Mile-High Mobile
By Jeff Goldman, Mon Nov 25 12:30:00 GMT 2002

The current prohibition against using mobile phones on airplanes is based more on anecdotal evidence than on hard facts. What's being done to clarify the dangers, and what will it take to make mobile phones a safe part of the in-flight experience?

Two months ago, 23-year-old Faiz Chopdat of Blackburn, England was sentenced to four months in jail for recklessly endangering the safety of an aircraft.

His crime? Playing Tetris on his mobile phone, while on a flight from Egypt to England.

At the sentencing in September, Judge Timothy Mort said Mr. Chopdat had endangered the lives of the aircraft's 200 other passengers by operating a device that could interfere with the plane's navigation systems. "The consequences could be fatal," Mort said. "Anyone who leaves a mobile phone switched on creates some risk."

How much risk a mobile phone actually creates, however, isn't clear. Dr. Gerry Purdy, Principal Analyst at MobileTrax, says there's no convincing evidence backing the idea that mobile phones and airplanes don't mix. "I just don't think there's a definitive connection between the navigation systems and a cell phone," he said.

Still, use of mobile phones is prohibited worldwide on airplanes in flight-and use of other portable electronic devices (PEDs) is prohibited during takeoff and landing. The reasons, though, remain ambiguous, and a number of solutions are being developed both to improve detection of interference and to enable the safe use of wireless devices in flight.

Do They Really Interfere?

There's a lot of anecdotal evidence in support of a connection between mobile phones and navigation systems. NASA's Aviation Safety Reporting Systems database lists a few PED-related flight problems each year, such as this one: in March 1997, a Cessna pilot received erroneous location readings, and when a passenger turned off his mobile phone, the readings corrected themselves.

The trouble is that those events couldn't be duplicated after the fact: when the airlines purchased the same devices and tried to replicate the problems, nothing happened. As a result, both the U.S. Federal Aviation Administration (FAA) and the British Civil Aviation Authority (CAA) have conducted studies to clarify the connection between PEDs and navigation systems.

But again, the results weren't conclusive. A 1996 study by RTCA, Inc., commissioned by the FAA, said there was only a low probability that PEDs would cause interference with aircraft systems. Still, the report stated that the use of PEDs during takeoff and landing "should be viewed as potentially hazardous and an unacceptable risk."

A more recent study by the CAA, looking specifically at mobile phones rather than at PEDs in general, did find that mobile phones can produce emissions in excess of certification levels for aircraft use-but they were unable to prove that such emissions would cause interference with specific navigation systems.

CAA spokesman Jonathan Nicholson says further research is planned in order to produce more specific results. "We've proved they interfere with aircraft systems, but we haven't proved which systems and how much," he said. "That's what we want to look at: we want to find out which equipment is susceptible, and how it's affected."

In the meantime, many remain unconvinced: MobileTrax's Purdy says he'd happily get on an airplane in which all passengers were using laptops and mobile phones. "I wouldn't feel bad at all, because I think it's already happening today," he said. "There's no way to tell somebody that their cell phone's on in Seat 3A or 20D-so I think it's a bit of a joke."

Would the Passenger in 20D Please...

Marshall Cross is Chairman and Vice President of Research and Development for MegaWave Corporation, a company that developed a PED detection system in 1998 with a grant from the FAA. And Cross is also skeptical. "I'm a pilot and also an electrical engineer, and I'm not convinced there's an interference issue," Cross said.

The point, Cross says, is that there might be an issue-and a system to pinpoint the specific location of any suspected interference could prove things one way or the other. "One of the points of building this system was to go out and get some data," he said. "When the flight crew reported an incident, we'd be able to correlate it."

The challenge in developing such a system is that an airplane in flight is already an RF-rich environment, with a wide range of signals hitting the airplane from the outside. In order to differentiate signals coming from inside the aircraft from those coming from outside, MegaWave's system works by installing small antennas on the overhead compartments, spaced a few rows apart, throughout the aircraft.

It then becomes relatively simple to determine the source of a signal. "If you see a signal on a certain frequency on all of the antennas, you know it's coming from outside the plane," Cross said. "If you see it just on a few of the antennas, you know it's coming from something on board."

And the system can be tuned to watch for the specific frequencies the aircraft is using at any given time. "If the pilot is doing a precision approach and the localizer is on 109.1 MHz, the system knows that's where he's vulnerable," Cross said. "So it can concentrate its processing power where the pilot is operating."

MegaWave's system was developed with $95,000 from the FAA, but the company was denied the additional $750,000 needed to complete the project and test it in the air. Cross says the FAA's reasoning was disturbingly simple. "They said they'll wait until there's a crash," he said.

As a result, MegaWave is left with a catch-22: it's impossible to prove a correlation between mobile phones and aircraft systems without a detection system in place, but the FAA won't fund such a system until a correlation is already proven. Future studies by other bodies like the CAA might make use of such systems-but at about $100,000 per aircraft, MegaWave's system won't be adopted lightly.

Dial 'Em If You Got 'Em

At the same time, others are looking at ways to render such a system obsolete, by simply making it safe to use a mobile phone in flight. Companies like the Colorado-based AirCell, Inc. are developing systems that could route onboard mobile phone signals to prevent them from interfering with the aircraft, and control their connection to the ground.

Such offerings are made much more complex by the fact that there's an additional reason for the prohibition against using mobile phones in the air, one that has nothing to do with the safety of the aircraft. When you turn on a mobile phone at 30,000 feet, it can connect to dozens of ground-based cell sites at once, causing both interference and busy signals for users on the ground.

To resolve this issue, AirCell uses the AMPS analog protocol to connect to its proprietary ground-based systems, which have been co-located at 134 preexisting cell sites throughout the United States. A translator/repeater on the aircraft intercepts the signals from the mobile phones on the plane and relays them to the sites on the ground.

According to Steve Cutbirth, AirCell's Director of Airline Sales, AMPS was selected because it serves as a common language among the many digital protocols in use in the United States. "Something like 84 percent of the phones used today have both a digital mode and an analog mode," he said. "Virtually everyone relies on AMPS as a backup mode to get coverage across the continental U.S."

Cutbirth explains that a number of patented methods are used to ensure that the system won't cause interference with terrestrial cellular. "We're horizontally polarized, which provides a tremendous amount of isolation," he said. "And we shifted the control channels so a ground-based terrestrial phone can't log onto our system, and we can't log onto theirs."

Customers who use their phones on AirCell's network will pay the equivalent of roaming charges, anticipated to be one or two dollars per minute. Compared to the astronomical rates charged for using seatback phones today, that's a significant discount, and Cutbirth expects it will make a great difference in usage.

AirCell expects to see its system in place by 2004, but it's got one enormous challenge remaining-it still has to convince the FAA that it's safe to use mobile phones in flight with its system. In attempting to do so, AirCell will face many of the same challenges others have already encountered in trying to prove conclusively whether or not mobile phones interfere with aircraft navigation systems.

In the meantime, the usual announcement will remain in place: "Please turn off all portable electronic devices..."

Jeff Goldman is a freelance writer covering a wide range of topics for a number of online journals. He currently writes regular articles for's ISP-Planet. Brought up in Belgium, Jeff spent the last decade in New York, Chicago and London; he now lives in Los Angeles.