New technology could facilitate safe in-flight cell phone use

Is it safe to use cell phones and other personal electronics on airplanes?

It depends on whom you ask, but researchers at Carnegie Mellon University say that the cacophony of electronic "noise" emitted by portable devices brought onboard by passengers indeed can cause dangerous interference with navigation sensors in the cockpit.

There is little evidence to suggest that a single cellphone accidentally left on in a briefcase poses much of a hazard in flight, but start using laptops, DVD players, cellphones, and other devices all at once and you've got a recipe for disaster, the scientists say.

"The problem isn't so much that any one piece of electronics will cause interference," said Bill Strauss, author of a study that sought to determine once and for all the probable risks associated with using portable personal electronics in flight. "When you have all of these devices operating simultaneously, through the magic of physics they can combine signals and overlap in frequency spectrums where they ordinarily shouldn't."

One of those spectrum ranges covers GPS, the satellite navigation system most airplanes use for primary navigation.

With the assistance of the FAA and three major U.S. airlines, Strauss and his colleagues at Carnegie Mellon installed spectrum analyzers aboard several airliners to try to discover whether passengers make calls after takeoff in violation of FCC rules. The researchers found that not only do passengers regularly use their cellphones in flight, but also that the cellphone signal activity observed was in the GPS band at levels that Strauss said could cause dangerous interference.

On each flight for which the group collected data, at least one phone was left on, and usually several were operating, Strauss said. In many cases, the phones appeared to have been left on unintentionally, posing little problem. But the data showed that passengers actually tried placing calls, on average one to four times per flight.

"We were surprised by what we saw. On every flight, we detected some kind of call energy," Strauss said. He added that he saw a young woman on one of the study flights talking on her cell phone during the airplane's takeoff roll and initial climb out. "She kept talking until the call finally dropped. Just amazing."

Under normal circumstances, cellphone transmissions and the GPS frequency band don't overlap. So why all the concern about cellphone use?

All electronic devices emit radio frequencies. This stray RF normally gets absorbed by the device itself or, if it can't be eliminated, manufacturers try to move the RF signals to noncritical bands. The trouble is, emissions from cellphones can "leak" over into the GPS band, or other aviation frequency bands, because of design flaws in a phone or what is known as "frequency intermodulation."

Intermodulation occurs when frequencies from two units combine to cause additional frequencies. You could be using a cellphone that's on a frequency well outside the GPS band, but that signal could combine with a frequency being emitted from some other passive system—say, a laptop computer—causing energy to be presented in the GPS frequency band. Few at the FCC or FAA even seem to recognize the problem exists, Strauss said.

"I'm worried that nobody will take this issue seriously until we have a crash," Strauss commented. "But will we even be able to prove an accident was caused because of a passenger's cellphone?"

The biggest danger, he said, would be some type of signal interference emitted from the passenger compartment while the airplane was on approach to an airport in bad weather or at night. In such a case, the pilots might not realize the airplane was straying dangerously off course until it was too late.

And it's not just cell phones that have the potential for interference, Strauss noted. He recalled an alarming incident where the flight crew aboard an airliner experienced a 30-degree navigational error that was immediately corrected after a passenger turned off a portable DVD player. Perhaps more frightening is the fact that the error recurred when the passenger was asked to turn the device back on.

"What's truly troubling is these devices all operate within FCC specifications," Strauss said. He and his colleagues are urging the FCC and the FAA to better coordinate their efforts to identify the types of portable electronics that pose the biggest risks in flight.

One potential solution could be the use of emerging technologies that would control the power levels emitted by cell phones. Such technology could even allow passengers to make cellphone calls after takeoff. Air France began trials of the concept aboard a few of its airplanes earlier this year and hopes to offer a commercial service for its passengers. Several other companies are working to bring the capability to business jets.

AirCell, a Colorado company that is developing a broadband Internet service for use in the cabin, believes personal cellphone calls in flight could be a reality soon. The idea is to fit the airplane with special equipment that communicates with the cellphone, instructing it to go to its lowest power setting. An antenna installed aboard the aircraft, called a "pico cell," would intercept the cellphone's call signal and route it through approved onboard communications gear. The benefit for companies like AirCell is that they would get to charge a premium of perhaps a few dollars per minute for this service. The benefit for passengers is that they would be able to make and receive calls using their phones, without fear of bringing down the airplane.