Author Topic: Cell phone location via triangulation  (Read 1863 times)

Fly320s

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Cell phone location via triangulation
« on: December 09, 2006, 09:54:01 AM »
This ties in with the lost Kim family in Oregon.

As I understand the events, the Kim family's cell phone was not within the cell tower coverage area so they could not get a signal to make a call.  I know that a cell phone signal can be located to it's source via triangulation when the cell phone is in contact with three or more cell towers, but can a cell phone be located when it is not in contact with a cell tower?

When a cell phone is first turned on it sends out radio signals to locate a suitable cell tower to work with.  Many cell phones will keep broadcasting that signal until they get a reply from a nearby cell tower.  Could Mr Kim's cell phone be located while it was transmitting that search signal?  Actually, I'm sure it can be located since it is a radio broadcast.  But do search and rescue teams have access to the type of equipment that can be used to find a transmitting cell phone?

Cell phones could become similar to emergency locator transmitters (ELTs) if the locating equipment is cheap enough for wide distribution.
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cfabe

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Re: Cell phone location via triangulation
« Reply #1 on: December 09, 2006, 10:21:49 AM »
I am not an expert, but I would think that rather than when a phone is unable to contact a tower, rather than send out a signal and wait for a reply, it's actually the other way round. It scans all appropriate frequencies and listens for an active channel, then tries to negotiate with the tower to gain access to the network. It would have to work this way, so that if you took your US phone to a foreign country and powered it up, it could not interfere with some other service that happened to be running on that frequency in the foreign country.

So, I think the short answer is no, they cannot be located if off-network.

Desertdog

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Re: Cell phone location via triangulation
« Reply #2 on: December 09, 2006, 11:13:38 AM »
From a lecture I was at a few months ago, they explained that the cell towers do plot the strongest signal location when you use the cell phone.

By tranulation from the locations of these 3 towers, they can do a good job of finding you.

Problem arise when there is only 1 or 2 cell towers.  With 2 cell towers you can plot a 2 directional line, which can do a fair job of plotting you location, just not as close as triaglelation.

With one tower, they can only plot a straight line to you.  No way to tell where ypu are on that straight line.

From an aricle I read explaining how they managed to find the Kim car, the Kims had sent a partial text message, which was recorded on 1 cell tower.  This gave them the area to start searching, ie: the straight line from that tower.

They also said that the cell phone informing the towers where they are is not recorded, only when they use use the cell phone.

Desertdog

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Turning cell phones into lifelines
« Reply #3 on: December 09, 2006, 11:30:55 AM »
Fairly long article about being traced by cell phones with, and without GPS systems.  This is the part of the article that explains how the Kim family was located.  The rest of the article is very informative.

Turning cell phones into lifelines
http://news.com.com/Turning+cell+phones+into+lifelines/2100-1039_3-6140794.html

Always connected
So how does it all work? Mobile devices, when they are within range, constantly let cell towers and the mobile switching center, which is connected to multiple towers, know of their location. The mobile switching center uses the location information to ensure that incoming calls and messages are routed to the tower nearest to the user.

If a subscriber is unable to get service, this location information is usually purged from the mobile switching center. But some location information may remain in call detail records. Some mobile operators may store the most recent communication between a device and a mobile switching center for a certain period of time, usually 24 hours.

When someone is missing, even this small bit of information can prove useful in determining the approximate location of a device using the updates from the mobile switching center. If the mobile subscriber is still within cell phone range, authorities can track his or her general movement by following the sequence of towers the phone has contacted or pinged. And if the cell phone goes out of range or runs out of battery power, the mobile operator may be able to use the last recorded location before the cell phone either lost its signal or lost power.

But the most useful information for locating people when they are lost comes when someone has initiated or received a call or text message on their phone. Mobile operators keep records of these events for billing purposes in what is known as a call data record, or CDR. And they can go back to these records to get a historical account of the cell phone's location.

This is actually what authorities used to locate the Kims' phone, according to Eric Anderson, director of engineering for Edge Wireless, a regional mobile operator that provides cellular phone service in the area where the Kims were stranded. One of Edge Wireless' cell phone towers briefly connected with one of the family's phones at about 1:30 a.m. November 26 near Glendale, Ore. The phone was connected long enough to the network to send a notice that there was a voice mail or text message waiting. But the connection didn't last long enough for the Kims to retrieve the message or initiate a call for help.

Still, the connection was long enough that two Edge Wireless engineers, Eric Fuqua and Noah Pugsley, were able to find this information in the CDR to determine that the family was in sector "Z" in the southwestern portion of the cell site's 26-mile radius. Wolf Peak's "Z" sector provides coverage to remote areas with little population and very little cell phone traffic. Using this information, authorities sent out rescue teams, which eventually located Kati Kim and her children.

Anderson said that the family was lucky that they were Cingular Wireless subscribers. Edge Wireless uses the same GSM network technology that provides roaming coverage to Cingular customers. If the Kims' phone had been with a different provider that didn't have roaming coverage with Edge Wireless, then the company might not have received any signal at all after they left the major highway, and the cell phone would have been of little use to authorities trying to rescue them.

"Where the Kims' car was found was on the fringe of our coverage area as it was," Anderson said. "So it was a miracle that the phone was able to lock onto the network at all."

Anderson said that if people ever find themselves in a similar situation--lost and having difficulty getting cell phone reception--they should search for the highest ground or area that may be in the line of sight to a tower. They should hold the phone away from their bodies or high so it has no obstructions to a possible tower. It may take up to two or three minutes for it to synchronize or connect with the cell tower and mobile switching center. Even if they can connect for a second or two, it could be long enough to register a voice mail or text message, which could ultimately help wireless engineers track their location.


280plus

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Re: Cell phone location via triangulation
« Reply #4 on: December 09, 2006, 12:05:47 PM »
Good info, thanks...
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Art Eatman

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Re: Cell phone location via triangulation
« Reply #5 on: December 10, 2006, 06:19:24 AM »
A lot of the Big Bend of Texas is well beyond the range of the very few cell towers located here, much to the dismay of a lot of city folks who take it for granted that their toys will always work.

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Fly320s

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Re: Cell phone location via triangulation
« Reply #6 on: December 10, 2006, 10:50:46 AM »
Art,

That's why I was wondering if the cell phone could be tracked when it is off-net.  Sort of a poor man's substitute for a personal ELT.

Obviously, personal ELTs would work much better, but they are a little more expensive.  I was thinking that if the technology is readily available, then searching for some lost person's cell phone signal might be a better alternative than a grid search with aircraft or people.
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Matthew Carberry

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Re: Cell phone location via triangulation
« Reply #7 on: December 10, 2006, 03:15:26 PM »
Just like with a radio, go for the high ground to get commo.

Helps if you have a decent map (topo) so you can figure line of sight.
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