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The Plan(s) Current Status Comments 19

Network Based Phone Based Poor PSAPs What Can You Do?

In 2001, it became clear that no one was going to meet the FCC's Phase II deadline for network-based location. Carriers applied for, and many were granted, waivers or extensions. These carriers were still looking for a solution that could meet the FCC's 100 meter requirement. Cingular and AT&T Wireless thought they had found a compliant technology (EOTD), and had begun installing it, only to find out it wasn't going to work. Cingular and AT&T were both fined for not meeting the FCC deadline with their equipment.

Finally, in 2002, carriers tested a network location solution that met the FCC's guidelines and began ordering equipment. Cingular, T-Mobile and some regional carriers built location networks based on U-TDOA technology. Carriers began installing this technology, and have nearly caught up to completing this installation in areas with Phase II ready PSAPs. The carriers will always lag slightly behind behind the number of Phase II compliant PSAPs because they are given 6 months to respond to a PSAP.

Many rural carriers which chose a network-based location scheme have recently been granted waivers by the FCC, allowing them to transmit only Phase I tower location data to emergency centers. These carriers typically install their cell towers in a line along major highways, making network based location impossible.

U-TDOA and other network based location technologies rely on triangulation to determine a handset's location. The handset is pinged by 3-4 towers and the time it takes for the phone to respond to each ping determines the phone's distance from each tower, and thus its location. In order to determine the phone's location, it needs to be between at least 3 towers on at least 2 axes. With towers along only 1 axis, such as a highway, networks could only determine the latitude but not longitude (or vice versa).

(See the illustration below for a visual explanation of triangulation.)

How Triangulation Works

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