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“The first thing I tackled in the 1970s was the task of computing the orbits of satellites very accurately to support satellite altimeter missions,” says Tapley. “We reached the point that we could compute orbits that had an accuracy of a few centimeters.” Eventually his computations hit a glitch that he couldn’t fix. The satellite orbits varied seasonally.
To relate changes in satellite motion to changes in gravity, Bettadpur and his team start by examining what the satellites’ paths ought to look like. They know where the mountains are, and where the oceans grow deep. They know the path of the Sun and the Moon, and the related fluctuation of ocean tides. They know what large weather systems have moved through the atmosphere. And they can determine how much all of these things should pull on the satellites.
In the decade since its launch, GRACE has observed a number of significant changes in the water cycle. GRACE revealed losses in ice mass on Greenland (where the loss is dramatic), Alaska, and Antarctica. The gravity measurements revealed how much the melting glaciers are contributing to sea level rise by recording both ice lost from land and the mass gained in the ocean.
THERE IS A DIAGRAM FOR WATER ANOMALY in CALIF CENTRAL VALLEY.