It's against FCC regulations to use a cell phone on an airplane. Most people seem to think the reason is that they're afraid it'll interfere with the navigation of the airplane. Here's the FCC's policy statement on the subject. Note in particular the second statement: they're worried about interference with the ground.
To understand this, it's important to understand how a cell phone works. Each phone has a radio transmitter/receiver, aka transceiver. It's capable of being tuned to any one of (depending on protocol) dozens or hundreds of channels. Whether it's digital or analog, 3G, 4G, TDM, packet, etc., what exactly constitutes a channel is immaterial to the issue. What's important is that there are a limited number of them.
The ground part of this consists of a network of transceivers mounted on poles or other high places, called cell towers, which can tune to the same channels. They are organized into cells, which is a region on the ground using wires or microwave links to connect to each other and the rest of the phone system. Each cell tower can use, at most, the number of channels in the protocol. Phones are low power radios and they actually reduce power so that they can't be heard by more than one or two cell towers at once. Because any bandwidth a phone uses must be reserved by every cell tower that can receive the signal. (this is not strictly true--there's some collision recovery in several of the digital protocols--but collisions reduce bandwidth, so the essential problem remains. for simplicity, let's pretend it's all channels and ignore packets and collisions and such)
Most of the time, most phones are not in use--they have a handshake with their local tower so the system knows which phone to ring, but this doesn't take much bandwidth. Phones that are in use use a lot of bandwidth. If an area has a lot of phone traffic--a big building for example, they'll put extra towers in to adapt.
But it's important to recognize the two dimensional design of the network. Most of the time you're fairly close to the center of a cell. you use two cells only when you're near the border of both. But if you're up in the sky, you're a lot farther from the closest cell, and the second closest might be only a little farther yet. Worse, there might be several others that are still close enough that any bandwidth you use on your phone must be reserved for all the cells. In a dense city and a plane at 10,000 feet, this could be a hundred cells or more.
Now imagine that a plane is flying over our group of cells, carrying 100 passengers who simultaneously want to call their spouse or client or whatever, telling them the plane is about to land, time to come pick me up at the airport. They've used 100 channels in every cell within range--which could be a band 10 or 15 miles wide and 50 miles long, under the landing pattern, which is usually in a big city. Not only that, there's another plane coming along a minute or two later doing the same thing, with an effect that's overlapping the first plane. Two or three such planes could exceed the entire capacity of the entire group of cells.
So they ban it. If one or two people call, it's no big deal; it's stealing those two channels from all those cells, but they have enough capacity they can handle it. If half of them leave their phones in ground mode by mistake, there's so little bandwidth used it hardly matters. It's only if a lot of them are using a lot of bandwidth simultaneously that it's a problem. Adding more cell towers doesn't solve it: they'd be in range too. what does solve it is a cell repeater on the airplane, which uses a different protocol to talk to the ground. FCC is happy to have people think it'll cause interference with the airplane they're on, because that'll keep them from cheating.
Similarly, the flight attendants don't have the time to figure out which electronic device might cause ground interference and which is harmless. The fact is that most are harmless, although there is a tiny chance that one of them might cause a bunch of RFI, and a real problem, either because it's non FCC compliant or because it's malfunctioning. The one they don't recognize might be the one that causes the problem. Simpler and safer to have a single rule and ban them all.
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