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I'm not sure if you're joking or not... Regardless, the ban does not affect cell phones.



Joking for sure. When I flew domestic in China people were using their phones the whole time.


I'm aware that using a phone on a plane does no harm, but I just wanted to make sure you didn't actually mean it.


Sure it does. To the cellphone networks. (Unless the aircraft carries its own picocell).

The big problem with cellphones on planes is that they overwhelm the signalling networks on the ground - when the carrier tries to determine which cell is best suited to handle your call, it basically polls the neighbouring cells for the received signal strength from that particular phone. When you're on a plane, your phone is seen from an awful lot of cells.

Also, some systems (GSM, for instance) relies on your velocity compared to the base station not being too large to allocate you a slot in the time-multiplexed channels. Depending on the geometry of the cell network, you may not be able to connect at all when on a plane - but your phone may create all sorts of grief for the network operator.

Or, put another way - if there really was a non-zero probability that a working cellphone on a plane would cause trouble, the TSA would simply collect the phones during boarding and being caught with one would give you the full terrorist treatment.

Given the abundance of cellphones and the sloppiness of people, I think it is probably safe to say that no commercial flight has taken off without at least one active cell-phone aboard for the past twenty years or so.


I'm skeptical that it really causes trouble for phone networks. In my experience flying small airplanes, getting a signal is easy below about 4,000ft, starts getting spotty up to 6,000ft, and is basically impossible above that. Which makes sense: antennas at the cell will be aimed sideways, not up, so the only cells you might be able to talk to would be really far away.

And this is in a fiberglass airplane that's basically radio transparent. Now try it in an aluminum airplane at 30,000+ft. I doubt the cells can even hear your phone in the first place.


-You're quite possibly right - I cannot recall having heard cell phones go off while at cruise, but during approaches it happens if not all the time, then at least every once in a while when I fly somewhere. (Me being a good boy, I've never tried to satisfy my curiosity by keeping the phone on during flights. :))

There's enough radio transparent openings in an aircraft fuselage for it to be a reasonably inefficient faraday cage - say, windows, for instance. (Or are those portholes on an aircraft, too?)


As far as I know they are still called windows. And yes, you'll be able to connect when close to the ground in many cases. The metal body will greatly attenuate signals, but won't block them completely, especially if you're near a window. The lower and slower you are, the more an airborne cell phone will look and act like a normal ground-bound one, so the less of a problem it'll cause for the network.


I wonder, don't cellphone networks have ways to handle it? E.g. if they detect a phone that behaves as if travelling 13km up at 900km/h, couldn't they just tag it as "in flight" and ignore it for the next few minutes?


I don't know, to be honest - I do suspect the problem is much smaller now that 3/4G is a thing, though - higher data rates means smaller cells means more directional antennas, and presumably carriers will not waste energy beaming skyward - so maybe airborne phones don't see as many cells as they used to (if any).

It would be non-trivial to determine the phone's altitude, but you could make a pretty good inference from the number and location of cells reporting that they 'see' it - and the velocity should be a no-brainer to figure out with a reasonable degree of accuracy. Hm.

Just a semi-educated guess. (I haven't looked at cellular systems since I was in university 15-20 years ago.)

Methinks you are probably right.


Except...the cell sites typically use antennas that have a high gain in the veritcal axis, meaning very low gain in the direction of high flying aircraft and a low probably that transmissions from phones inside a faraday cage at said high altitude will rise above its receiver noise threshold.


That's funny because in China it's forbidden to use it even on airplane mode.


Sometimes in China, the thing is not lack for things being forbidden, it's the lack of enforcement culture.




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