Satellites map subtle variations in Earth’s gravitational field (2)

Grace in Space

A pair of satellites map subtle variations in Earth’s gravitational field, revealing secret craters, undersea mountains, and the impact of climate change.

by Sam Flamsteed

Unfortunately, the variation in distance between the two satellites is so small that in the early 1960s it would have been virtually impossible to detect using any technology then available. In 1976 NASA launched a satellite called LAGEOS (Laser Geodynamics Satellite), which began to address the problem, albeit crudely. It carried no instruments at all. In essence, LAGEOS was a two-foot-diameter shiny brass golf ball; by bouncing laser beams off the satellite from different places on the surface of Earth, geologists could measure the precise distances between widely separated places on the planet. They could, for example, see the gradual separation of continents, due to plate tectonics, year by year.

In the early 1990s the TOPEX (Topography Experiment for Ocean Circulation)/Poseidon satellite, a joint American-French mission, shot into orbit armed with radar altimeters to measure the height of the sea surface. “What they’ve basically done,” Watkins says, “is to look at changes in the sea surface over time, on the assumption the geoid itself doesn’t change.” Except that sometimes it does. Along with its measurements of continental drift, LAGEOS also detected a very gradual change in the gravity field over Canada and northern Europe as the crust continues to rebound—10,000 years later—from the weight of the massive glaciers that pinned it down during the last ice age. It also revealed annual variations in local gravity due to the natural storage and depletion of water during rainy and dry seasons in different parts of the world.

Laser beams fired at LAGEOS were not sensitive enough to pinpoint variations in orbit smaller than a centimeter or so and were too imprecise to pick out the subtler differences in gravity. For that, a double-satellite mission was needed. Finally, in the mid-1990s, the technology to pull it off became available in two forms. The first was microwave transmitters and receivers small, efficient, and reliable enough to be to mounted on small spacecraft and used to gauge the distance between the satellites. The second: the Global Positioning System (GPS). “If I’m sending a signal from me to you,” says Watkins, “and I want to know the time of flight, it’s crucial that our clocks be perfectly synchronized.” By checking in constantly with whatever GPS satellite is in view at a given time, a pair of gravity satellites can use its single clock rather than trying to synchronize their own.

With the technology finally in place, Watkins, together with ­aerospace engineer Byron Tapley of the University of Texas at Austin and several other scientists and engineers, proposed the GRACE mission. In partnership with the German space agency, NASA sent the dual GRACE satellites into orbit in March 2002. Since then, they have been zipping around Earth in a polar orbit, one satellite about 137 miles ahead of the other. To an observer in space, they would appear to be tracing out the same circle over and over, but since the planet is continuously rotating beneath them, the intrepid satellites orbit over every slice of the surface once every 30 days.

GRACE’s data show that the ice sheet covering Antarctica has lost an average of 36 cubic miles of ice per year

Their instruments measure not the distance between the two satellites but rather the change in distance, and thus the acceleration due to gravity. They do it through interferometry—watching how beams of microwaves interfere with each other. One satellite shoots out a continuous stream of microwaves, which is received by the second satellite and both are sent to the ground. The outgoing and incoming beams are superimposed, creating an interference pattern that varies depending on how close the waves are to being perfectly in phase—that is, how close the waves’ peaks and valleys are lined up. A tiny difference in satellite-to-satellite distance—and thus an increase or decrease in gravitational pull from Earth’s surface—makes a marked difference in the interference pattern. If the satellites are moving together or apart at as little as 150 nanometers per second, the GRACE scientists can see it.

(Click here for a larger version.)

A map of gravity anomalies created by GRACE shows
mountain ranges and other large masses in red—
even those under the sea.

That is not quite the end of the story. Even though 310 miles up is technically outer space, a few air molecules still float around—not enough to make the slightest difference to astronauts on a space shuttle or the space station, which orbit considerably lower, but sufficient to slow the GRACE satellites perceptibly. A clump of air molecules could fool an observer into thinking that something lies below—perhaps a glacier—so each satellite has what’s known as a “proof mass” floating in a chamber inside, untethered to the satellite itself. The proof mass is itself in orbit, so when one of the satellites speeds up or slows down due to gravity variations, the mass does too. But when a satellite slows due to air drag, the proof mass inside, blissfully unaware, keeps moving at its original speed. It doesn’t hit the interior wall of the satellite because onboard electric plates keep it from doing so—but sensitive electronics keep track of the discrepancy so the engineers can subtract it from the real signal.