The Gravity Recovery and Climate Experiment (GRACE) is carried out by twin satellites in near polar orbit 500km above the earth. About 220km apart, they make 16 orbits of the earth a day. Each satellite is equipped with microwave sensors which measure the distance between them with extraordinary accuracy of ±5 microns. Other sensors linked to GPS satellites record the precise position of GRACE over the Earth’s surface.
The two satellites constantly maintain a two-way microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. As a cross-check, the vehicles measure their own movements using accelerometers.
As the satellites circle the globe they sense minute variations in Earth's gravitational pull. When the first satellite passes over a region of slightly stronger gravity, an anomaly, it is pulled ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft passes the anomaly, then slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point.
By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from GPS, scientists construct a detailed map of Earth's gravity at a particular point in time, say monthly.
All of this information is then downloaded to ground stations. To establish baseline positions and fulfill housekeeping functions, the satellites also use star cameras and magnetometers.
Why record gravity?
Changes in gravity occur when the density or mass of an object changes. In other words, changes on or under the Earth’s surface can be detected by measuring the strength of gravity and comparing that measurement with a previous or subsequent measurement.
The gravitational pull exerted by a mountain range, an open plain or an ocean will remain constant over a long period since it usually takes millennia for mass and density to change. This measurement is known as mean or long-term gravity. This does not mean that mass can not change more rapidly. It does, primarily through the accumulation or loss of water in the form of snow, ice or as a liquid which is subject to movement by melting, through evaporation or by flowing to other locations. These shorter term changes are known as time variable gravity.
Over a few centuries, snow falling on a mountain can compact into ice and the ice can accumulate into an ice sheet or flow into glaciers, increasing the mass of the mountain and therefore its gravitational pull. Similarly, an aquifer may shrink in size and that change can be detected by measuring the size of change in gravity occurring at that location. The presence of water will cause an increase in gravity while a reduction in its volume reduces its mass and therefore its gravitational pull.
By measuring gravity at the same locations at different times, GRACE measures changes in time variable gravity producing data that can be used to measure:
- changes in mass of polar ice caps;
- change in other land based ice and snow cover;
- rising sea level resulting from ocean temperature and changes in mass;
- changes in water resources on and under land;
- shallow and deep ocean current transport; and
- atmosphere-ocean mass exchange.
This enhanced knowledge is expected to result in a better understanding of the forces that drive El Niño and La Niña, more accurate seasonal forecasts of Earth’s weather patterns, ability to track the changing distribution of water resources and in improved forecasting.
What GRACE tells us
Since its launch in 2002, GRACE has produced data which have been used to calculate changes in aquifers, the rate at which polar ice is melting and the loss of mountain ice and water from aquifers. From this source it has been confirmed that:
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