Working in information technology department of a large 9-1-1 center has all kinds of benefits. One of them is the understanding of how the public switched phone system works and how ill prepared the network is for a major catastrophe. I don’t want to induce panic but there are limitations to what the public switched phone network and 911 call centers can handle. My topic of limitation is cell phone location technologies.
When a caller calls 9-1-1 the most important thing to convey to the call taker is location location location. Many times the caller can give this information but in some cases they can’t. This is where call location technology comes into play.
Information coded inside the call sends location data to the 9-1-1 call taker.
For the past 30 plus years the public has become accustom to the fact that when you call 911 from a land line that your address (Automatic Location Identifier or ALI) and phone number (Automatic Number Identification or ANI) information is displayed for the call taker on the other end of 9-1-1. This has given the public and first responders a level of comfort that even if you can’t speak if you can dial 9-1-1 we will find you. This is true.
Enter cell phone technologies-
Cell phones do not have nor have they been required to keep the same level of accuracy for location.
The FCC released a decision in late 2007 that requires cell phone companies to meet the location accuracy requirements that are more stringent than have been in place. This is known as Phase II or Enhanced 911.
There are two different methods for obtaining location information from cell phones.
The first is a ‘Network-based’ technology that uses Time Difference of Arrival (TDOA). In simple terms it measures the signal travel time from phone to several different towers and uses a formula to determine location. The down side to this is the number of towers that can receive your signal. In urban environment there are many in a rural environment not as many and thus less accurate.
The second technology is ‘Handset-Based’. This means that the phone itself has a gps inside of the receiver. Location is determined by a minimum of three satellites. This works great in the open not so good in the city where buildings etc block the sky. Being indoors also creates a problem. It doesn’t know if you are on the first floor or the 50th floor of a building.
Where the problem lies is in the accuracy.
For network-based technologies the required accuracy must be within 100 meters for 67 percent of calls, 300 meters for 95 percent of calls. For handset-based technologies the accuracy must be within 50 meters for 67 percent of calls, 150 meters for 95 percent of calls. The remaining 5 percent of calls are handled on a’best efforts’ basis. FCC Requirements
To put this in perspective my coworker Joe Borgione of Alpine Geographic put it into a map.
First there is a huge discrepancy in the technology gap as well as accuracy in a real world scenario. If seconds count and you need 9-1-1 services, you should use your land line first and use your cell phone as a last resort. If you have a cell phone I would move to a carrier that has a hand set and is GPS based.
What some of the wireless companies don’t want you to know? If you think ATT Wireless is GPS based because it offers GPS phones like the Apple IPhone, you are mistaken, as ATT use network triangulation to determine location which is the least accurate. Sprint uses GPS based handsets and are far more accurate. With that said a land line is still your best option.
Outside of the Technology there are also other inaccuracies in the set up of 9-1-1.
Cell phone companies claim that they meet the FCC accuracy requirements. But the national operators (AT&T, Sprint, Verizon and T-Mobile) have been meeting the requirements on a national average basis. Now the FCC has decided that they must meet the accuracy requirement within the geographical coverage area of every Public Safety Answering Point (PSAP). According to Consumer Reports magazine, accurate location information is not delivered at the PSAP level in nearly half of the country. According to a list maintained by the FCC, there are around 6,000 to 7,000 PSAPs in the U.S. Verizon says it has deployed E911 wireless location capability in areas served by more than 2,800 PSAPs. But it has run into cases where cities and counties conflict with one another, each claiming PSAP jurisdiction over the same area. And PSAP service areas vary in size and shape. But the FCC said,’We recognize that geographical variations in service areas can present challenges to the provision of E911 service, but in the interest of public safety, we cannot permit those challenges to justify diminished location accuracy.’ -Jeffrey Krauss CED Magazine January 2008
The next time you need to dial 9-1-1 hopefully you will be prepared with your location information, if not you are at the mercy of technology.