Global Positioning System at the Mercy of Solar Flares

WASHINGTON, DC, April 9, 2007 (ENS) - Strong solar flares can cause Global Positioning System, GPS, receivers on Earth to fail, Cornell researchers have discovered. They predict that the large solar flares expected in five years could produce outages of all GPS receivers on the day side of Earth with devastating consequences for aviation and maritime operations, disaster relief, and telecommunications.

The findings were announced Thursday in Washington, DC, at the first Space Weather Enterprise Forum, a gathering of academic, government and private sector scientists who study the Earthís vulnerability to the impacts of space weather.

"If you're driving to the beach using your car's navigation system, you'll be OK. If you're on a commercial airplane in zero visibility weather, maybe not," said Paul Kintner Jr., professor of electrical and computer engineering at Cornell and head of Cornell's GPS Laboratory.

The Global GPS Network is a set of precise GPS receivers used for scientific and real-time applications including a positioning service that can provide a userís position with 10 to 20 centimeter accuracy anywhere in the world, on land, in the air or in Earthís orbit.

Solar flares are accompanied by solar radio bursts. Because the bursts occur over the same frequency bands at which GPS satellites transmit, the radio waves act as noise, leading to a loss of signal.

Alessandro Cerruti, a graduate student working with Kintner, accidentally discovered the effect on September 7, 2005, while operating a Cornel GPS receiver at Arecibo Observatory in Puerto Rico. Cerruti was investigating a phenomenon unrelated to solar flares when a flare occurred, causing the receiver's signal to drop significantly.

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A close-up of the September 7, 2005 solar flare taken with NASA's Transition Region and Coronal Explorer spacecraft. (Photo courtesy NASA)
To be sure of the effect, Cerruti obtained data from receivers operated by the Federal Aviation Administration, FAA, and the Brazilian Air Force, among others. He found that all receivers on the sunlit side of the Earth had been affected.

Kintner and Cerruti confirmed the effect on December 6, 2006, when a huge solar flare created an unprecedented intense solar radio burst causing large numbers of receivers to stop tracking the GPS signal.

"In December, we found the effect on GPS receivers were more profound and wide spread than we expected," said Kintner. "Now we are concerned more severe consequences will occur during the next solar maximum."

The next solar maximum, the highest point of the Sun's 11 year activity cycle, is due in 2011. The most recent solar minimum occurred in 2006, but even during minimums the Sun can emit giant flares and radio bursts.

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Sunspot counts and solar X-ray flares during the last three solar cycles. Solar activity continues even during solar minimums. (Graph courtesy David Hathaway, NASA/NSSTC)
"This solar radio burst occurred during the solar minimum, yet produced as much as 10 times more radio noise than the previous record," said Dale Gary, Ph.D., chair and professor of the physics department at New Jersey Institute of Technology, NJIT.

"Measurements with NJITís solar radiotelescope confirmed, at its peak, the burst produced 20,000 times more radio emission than the entire rest of the Sun. This was enough to swamp GPS receivers over the entire sunlit side of Earth," he said.

"Soon the FAA will require that every plane have a GPS receiver transmitting its position to air traffic controllers on the ground," warned Cerruti. "But suppose one day you are on an aircraft and a solar radio burst occurs. There's an outage, and the GPS receiver cannot produce a location."

"It's a nightmare situation," Cerruti said. "But now that we know the burst's severity, we might be able to mitigate the problem."

The only solutions, suggested Kintner, are to equip receivers with weak signal-tracking algorithms or to increase the signal power from the satellites. But the first idea requires compromises to receiver design, and the other suggestion requires a new satellite design that neither exists nor is planned.

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Alessandro Cerruti, left, and Professor Paul Kintner work on the antenna on the roof of Cornell's Phillips Hall. (Photo courtesy Cornell University)
"I think the best remedy is to be aware of the problem and operate GPS systems with the knowledge that they may fail during a solar flare," Kintner said.

At the National Aeronautics and Space Administration, NASA, the Global Positioning System provides key positioning, navigation and timing capabilities onboard the Space Shuttle, the International Space Station, and on numerous science satellites.

NASA Administrator Michael Griffin says GPS "is a national asset that will be used to further America's long-term space exploration objectives under the Vision for Space Exploration."

Looking into the future, Griffin said, "GPS precision time signals may become the electronic foundation for the communication and navigation systems of human and robotic explorers and enable an "interplanetary Internet" for the global community to use."

On Earth, GPS signals support entire industries such as aviation, maritime, disaster relief, surveying, banking, and telecommunications.

"NASA wants to better understand this solar phenomenon so we can limit the adverse impacts on real-time systems," said Tony Mannucci, Ph.D., principal technical staff and supervisor, Ionospheric and Atmospheric Remote Sensing Group at the NASA Jet Propulsion Laboratory.

There are three important points to remember about solar radio bursts, said Anthea Coster, Ph.D., of the Massachusetts Institute of Technology's Haystack Observatory.

"First, society cannot become overly reliant on technology without an awareness and understanding of the effects of future space weather disruptions."

"Second," she said, "the December 6 event dramatically shows the effect of solar radio bursts is global and instantaneous."

"Third, and equally important, the size and timing of this burst were completely unexpected and the largest ever detected," said Coster. "We do not know how often we can expect solar radio bursts of this size or even larger."