We all know the procedure. Warning signs, “The use of mobile phones is strictly prohibted during your entire stay onboard!”-speaches, the threat of prosecution. The official reasoning is that cellphones could interfere with the aircrafts electronics and risk a breakdown of navigation or communication equipment. Given the other sources of intereference on board and the general construction of modern aircraft (all controls go over fibreglas cables that are immune to radio interference), this seemed like a rahter paranoid measure. Indeed, if you study experiments closely where they actively tried to achieve interference in a commercial aircraft, the concusion is that it is in theory possible. But only if the pilot itself directly holds the phone transmitting at peak power very near to the instruments.

So, the popular theory was that the GSM networks itself requested the ban, in order to avoid network confusion from phones seeing to many cells at once from the great altitude. But on closer inspection this also proves to be nonsense. The networks handover strategy could be easily adapted to that risk, and in fact it apparently has already been implemented. Using a GSM cellphone at the normal speed and altitude does not produce any meaningfull results anyway, except during takeoff and landing. Everybody who has traveled with a small private airplane will also know that the pilots routinely use GSM cellphones to advise their landing times to their target airstrip by SMS, as those tend to work best (and, oh wonder, they dont fall down while doing that!)

Now I had recently a couple of beers with someone seems to be really in the know. And what he told me sounds rather logical. The real reason GSM got banned in airplanes are flashing mobile phone antennas. Yes. These blinkety things that were modern some years ago and are still chick in some parts of the world. The physics works as follows: the LED(s) is a semiconductor, a diode to be precise. it is positioned very near to the antenna and gets a lot of radio energy through its own little antenna, enough that it starts to light up and blink. Now by the magic of radio physics, part of that energy gets re-radiated, but on a different frequency, determined by size, shape and position of the antennas to eachother and the characteristics of the LED. Unfortunatelly the re-radiation happens in the frequency range of the aircrafts navigation and ground communication radios, effectively creating the risk of a jamming signal. Now the engineers said “lets ban flashing antennas on aircraft”, but the security folks new they would never ever be able to enforce it. So they banned the use of GSM mobiles altogether…

I will try to confirm this story with own measurements, as soon as I get my hands on a spectrum analyzer. Could be a while, but I wont forget it.