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How do I turned the Enhanced 911 feature off my cell phone?
December 21, 2007 2:22 PM   Subscribe

I would like to disable the Enhanced 911 feature of my cell phone that broadcasts my position continually via GPS tracking. As my phone doesn't allow me to turn this feature off through the settings, how can I modify the circuit to bypass the GPS transmitter?
posted by moooshy to Grab Bag (13 answers total)
 
I wouldn't say it broadcasts continually - only while a 911 call is in progress, and even then, I have to hit "Ok" on my phone.


Listing the phone you have will make it easier to get a proper answer.
posted by niles at 2:25 PM on December 21, 2007


It doesn't broadcast your position constantly, and you can't disable it.

Also, there is no "GPS transmitter". There's a GPS receiver, and during a 911 call, when the 911 center sends a query, it takes a position fix and then briefly interrupts the voice traffic to send a data packet with your location.
posted by Steven C. Den Beste at 2:30 PM on December 21, 2007


There is no broadcast. E911/agps position is determined by your position relative to the last few radio towers you have connected to. We know the gps of those towers and can do a little triangulation (and phone wizardy) and figure out roughly where you are. Its also federal law to have this working, so you cant call your cell provider and ask them to remove this.

Phones and pdas do have a function to block this data from other applications running on the phone but not from the cell provider. You can get a pre-e911 phone but you can be as easily tracked through other methods. see this wikipedia article for more info.
posted by damn dirty ape at 2:38 PM on December 21, 2007


OK, I swear I'm not making this up (based on the first few responses):
http://www.wired.com/science/discoveries/news/1998/01/9502

The bit about transmitter vs. receiver-that-transmits seems like
splitting hairs; either way, I want to disable it. :P

Oh well. Better to come home empty-handed than to never go fishing.
posted by moooshy at 2:40 PM on December 21, 2007


The way your position is broadcast constantly is through the phone's normal operating signal. It allows the network to know which cell tower to use to route calls to you. There is also a way they can track you with more detail about your position by measuring the strength of the signal between multiple towers but unless you're being tracked by the Feds, no one is doing this to you.

The simple solution is to turn your phone off, or some would say turn it off and take the battery out.

I tried to get a pre-enhanced 911 phone activated on a network, any network, just because I liked the phone. I couldn't do it because it was explained to me that there were huge fines involved for companies that allowed it to happen.
posted by 517 at 2:50 PM on December 21, 2007


There is no broadcast. E911/agps position is determined by your position relative to the last few radio towers you have connected to.

Thats not true. Phones with GPS will attempt to get a fix and transmit that location to the emergency services if a 112/911 call is made.

OK, I swear I'm not making this up (based on the first few responses):
http://www.wired.com/science/discoveries/news/1998/01/9502


That article basically just says that the e911 legislation could be abused in future for surveillance purposes. Currently handsets with GPS will bring up a dialog saying "Your position was sent to the emergency services" when emergency mode is activated. Thats pretty useless for spying on people.

Telephone networks have always been able to log what cell ids your phone has been using. While this usually isn't enough to get an ambulance to your location I suspect it is enough for a government agency that wants to keep tabs on someone.

If you're paranoid about being followed you're better off not having a mobile phone, whether it has a gps receiver or not makes no difference.
posted by Olli at 2:55 PM on December 21, 2007


The way AGPS works is by transmitting the GPS signal up to the cell towers where their faster heavier more power hungry computers can compute it.

If your phone has AGPS on all the time, you will be transmitting basically your exact position all the time. Even without that the cell provider can probably get a rough sense of where you are just based on what cell towers you're talking to.

I am not 100% sure about this but it looks like AGPS uses a separate GPS antenna. If this is true, you might be able to disable that, if you can find it. That would of course disable 911 geopositioning too. I don't know if installing a switch would ruin the antenna - GPS antennae tend to be really sensitive to cabling issues.
posted by aubilenon at 3:13 PM on December 21, 2007


E911/agps position is determined by your position relative to the last few radio towers you have connected to. We know the gps of those towers and can do a little triangulation (and phone wizardy) and figure out roughly where you are. Its also federal law to have this working, so you cant call your cell provider and ask them to remove this.

DDA, your information is wrong. There is a GPS receiver inside the phone, for CDMA2K phones at least (Verizon and Sprint), because there turned out to be no way to guarantee to locate the phone with sufficient accuracy without it. You can't triangulate because often only one tower can receive the phone's signal. (And even if two could, there are other issues involved.)

It's also true that whenever your phone is on the cell system has at least a general idea where you are. That's necessary so that it knows which tower(s) to use to transmit a page when you have an incoming call.

