The goal of the Gravity Recovery And Climate Experiment (GRACE) space mission is to obtain accurate global and high-resolution determination of both the static and the time-variable components of the Earth's gravity field.
GRACE will map variations in the Earth's gravity field over its 5-year lifetime with its two identical spacecraft flying about 220 kilometers apart in a polar orbit 500 kilometers above the Earth. The twin GRACE satellites were launched from Russia on March 17, 2002.
The two satellites constantly maintain a two-way radio-ranging link between them, as well as measuring their own movements using accelerometers and star cameras and by listening to GPS satellite broadcasts. All of this information is then downloaded to ground stations.
The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations.
GRACE is intended to enable precise measurement of Earth's shifting water masses by detecting their effects on our planet's gravity field, allowing the study of global climatic issues by enabling a better understanding of ocean surface currents and heat transport, measuring changes in sea-floor pressure, watching the mass of the oceans change, and by monitoring changes in the storage of water and snow on the continents.
The data so far obtained by GRACE is the most precise gravimetric data yet recorded: it has been used to re-analyse data obtained from the LAGEOS experiment to try to measure the relativistic frame-dragging effect.
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