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I've heard that mobile phones send signals to the tower even when the mobile goes off so your location can be detected. Is this possible?

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Welcome to Skeptics! We want to focus our attention on doubtful claims that are widely held or are made by notable people. Please provide some references to places where this claim is being made. In particular, I'd like to see what people mean when they say "off". Not on a call? turned off? battery removed? –  Oddthinking Apr 12 '13 at 3:59
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it's certainly a doubtful claim, in fact it's a totally bogus claim UNLESS with "off" he means turning the screen off which leaves the device switched on and capable of receiving calls, which of course requires it to send keep-alive signals to the network which can be used to triangulate its position. –  jwenting Apr 12 '13 at 5:33
    
It is obviously a confusion of the sort alluded to by jwenting. Airlines require phones be turned off to stop their transmissions interfering with avionics. Phones with the display off are still on and are keeping in touch with base-stations so that they hand-over contact to the nearest one as they move, so that they can receive calls and messages - not primarily so that telcos can monitor your location. –  RedGrittyBrick Apr 12 '13 at 9:28
    
I have to say that I have heard this story too: you have to remove the battery for the cellphone to stop sending signals. Of course whoever was telling this factoid could never give me a real explanation for it... –  nico Apr 12 '13 at 16:43
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I know we can do better than this! So far, we have a far-fetched claim with no notability, arguments over what the claim really means, an accepted answer with no references at all, and answer to a different question, and an answer with a single, inappropriate reference. This is a broken window; let's fix it. –  Oddthinking Apr 15 '13 at 16:12
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4 Answers 4

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This is not true.

OK, let me back that up very slightly. First, let's assume we are talking about a normal phone as supplied. Phones can obviously be doctored to look like they are off when they are on, or have additional listing devices planted in them. But this isn't about that.

Second, it does depend on what you mean by 'off'. Many phones will turn the display off when they are not being used. However when that happens, the phone is still doing everything it is supposed to, it's just not displaying anything on the screen. Some people might think of this as 'off', but it really isn't. A simple test - try to call the phone. If it rings, or vibrates, or shows on the screen that you have a call, then it isn't 'off'. In that mode it is still sending messages to cell towers, and someone with the right knowledge and permission can find your location.

Let's consider some other definitions of 'off'. Most phones can switch off their connection to the mobile network (i.e. cellphone network). In that mode, the phone is not communicating at all with the cell towers. No messages are being passed. The phone company cannot deduce your whereabouts in this mode. Of course you might claim that secretly, behind the scenes, the phone is passing information to the cell towers even when switched off. How do we know this isn't happening? Because phones are tested by the manufacturer, and one of the tests is that they emit no radio waves when switched off. For these tests to be faked, hundreds of people in many phone manufacturers, all in different countries, would have to be in on it - and not one would ever have to have leaked it. As well as that, phones are also tested by the carriers - hundreds of carriers in dozens of countries - and they would all have to be in on it too. Most governments also do testing, and they would have to be in on the conspiracy - and this is all governments, from the US to France to India to Russia, all working together to suppress this information.

As an aside, if connection to the mobile network is off, you can sometimes deduce a location of a phone based on a WLAN (Wireless LAN) connection. That assumes of course that wireless is enabled, with all the same caveats about the mobile network. If there is no wireless connection then you can't do that either.

The most normal definition of 'off' is when the phone is powered down. Everything in the above paragraph applies double to the state when the phone is powered down. No radio frequency communication takes place, and this is tested by many, many people worldwide. No phone can reports its position like this. In some phones you can remove the battery, and that makes it triply certain.

So in short: this conspiracy theory is on the same level of probability as the world-controlling Illuminati, or the fake moon landings.

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There are quite a few unreferenced claims in here (e.g. these government and carrier tests on turned off phones). –  Oddthinking Apr 13 '13 at 5:48
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The explanation might be simpler if you introduce the term 'standby'. Side note: Even when the (primary) battery is removed, some functions still work, such as the clock... so how sure can we be? –  Oddthinking Apr 13 '13 at 5:53
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Phone companies put huge amounts of effort into minimizing the draining on the (primary) battery when the device is connected to the mobile net, and even so they usually last only a day or two. The clock battery is hundreds of times smaller than the primary battery. It could conceivably power the radio for a few minutes at most. –  DJClayworth Apr 13 '13 at 15:03
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Sorry Rory, but Ilyas answer is incorrect if you follow the sources back. The Kaplan opinion was about a bug planted in the phone. Obviously a phone with a bug in it can transmit whatever it likes. –  DJClayworth Apr 14 '13 at 21:54
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@RoryAlsop thanks for the ad hominem attack against an argument you can't refute. I've worked in the mobile phone industry, I know what data they receive from phones and none is received when the phone is turned off. You may be paranoid, but so am I. Yet I know my phone isn't going to mysteriously send my location to "the government" when it's turned off, the fact that the battery doesn't drain when it's turned off is indication enough of that. –  jwenting Apr 15 '13 at 13:09
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These articles state it can be done:

