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Friday, May 25, 2007

Running interference on a 747

To my friends, I am something of an ambassador from the world of technology to their world, which is one of stone-age tools, grunts and unashamed nakedness. I am asked often to recommend computers (standard answer: a Dell laptop in the $1000 range; choose the slowest processor, get an external hard drive to make upgrading easy, skip the service plan, and reformat it completely as soon as you get it), digital cameras (HP Photosmart in the $200 range; current one is the R817), mp3 players (get an old iRiver H140 on eBay and install RockBox), and other electronic devices (whenever possible, get a Sanyo). And if I don't know the answer to a question, I'm always ready and willing to just bullshit my way through an answer.

My friend Babble likes to challenge me to explain random phenomena such as the difference between the two sides of tin foil (it's just a by-product of the way it comes off the roller; both sides will dissolve equally quickly when in contact with acidic foods like tomatoes). But I had to do some sincere research when he asked me a tough one: do cell phones really interfere with airplane instruments?

The short answer is yes, but it probably hasn't caused any accidents. From Phil Windley's Technomatria blog:
Bill Strauss, M. Granger Morgan, Jay Apt, and Daniel D. Stancil measured the RF spectrum inside commercial aircraft cabins during 37 real flights over the course of three months in late 2003. They found that a cell phone was illegally used on average at least once per flight. In addition, at least one passenger neglects to turn off their cell phone on any given flight. They also found that cell phones and even laptops with Wi-Fi cards can interrupt the normal operation of key cockpit instruments—especially Global Positioning System (GPS) receivers...

The radio frequency data was collected, with the permission of the airlines and the FAA using gear concealed in a standard carry-on bag stowed in the overhead luggage bin.
The paper these researchers published concludes:
...we estimate that the average number of reported interference events might be as high as 23 per year...

In one telling incident, a flight crew stated that a 30-degree navigation error was immediately corrected after a passenger turned off a DVD player and that the error reoccurred when the curious crew asked the passenger to switch the player on again. Game electronics and laptops were the culprits in other reports in which the crew verified in the same way that a particular PED caused erratic navigation indications...

So what about accidents? We can extrapolate by looking at the existence of interference. Beginning in the 1930s, industrial safety pioneer H.W. Heinrich found—across many industries—that the ratio of incidents to accidents is about 300 to 1. Since then, this ratio has been approximately confirmed in a number of studies, including ones by the U.S. Air Force in the early 1970s. If this ratio holds true for the aviation industry, then we would expect PED interference to be a factor in an accident about once every 12 years...
So dangerous interference does happen, but why? After all, cellular frequencies are different from GPS frequencies, and devices like DVD players aren't supposed to emit external signals at all.

The truth is that we're still learning about how interference works, but one thing is certain: the world of electromagnetic radiation is a messy and sloppy one. Between personal electronic devices (PEDs), power lines, the television and radio waves that pass through us constantly, radiation from the sun and earth and the universe's background "noise", there's interference going on all the time, but usually it is harmless or essentially cancels out.

That's pretty wild when you think about what radio and other waves used in communications actually are. They're just light--the same as light you see with your eyes, just tuned to frequencies that our eyes can't see. Different animals, and to a small extent different people, can see farther than others in the infrared and ultraviolet directions, beyond the end of what others can see.

Imagine an extreme example of such a person, perhaps the beneficiary of an eye mutation, who can see the entire spectrum. What would this mutant see, if he stared at an AM radio tower? AM means "amplitude modulation"; in other words, the frequency--which we experience as color when the frequency is in the visible light range--is fixed, and the information in the signal is sent by making the light brighter and darker, in patterns. So our eagle-eyed person would see, say, a bright green light shining from the radio tower, and this light would quickly blink from bright to dim in an uneven pattern.

What about FM? That's "frequency modulation", and frequency corresponds to color in our perception. So the person would see a light whose brightness is constant but whose color, instead, is changing rapidly; imagine a light that alternates between lime green, forest green and aqua.

What if there was someone else in the way, standing between our friend and the tower? Our optically-endowed pal would still see lots of light reflecting off the ground and the air, the same way that you can still see the ambient light from the moon if it's hidden from view by a tree. In addition, most of the light hitting the person in the way would just pass through them; light at the low frequencies used by television and radio isn't powerful enough to be absorbed by flesh and bones, which would require it to bump electrons up to higher states. This is why radio waves are able to go through walls and reach radios inside buildings. Some amount of these photons do get absorbed in the process of passing through solids, however, and thickness and density affect how much gets through; hence losing radio reception while in a tunnel, or bones blocking x-rays.

(X-rays pass through your body for the same reason radio waves do, except in reverse: they are too powerful to be satisfied by bumping electrons up to higher energy levels, and that is why they pass through flesh so easily; but when they do hit an atom, they disrupt its electrons violently, which is why x-rays are dangerous.)

What if our buddy the hawkeye was standing a mile from the radio tower? She might not see the tower directly, but she would see its light shining off other objects, even off of clouds and the sky in general. Think of how long you can still see sunlight after the sun sets; our friend would see the sky flickering with radio light from towers below the horizon, thanks to waves which have curved around the earth by bouncing off the atmosphere and back. This is how you get long-distance shortwave radio signals; they can't go through the dense earth, but they can curve around it by bouncing back and forth between the ground and atmosphere.

Back to the airplane. If I make a call on my cell phone, it starts sending out light in every direction, at a certain frequency. Let's assume that this frequency of light appears orange to our perceptive friend. But it won't be perfectly orange, as the device's frequency specifications call for it to be; it may be slightly off, due to design error or age, and might have a bit of a yellow tint. And if I use my laptop or GameBoy, those will send out a little light as well, as a by-product of their circuitry; perhaps this too is yellowish.

The airplane's devices are designed not to interact with photons a the orange frequency, which is reserved for consumer products and cell phones; perhaps they stick to chartreuse. Problem is, there's a few small components that erroneously respond to broader shades of yellow and green, and these can see the light from the cell phone and GameBoy. They are shielded a bit by heavy metal casing, but maybe some bouncing light gets in through the glass surface of the instrument panel; you can't design away every possibility of exposure to outside radiation. It's not bad if they get a signal here or there from your call to check on your brood, but if events conspire and error-checking isn't done well, such a mistake could snowball and lead to disaster.

So do turn off your cell phone and stop playing Snood while the plane lands, and imagine all the conversations, free XM and internet porn you could see bouncing off buildings in blinking colors if you were an X-Man.