Everything you wanted to know about mobile networks but were afraid to ask: Airplanes & Mobile Phones

Thank you once again to contributor Steve Kennedy. It’s not often that we’re lucky enough to get a contribution from him (his last was on the Ofcom white space issue) so I’m pleased to bring you not one, but a series of posts from Steve.

This series is dedicated to those things you sort-of know about. Well, me at least. I like to think I’ve got a good, broad working knowledge of everything mobile — but I do very much enjoy learning more about things. Which is where the utter geek that is Steve comes in.

If you’ve ever met Steve around the UK tech scene, chances are you’ve come away from the encounter with at least one factoid that you definitely didn’t know. At the P0wer0f1 event recently, I bumped into him. I can’t remember how we got on to the topic of planes and Heathrow but all of a sudden Steve was explaining why it’s seriously necessary, in this day and age, to require passengers to turn off their mobile phones. Surely it’s nothing to do with radio signals interfering with plane systems, right?

Over to Steve.

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Mobile Networks are just radio systems that just sit there and allow you to make calls. Most people just take them for granted as they just work (well most of the time). But mobile networks are actually quite fragile beasts and they can get upset quite easily.

One of the first things you’re warned about when flying is to “Make sure you turn off your mobile phone, it can interfere with the plane’s electronics” (or that might be guidance systems or other systems on board), you’re also told to turn off all electronic devices during take-off or landing.

The electronics thing used to make some sense as old electronic devices were clunky and may well have produced weird harmonics and done strange things (they used to have big batteries and used a lot of power, much of which would be radiated into the surrounding space). Nowadays even if you turn off your electronic device (especially your phone) it’s not really off, but in a more sleepy state, the power button on most phones is a ‘soft’ button that just triggers the devices to wake-up properly. Unless you take the battery out, the phone is on (as is your iPod Touch, your Walkman etc).

Mobile phones transmit at very low power (or the battery life would be a lot worse than it is now), in urban areas it’s likely that there’s a GSM or 3G cell site not far away (which is better for your battery as the signal doesn’t have to go so far to reach it). In rural areas your battery life will actually be a lot less as rural cells can be tens of miles apart.

So say you’re in Heathrow airport you’ll be in close proximity to at least 5 cell sites (assume each network has at least one site in there, maybe a couple as the’ve extended the airport with Terminal 4 and 5). Each cell site is pounding the planes with magnitudes more power than a humble mobile phone can offer (and yes signals pass through windows or your mobile wouldn’t work at all while the plane’s stationary on the ground).

So why the big fuss about having your phone on when air-bound? It’s all about cell handover (and in the US about phone cloning as the IMEI on a CDMA phone is part of the phone itself).

Take the UK as an example: You take-off from Heathrow and head south, rapidly ascending. Suddenly your phone is visible to cell sites in Heathrow, all the neighbouring villages, probably most of London too. Networks have special algorithms which work out which cells to hand-off to, so if you are travelling on the ground the network can easily predict what cell sites should be able to see you next and they can optimise traffic levels by only allowing you to hand-off from one cell to the predicted one(s). Otherwise every cell would have to look for your signal and that is very costly to the network. If your phone doesn’t hand-off correctly then — obviously — the call drops.

When you’re in the air and jumping from random cell to random cell it confuses the hell out of the network hand-off algorithms. Imagine the problems for the network if you’re in Heathrow one moment and then 45 seconds later you appear to be visible to cells in Kent somewhere. To make matters worse, the higher you are, the more cells are visible!

So this is one primary reason why you’re asked to keep your handsets in airplane mode whilst on board.

In the future we — as we’ve already seen with WiFi — airlines are realising they can make money out of mobile service in the air. To deliver cell service on board (and sell you some seriously expensive minutes and texts), all that’s needed is a pico cell with some kind of backhaul connection. Your phone will then roam on to this and therefore won’t need to bother looking for cells elsewhere — and will use minimal power. They’ll still stop you using your phone during take-off and landing to avoid confusing the ground based systems.

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Thanks very much for this Steve.

Standby for Steve’s next post in the series coming soon.