By September 2004, a new NSA technique enabled the agency to find cellphones even when they were turned off. JSOC troops called this “The Find,” and it gave them thousands of new targets, including members of a burgeoning al-Qaeda-sponsored insurgency in Iraq, according to members of the unit.

A secondary, ulta low power "baseband" processor remains "on" to listen to the cell tower. When the baseband processor detects an incoming call, it turns the rest of the phone back "on". Especially with older "feature phones", turning the phone "completely off" would sometimes leave the baseband processor still "on", thus allowing you to be tracked. For example, sometimes the phone had a timing circuit that will occasionally turn on the baseband to grab SMS messages every 10 minutes

On most Nokia phones, the alarm still rings even when the phone is turned "off."

A cellular telephone can be turned into a microphone and transmitter for the purpose of listening to conversations in the vicinity of the phone. This is done by transmitting to the cell phone a maintenance command on the control channel. This command places the cellular telephone in the "diagnostic mode." When this is done, conversations in the immediate area of the telephone can be monitored over the voice channel.

And the police can potentially push updates onto your phone that backdoor it and allow it to be turned into a microphone remotely, and do other stuff like that

Simply put, yes, it can be done. Technical details aside, your cell phone can be remotely powered on and off without either your consent or your knowledge.

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Need better sources on the ones that say the phone can't be tracked when off. The more reliable sources don't quite address the question. But I do recall having a Nokia alarm ringing after being switched "off". –  Muz Jul 29 '13 at 4:35
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When there's no power in electronic components, they do nothing. However, and that's the tricky point, it is nigh impossible to "switch off" modern mobile phones (and computers, for that matter). The power switch is no longer a real, genuine, true-to-the-core power switch which switches power; it is a normal switch which sends a signal to some CPU which then decides to kill power in most circuits, or restore it. A corollary is that even when the device is formally "off", it still draws a tiny bit of current in order to process the power switch, should it be pressed.

Removing the battery, or waiting for a complete discharge, really kills power and thus make the phone emit no more electromagnetic waves than a brick (i.e. a bit of infrared, as is expected, unless you keep the phone in liquid nitrogen, which might void the warranty).

As an indirect symptom, turn the phone off, then plug it on the USB port of a computer (I assume the phone has a USB-compatible plug). It should soon light the screen and display a "charging" icon (so, at that point, the CPU in the phone is indubitably "on", if only to manage the nifty icon animation). Now try again, but only after the battery has been thoroughly discharged (you kept the phone off until it shuts down spontaneously, then power it up again, and again, until the power-up sequence does not produce any visible behaviour at all). You plug this empty phone in the USB port of a computer; the "charge" icon appears again, but this time after a muuuuch longer delay. What has happened ? In the first case, the phone was not completely off; in particular, the RAM contents were maintained. So that when the USB port began delivering some current, whatever circuitry handles the USB protocol sent the wake-up signal to the CPU, which promptly recovered a working state and could proceed with the charging and animation. In the second case, the CPU (the main one, or at least a secondary CPU) first had to be booted up from the newly available power, and the boot sequence takes some time, as is observed.


So mobile phones are not really "off"; the power button does not kill them completely. What do they do while in such a coma ? It... depends.

When the first iPhone was put on market, a few horror stories circulated about people who brought their phone with them while on a trip abroad, and found huge bills (thousands of dollars) upon coming back home. For instance, this story. Interestingly, the report says that:

The iPhone regularly updates e-mail, even while it's off, so that all the messages will be available when the user turns it on.

(Emphasis mine.)

This is slightly weird, because exchanging data over radio takes some non-negligible power. A basic phone (not a smartphone with its zillions of applications) can last about ten days on its battery, if not call is performed or received. During that time, the phone will receive signals from cell towers almost continuously (if only to decide whether a given frame is for it or not), and send signals of its own on a regular basis, about once every 30 seconds or so. It is the sending which draws the most power, not the receiving. For checking on emails, the phone must do some TCP/IP, and probably a SSL handshake, so it will have to send a few kilobytes of data: we can estimate this activity as costing at least 10 times as much as the normal "keep-alive" signal of an active phone. This means that even with some good signal optimization, an "off" iPhone would discharge its battery in three weeks.