Your phone "registers" periodically. The interval is a parameter set in the cell but usually it's once every ten minutes. That means the phone wakes up briefly and sends a small message to the cell on the paging channel to tell the system that the phone is present and is turned on.

When you get an incoming call, the cell system transmits a challenge on the paging channel for all the towers in the region you were last seen in, which your phone responds to with another registration. That tells the system which sector you're in, and it then uses that sector to create a traffic channel to your phone -- after which your phone rings.

It isn't possible for the system to operate without tracking the general locations of the phones in it. And the system can send a challenge any time -- including under operator control. That's how OJ Simpson was tracked during his "thunder run" through LA. The cell system transmitted registration challenges to his phone every couple of minutes, and the cell company told the police where, in general, he was.

And that was an AMPS phone.

They bagged the "happy face" bomber the same way. He had been smart enough to turn his phone off, after he became aware that the police were on to his identity, and left the area. In northern Nevada, his phone got turned on somehow, and registered with the local phone system, which sent that to his home system (which is necessary to make roaming work), and his home system told the cops where he was. After that, it didn't take long to figure out that he was on a particular stretch of a highway, and the Nevada state police found him.

If you're paranoid about being tracked by The Matrix, the only way to prevent that is to turn your phone off. Of course, that means you can't receive incoming calls.
posted by Steven C. Den Beste at 3:27 PM on December 21, 2007 [2 favorites]


Two crude ways to do it - snip the GPS antenna (it will almost certainly use a seperate antenna from the cell phone), or snip the GPS chip.

Cutting traces to the GPS chip risks bricking the phone if it checks this and so fails its boot sequence self-test, but cutting the antenna should just result in the GPS chip never managing to get a lock on any satellites, and everything works as normal. As far as the GPS knows, your phone is simply indoors and thus unable to see any satellites right now.

Go online, and learn what the different kinds of GPS antenna look like so you don't cut the cellular one by mistake. If you want to be extra sure, look up the ID numbers of all the chips, as the GPS is probably handled by a dedicated chip, and confirm that this is connected to the antenna you think is the GPS one.
posted by -harlequin- at 5:25 PM on December 21, 2007


If you have to ask AskMe for advice you should NOT try to modify your phone. Pulling this off successfully is tricky for someone experienced in working with modern high-density circuit boards and a schematic.
posted by b1tr0t at 8:23 PM on December 21, 2007


I wouldn't say it broadcasts continually - only while a 911 call is in progress, and even then, I have to hit "Ok" on my phone.

I'm pretty sure 911 can get your position just by calling you, regardless of whether or not you pick up. I seem to recall a news story about a woman who's body was found that way.
posted by delmoi at 10:12 PM on December 21, 2007


If you're using a CDMA phone, there's at least one chipset that combines the GPS and celphone receiver into one unit - The block diagram for this chipset shows seperate GPS and Cel antennas, however, so the cut-the-antenna trick would work for this model. This does not stop tower triangulation (Both methods are exsistant in the e911 network, I believe), but even on a good day, triangulation isn't very accurate.

Agree with above that, if you're asking, you're probably going to brick your phone trying this - Don't cry if it happens.
posted by Orb2069 at 12:38 PM on December 22, 2007


If you are concerned about your position being known, do not turn on your phone. It is that simple. What the posters above have said is correct about your (general) position being known every time your phone makes contact with or is interrogated by the cell network.

A couple months ago I met a hardware engineer who worked on E911 and questioned him at length about the capabilities of the system. Assuming no GPS data, if you're in the middle of nowhere, your local cell site is probably just a single omnidirectional antenna, and the only location data that can be provided is your distance from the antenna, based on the round-trip time for packets sent to/from your phone. If you have line-of-sight to two cell sites, the system can deduce that you are in one of two possible points (in 2D space), at the intersection of the distance-circles drawn from each antenna. Along major highways, the cell sites are typically located right next to the highway, and this has the consequence that a distance report from two cell sites, or a position report from one cell site along with the direction its antenna is facing, is sufficient to pinpoint the location of the caller.

In more populated areas, a very precise 2D position is possible, taking advantage of the additional data points from the other nearby cell towers. In some scenarios, a precise 3D position can be triangulated. The engineer I spoke to said in a major city with lots of cell towers, it was entirely possible for them to tell the responding authorities on what floor they were likely to find the caller, along with the general 2D location (e.g. northwest corner). The 911 operator also receives an estimate of how accurate the location is.

So while the overlords don't yet have the pinpoint-precision-of-everyone-all-the-time capabilities you see on TV and the movies, radiolocation of your position is possible in many areas, GPS or no GPS, with or without your knowledge.
posted by brain at 1:06 AM on December 24, 2007


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