Tag Archives: E-911

How mobile phones reveal your location

Obviously, when your phone’s GPS receiver is on, your location within 30 feet or so is usually available.

Cellular antenna system on monopole
photo Steve Kazella

There’s another way that remotes, your cellular service provider, 9-1-1 call centers [also known as Public Safety Answering Points (PSAPs)], and law enforcement can determine your phone’s location, even when your GPS is off, or even if your plain-Jane flip-phone has no GPS receiver. It’s called Uplink-Time Difference of Arrival U-TDOA). Here’s a brief simplified video description. Each cell tower has an antenna array with three or four 90 or 120 degree (when viewed from above) antenna sectors. Each tower knows, by comparing your phone’s received signal strength in each sector, which sector your phone is in. By measuring the propagation time for a “ping” to travel between the tower, your phone, and back again, it also knows the range to your phone. In a populated area your phone is likely to be talking with more than one tower, so all that’s needed is to know the bearing and range to your phone relative to two or more towers, and your location can be estimated within maybe a 100 foot radius. (You will be at the intersection of the two or more arcs.)

Even with only one tower talking to your phone, it knows that you are located somewhere along that 90 or 120 degree arc within the sector with the strongest signal. U-TDOA is used in Enhanced 9-1-1 Phase II systems so that first responders may be dispatched to wherever your cell phone is located when you place a 911 call for emergency assistance.

The only way to stop this is to remove the battery from your phone. (Oops. Sorry, iPhone users.) Switching it off won’t stop the communication. Switching it to Airplane Mode will prolly stop it, but there are no guarantees.

Update Here’s a clear explanation of mobile phone positioning techniques.

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© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695

T-Mobile’s VoIP 9-1-1 Strategy

Yesterday, my T-Mobile home page included a link to their page titled Wi-Fi Calling and 9-1-1 Address. This describes their method of providing 911 caller geo-location information when using a T-Mobile phone that’s connected via a Wi-Fi access point. [“Wi-Fi Calling” is their tradename for VoIP (Voice over Internet Protocol).]

Like their cellular phone 9-1-1 location strategy, it doesn’t rely on the caller phone’s GPS receiver. Also like their cellular geo-location strategy, it tries the most precise method first, and falls back to progressively less precise methods. I guess that the other U.S. cellular companies employ a similar VoIP 9-1-1 strategy.

Visit my website: http://russbellew.com
© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695

Mobile phone E-911 deployment

On their mobile homepage, T-Mobile recently linked to T-Mobile’s 911 geolocation commitment document. Locating landline phones during a 911 call is easy: each landline phone number is associated with a geographic location, so the 911 operator can quickly dispatch help to the correct location.

Landline or not, you should ensure that the correct location is associated with your phone number. Check with your phone service provider. Most provide a private webpage with street address form that’s accessible from your account. Having the correct location on file could save your life.

Click to enlarge

9-1-1 System (click to enlarge) · illustration by Evan Mason

Mobile phones complicate this system, because they’re, well, mobile. Enhanced 9-1-1 Phase I service identifies the caller’s cell-site and sector, which provides a general location to the 911 operator. Enhanced 9-1-1 Phase II service provides more precise location information to the 911 operator. T-Mobile explains,

Since early 2004, T-Mobile has been deploying a network-based Phase II solution. . . T-Mobile’s solution is referred to as U-TDOA, Uplink-Time Difference of Arrival. U-TDOA does not require customers to purchase a new handset. [It also does not require that the phone contain a GPS receiver and is accurate to within about 150 feet. -RB].

VoIP (Voice over Internet Protocol) phones, because of their mobility, present similar geo-location obstacles during a 911 call.

Next Generation 9-1-1

The next generation 9-1-1 system (NG9-1-1) will allow “calls” to be initiated by any Internet-connected device. Calls for help may contain text, pictures, or video. Crashed vehicles will initiate “calls” and send crash data to 9-1-1 centers [also known as Public Safety Answering Points (PSAPs)] without human intervention.

Visit my website: http://russbellew.com
© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695