That's the part I find weird. Doing a lot of magic transparently so that the user finds his emails when he wakes up, that would be quite typical of an Apple product. But squandering that much power on a device which is known to be power starved, that would be peculiar. I cannot dismiss the possibility, but I give it a 10% chance of being real, tops.

On the other hand, I have a rather old mobile phone (from 2003) which works only on another continent than the one where I live. I use that phone about once every six months; the rest of the time, it sits in "off" state in a drawer. When I power it on, after six months, the battery is not empty. This means that this particular phone cannot emit signals while off, or at least not often.


In some official circles, they insist on participants to "sensitive" meetings to remove the battery of their phones. This makes sense: they fear that a phone could be bugged, from software, emulating a normal "off" state but actually being in full microphone-and-radio mode.

We can totally imagine that a given smartphone could be infected with some malware which makes it emit localization signals when switched off (i.e. the phone is not switched off, it just blackens the screen and emulates an "off" state; but it remains somewhat active internally). I rather doubt that mass-produced phones would be systematically equipped with such a feature, but I am known to be an optimist.

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This post does not cite any references or sources. Please help improve this post by adding citations to reliable sources. Unsourced material may be challenged and removed.

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One post on Democratic Underground is not a credible source. –  DJClayworth Apr 14 '13 at 23:57
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Please provide some references to support your claims - including that current is required to service a power switch, which is about where I started skimming. –  Oddthinking Apr 14 '13 at 23:58
    
@odd - mobile phones do not have a power 'switch' as you understand it, but have a circuit which you can send a trigger impulse to, for it to decide whether to power the whole device. –  Rory Alsop Apr 15 '13 at 15:51
    
@Rory: Maybe you are correct. Or maybe it is between the two extremes - pressing the momentary-close switch could send not just a trigger, but enough current to wake up a circuit which could then decide to power the whole device. More importantly, we are speculating without evidence. –  Oddthinking Apr 15 '13 at 15:58
    
I will see whether I can provide sources of information suitable for you @odd. I don't know whether any useful info is in the public domain. –  Rory Alsop Apr 15 '13 at 16:57
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It can, if someone with the needed technology, like the FBI, wants it to and installs the needed device or software without your knowledge. This can be avoided by cutting off the power supply to the device, i.e. remove the battery from the device.

Cell phones can be taped and used as a listening device as long as their buttery is in, even if they are powered off. As this article by cnet demonstrates:

The surveillance technique came to light in an opinion published this week by U.S. District Judge Lewis Kaplan ...

Kaplan's opinion said that the eavesdropping technique "functioned whether the phone was powered on or off." Some handsets can't be fully powered down without removing the battery; for instance, some Nokia models will wake up when turned off if an alarm is set. While the Genovese crime family prosecution appears to be the first time a remote-eavesdropping mechanism has been used in a criminal case, the technique has been discussed in security circles for years.

Countermeasures against it are taken in the IDF (Israeli Army). I have experienced in first hand that to classes, briefings or meetings where confidential material is talked about or revealed, all participants must suppurate their phone from its buttery, and if your phone doesn't allow this, e.g., the Iphone, then you leave it outside of the room. Here is an article (in Hebrew) talking about information safety in the IDF that tells the same thing:

בדיונים חשובים משאירים [...] את הסלולרי אצל הפקידה בחוץ (בהתאם לפקודה האוסרת שימוש בטלפונים סלולריים, למעט "ורד הרים", במהלך אימונים, פעילות מבצעית ודיונים מסווגים)

Which translates to:

During important discussions, cellular phones are left with the secretary outside (With compliance with the order that prohibits the use of cellular phones, except for "mountain roses"*, during training, operations and classified discussions).

*: "Mountain Rose" is a secure IDF cellular network.

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Sorry to burst your bubble, but if you trace the sources the Kaplan opinion (and the cnet article) refers to a "listening device installed in the phone" which functioned whether the phone was on or off. In other words the phone was bugged - the phone itself didn't transmit. The other vulnerabilities were to either rogue software installed in the phone, or monitoring conversations. Such vulnerabilities easily explain the reason why phones need to be left outside secure environments. –  DJClayworth Apr 14 '13 at 21:52
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Also I believe the question was about location information, not voice monitoring. –  DJClayworth Apr 14 '13 at 22:04
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