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Will GPS Systems Be Affected by Japan's Earthquake?

/ Source: TechNewsDaily

Apart from the horror of the recent earthquakes and tsunami waves that devastated Japan is a sense of awe over geophysical changes wrought by the event. Northeastern parts of Honshu, Japan's main island, moved eastward about 13 feet (4 meters) and dropped about 2 feet (0.6 meter), while the Earth's figure axis shifted by about 6.5 inches (16.5 centimeters).

As dramatic as those alterations seem, however — and despite Internet rumors to the contrary — the worldwide Global Positioning System (GPS) has not been affected by the disaster in Japan, experts say.

"The northern Honshu displacements are large and significant, and yet this warping of the Earth's crust is something that can be readily compensated for," said Ken Hudnut, a geophysicist with the United States Geological Survey.

GPS is a network of satellites and ground stations, and together they provide accurate locational information around the clock for people and objects on Earth.

Ever since the United States brought the first (and so far, the only complete) satellite network into being in 1994, GPS has become an important economic tool with key applications in navigation, mobile phone operations and surveying.

Getting your bearings

GPS relies on two interdependent reference systems — one in the sky and one on the ground, said Adrian Borsa, a geophysicist with UNAVCO, a geodesy consortium based in Boulder, Colo. (Geodesy is the science of determining the exact position of geographical points.)

"There are two reference systems coming into play here," said Borsa. "One consists of satellites orbiting the Earth. We triangulate with these satellites, and we can say where I am relative to these satellites on the ground."

Twenty-four operational satellites comprise the Earth-orbiting constellation of GPS spacecraft. The positions of the satellites are known quite accurately, Borsa said, with respect to "certain places on Earth where we tie lots of reference frames together," called, appropriately enough, IGS (International GNSS Service) reference frame stations.

Reference frames relied on by IGS stations include celestial reckoning points such as quasars — the hyperenergetic cores of distant galaxies. To our radio telescopes, these objects appear fixed in the sky due to their extreme distances, and therefore act as "a backdrop with which to orient Earth," Borsa said. Sometimes " laser ranging " is performed at IGS facilities, which consists of reflecting a light signal off of a satellite and timing the bounce-back rate to calculate distance.

There are more than 100 IGS reference frame stations currently operating all over the world, and about 200 more offer supplementary data.

Local distortion

Japan, for its part, has more than 1,200 stations that make up a regional network called GEONET (GPS Earth Observation Network). The earthquake-prone country has invested in this infrastructure in order to better track and study the seismic activity that often rattles the island chain.

At least three GEONET stations were damaged by the earthquake and tsunami, and as of March 17, another 25 near the epicenter were offline per power or communications outages, a Japanese official said. [Read: Q&A: Nuclear Power Expert Explains Japan's Crisis ]

The sudden eastward jarring during the earthquake of the landmass upon which several GEONET stations are built might have temporarily thrown off driving directions or even property records tied to the local geodetic grid, Hudnut said.

But recalibration would not take long given the hundreds of other reference points available within Japan's regional network and the resiliency of the worldwide GPS system.

"Basically, it's all done already," Hudnut said.

The satellites can deal

Despite the Earth's post-earthquake figure axial shift of 6.5 inches, the orbits of the satellites wheeling overhead our planet were not significantly affected, experts said.

The figure axis is an imaginary line around which the Earth's mass is balanced. As a result of the earthquake, the planet became a bit more compact. This change in the distribution of the Earth's mass has sped up the rotation of the planet such that a day is 1.8 microseconds shorter than before the quake, and the planet will wobble a bit differently.

But according to a press release from Richard Gross, a research scientist at NASA's Jet Propulsion Laboratory, the Earth's figure axis varies by about 1 meter (3.3 feet) over the course of a year — much more than the temblor's influence.

"The change to the figure axis of the Earth from the quake was about six times less than its annual variation due to atmospheric and ocean mass movements," Borsa said, adding that GPS is equipped to handle these sorts of natural whims.

GPS must be flexible

Occasional adjustments to GPS-tied records are necessary due to the incessant movement of the tectonic plates that make up Earth's crust (the phenomena behind earthquakes). These plates usually creep along on the order of millimeters a year, grinding against each other or splaying apart.

In seismically active places, GPS tweaking happens more often, Hudnut said.

"These tectonic changes are going on all the time, and surveyors in California and especially Japan who work in an actively deforming area have to adjust to them," Hudnut said.

Part of Japan's recovery will be trying to return to a normal, pre-disaster livelihood. So it is small, yet significant solace that GPS, which is so depended on in modern societies — especially industrialized ones such as Japan's — can readily cope with the terrible outcome of a geological event.

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