Solar flare puts GPS off the air

It was like something outof a sci-fi movie. Mysteriously,on Dec. 6, 2006, Global PositioningSystem devices suddenlymalfunctioned acrosslarge swaths of the planet.The good news is that thecause was not invading aliens.It was an intense burst of radioenergy, called a solar flare,emitting from the sun's surface.And there's more good news 'although the event temporarilyknocked out many GPS receivers,no airplanes fell fromthe sky, and no ships lost theirway at sea.But the event nonethelessgenerated concern among scientists.Although they wereaware that radio bursts generatedby solar flares could affectGPS equipment, they were surprisedthat this large an eventoccurred during a period of relativelylow solar-flare activityand that its impact was asstrong as it was.'It's more serious than wethought. We didn't think thiswas going to happen until thenext solar maximum, which isabout 2011,' said Paul KintnerJr., professor of electrical andcomputer engineering at CornellUniversity and head ofCornell's GPS Laboratory.'We've been monitoring solarflares for four years. [The Dec.6 event] suggests that monitoringhas been inaccurate.And we don't have a good historicalbasis for predictingwhat's going to happen, sowe're concerned.'The radio bursts don't actuallydamage equipment but onlyinterfere with transmissionsbetween GPS satellites and receivers.Some GPS equipmentmay be affected for about 30minutes, Kintner said, 'butthat's really an upper limit.'This means that the risks aregreater for some GPS usersthan for others.'This past event was sort oftwo really intense pulses thatlasted three to four minuteseach, so this is not a major outageif you're driving a car orhiking,' Kintner said. 'It's reallya concern for people whowant uninterrupted operation.'People, for instance, operatingfloating offshore oil platforms.Such platforms constantlymonitor themselves,navigating with GPS to staywithin a about a meter overwhere the pipe enters the seafloor. 'They have very demanding,precision GPS requirements,'Kintner said. 'A fewminutes of outage could breakthem.'Airliners also depend on continuoussignals, employing theWAAS ' Wide Area AugmentationSystem ' for GPS services.The system improves thestandard GPS signal to lowerthe margin of error from about15 meters to 2 meters. That'sgood enough that WAAS isused not only for in-flight reportingof location and terminaloperations but also forguided-approach services,which bring aircraft down toabout 200 feet above a runwaybefore the pilot has to takeover.How did WAAS fare throughthe Dec. 6 event? According toPatricia Doherty, senior researchscientist at the Institutefor Scientific Research atBoston College and a consultantto the Federal Aviation Administration,the results weremixed.Although the nonprecisionservices used for in-flight locationand terminal operationswere not affected, Dohertysaid, the precision-guided-approachsystem was compromised.'The system still operated,but we lost use of that guidedapproachservice for about 15minutes,' she said.Doherty said, however, thatpeople shouldn't worry aboutairplanes falling from the sky.'WAAS doesn't consider [suchoutages] a problem because itoften loses use of that systemdue to other operational reasons,such as satellite maintenance.It wasn't a real worry tothem,' she said. 'If a solarradio burst hits, it's detected bythe ground system. Theground system then alerts theaircraft that you can't use thesystem because there's a problemwith the GPS. There arealternatives.'If another large radio bursthappens, and the guided-approachservice is unavailable,Doherty said, the most likelyresult is that those airplanestrying to land in areas wherevisibility is very bad may haveto circle a while to get theirturn to land on one of the fewother runways that have non-GPS blind navigation systemsin place.'The reason that WAAS did alot better than some of theseother networks is that theyhave been built with a very robustreceiver design,' she said.'The receivers have somethingcalled 'interference rejection' inthem. When a solar radio burstbombards a receiver, it bombardsit with noise on the samefrequency as the GPS. TheWAAS receivers filter out externalnoise. It's a lot more expensive,absolutely.'Kintner said there are twobasic ways to keep solar radiobursts from interfering withGPS devices. One is to increasethe strength of the signalbroadcast by GPS satellites.The other way is toimprove the performance ofGPS receivers so they can betterfilter out interference.Unfortunately, 'I don't thinkeither is likely to be very effectiveby the next solar maximum,'Kintner said, referringto the period of heightenedsolar flare activity expected in2011.A third strategy ' and onescientists had been counting onto help the situation until affordabletechnology comes tothe rescue ' is to be able topredict solar radio bursts sothose who depend on continuousGPS can prepare to getthrough a period without it.That's why the Dec. 6 solarevent was so unsettling tosome scientists.'The relationship betweensunspots and solar flares andradio bursts is qualitative,' saidKintner. 'So larger solar flaresdon't necessarily produce largersolar radio bursts. The relationshipis not one to one.That's part of what we're tryingto understand.'