the two satellites to vary as they fly over the Earth.


Measurements:
GRACE is different from most Earth observing satellite missions. This mission will not carry a suite or independent scientific instruments on board. The two twin GRACE satellites themselves act in unison as the primary instrument. Instantaneous changes in the distance between the two satellites are used to make an extremely precise gravitational field measurement. With the K-band ranging instrument, the satellites are able to measure the distance between each other with a precision better than the width of a human hair. By monitoring this distance, GRACE will be able to detect minor alterations in the gravitational field, and therefore differences in the density of the Earth's surface beneath the satellites. Combining GPS data with data from GRACE, the satellites will be able to produce a map of the gravity field approximately once a month.

GRACE measurements will enable scientists to:

  • Measure changes in the Greenland or portions of Antarctic ice sheets to less than 1 mm/year. This uncertainty corresponds to a contribution to global sea-level rise of less than 0.05 mm/yr.


  • Measure changes in large aquifers (e.g., the Ogallalla aquifer in Nebraska) to 1 to 2 mm/yr water equivalent (1% to 2% of estimated Ogallalla deplation rate).


  • Isolate geoid changes due to changes in global-sea-level better than 0.1 mm/yr. In conjunction with altimetry, this allows us to separate the effect of changes in volume from changes in the mass of water in the oceans.


    • Grace gravity-field data will enable oceanographers to estimate deep sea ocean currents to better than 1 mm/s at a few hundred meters above the ocean floor.

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