There are two totally different technologies being deployed to deliver your calling location when you dial 911 from your cellphone. One requires that a handset contain circuitry that receives signals from the constellation of Global Positioning Satellites (GPS), processes those signals to determine your location, and then transmits your location to the network. The other requires that network operators install special receivers throughout their network to listen for special signals from your cellphone, and add processing technology that compares the times of arrival of these special signals to determine the location of your cellphone. This approach is called Time Difference of Arrival, or TDOA.
Both of these approaches overcame technical challenges. The GPS approach needs help when the cellphone is inside a building and the satellite signals are blocked, so this approach is sometimes called Assisted GPS (A-GPS). You can check out http://www.snaptrack.com/ for more details. The network TDOA approach needs to deal with multipath–signals that are reflected before reaching the special receivers. Take a look at http://www.trueposition.com/ for more details on this technology.
The bottom line, though, is that both of these approaches work, and they both satisfy the FCC's accuracy requirements.
Last year, the FCC imposed Consent Decrees on the major wireless carriers. These Consent Decrees imposed specific deployment benchmarks. For carriers using TDOA, the benchmarks required that specific numbers of receivers be deployed by certain dates. For carriers using A-GPS, the benchmarks required that specific percentages of GPS-enabled handsets be deployed by certain dates. And the carriers must file quarterly reports with the FCC.
So, for example, as of earlier this year, T-Mobile and Cingular had each deployed more than 5,000 TDOA receivers throughout their networks, in compliance with the benchmarks the FCC imposed last year. AT&T Wireless claims to have met the FCC benchmarks, but did not report summaries like the other carriers did. TruePosition, a TDOA vendor, reported to the FCC that its technology was deployed at 10,000 sites, and Andrew Corp., another TDOA vendor, reported 18,000 deployments.
All new cellphones that Sprint and Verizon sell are now GPS-enabled. For Nextel, all current models except a Blackberry are GPS-enabled.
But the real measure of progress is the number of Public Safety Answering Points (PSAPs) that are receiving and processing the location information. In order for this to work, the PSAPs need new software and new databases and new data communications links to the wireless carriers. That part of the deployment has been slow.
Sprint claims that it is providing A-GPS location information to 699 PSAPs in 30 different states. Verizon provides A-GPS location information to 1,162 PSAPs serving about 100 million residents. Nextel and its affiliate Nextel Partners are providing A-GPS location information to 237 PSAPs. The TDOA carriers, AT&T, Cingular and T-Mobile, are not specific about the number of PSAPs they serve, but Cingular reported that it has received requests for service from 713 PSAPs.
These numbers don't tell the whole story. While these six carriers are unquestionably the biggest, the wireless trade association CTIA has about 70 wireless carriers as members, and throughout the U.S., there are actually around 3,000 wireless systems.
But even more to the point, the FCC publishes a Master PSAP Registry that contains entries for about 8,000 PSAPs. Admittedly, some are duplicates. But even so, there are still thousands of PSAPs that do not or cannot receive E911 location information, either because the PSAPs are not ready for it or the wireless carriers are not ready.
The situation is worse for the smaller wireless carriers than the big ones. The same FCC file that contains the fairly optimistic reports from Verizon and Sprint also contains pretty depressing reports from small carriers like South Canaan Cellular (Pennsylvania), Litchfield County Cellular (Kentucky), Copper Valley Wireless (Alaska), and many others. Mostly, these small rural carriers say they may be able to provide E911 locations by 2005 or 2006–if they can afford to replace their obsolete analog networks with digital technologies, and if they can convince their subscribers to switch to new digital handsets.
So in practice, not much has changed. Except for a few jurisdictions, when you call 911 on your cellphone, you'd better know where you are. Don't count on the PSAP getting your location data from the wireless operator.
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