Until the rule change last month, all my adult life portable electronic devices (PEDs) have not been allowed on commercial flights below 10,000 feet. Electronic devices were less common in the 80s and 90s, so people paid less attention the rule. It affected Walkman cassette players and portable CD players. My understanding was you weren’t allowed to use a receiver on a Walkman at any altitude, although this was moot point since you wouldn’t receive much at high altitude inside the plane.
This made me, as a teenager, immediately try listening with my handheld transceiver on the plane. On the FM band I could hear the expected cacophony of broadcast stations within line of sight. I suspect most of the signal came in through the flight deck windows, since the passenger windows were smaller than half a wavelength. The VHF public safety/service band was the same. The UHF band was much active. With a wavelength of 65cm [26”], the signal apparently came through the passenger windows. I heard a station every 12.5kHz, more stations even than I had heard on the Sears Tower.
What is the next thought a teenager has? It’s not concern that tiny signals from the local oscillator may be radiating out my antenna and interfering with the plane. It’s “I bet I can hit some of these machines!” (In other words, “I bet I can transmit through various repeater systems on the ground designed for local use by businesses, ham radio operators, and public safety.”) With the transceiver hidden under my coat I transmitted 4W into a rubber duck. Nothing bad happened with the plane, but a repeater responded. When I tried different PL tones different machines responded for each tone.
I figured all this was safe because I wasn’t intentionally transmitting in the aircraft or glide slope bands, and I wasn’t transmitting long enough bursts to disrupt public safety communication. I also figured being the bad-boy with cool toys like a multi-band handheld transceiver should impress the girls. So much of my 15-year-old thought processes didn’t make sense. The pilot should have come back and yelled at me. Fortunately, my actions apparently caused no harm, and no one appeared to notice.
Now over 20 years later PEDs are widespread. One argument for lifting the ban is that the devices are often left on or in standby mode by accident. On a business trip eight years ago I accidentally left my laptop in sleep mode instead of hibernate during landing. I realized that on top of the risk of being on a flight with some idiot with a 4W FM transmitter there was the risk of many people not even knowing their phones had airplane mode or that the computer’s clock keeps running in sleep mode.
Research corroborates the view that these devices often remain turned on on plane. Spectrum reported on tests with hidden spectrum analyzers. This plot from the article shows a narrow band mobile phone developing -55dBm on the spectrum analyzer's antenna. This 2006 article by EMC experts suggest flights monitor for this interference and ask people to turn the equipment off. This sounds like a very good idea. (Not the graphic says power relative to 1 megawatt, but the values are obviously relative to 1mW. If we took it literally, the narrow-band phone is delivering 1W to the spectrum analyzer, which would make this an article on wireless power!)
It stands out that most of the concern is about interfering with GPS. GPS receivers are susceptible to interference because they rely on signals in the -120dBm range. I have never worked on GPS equipment, but it’s hard for me to understand why aircraft rely on anything that uses signals that weak. There are too many ways some random failure modes in an electronic device can emit signals in that range.
The new rules announced last month still require devices to remain in aircraft mode under 10,000 feet. This seems like a sensible requirement. It would be good if planes also had equipment to detect unsophisticated people forgetting to use aircraft mode. A strict ban on anything electronic below 10,000 feet no longer makes sense.
