Location Accuracy and Mobile Phones ( How does GPS work for 911?)
If a Public Safety Answering Point (PSAP) has inaugurated what is called “Phase II” service, a wireless service provider must attempt to generate location information (x and y coordinates) on each 9-1-1 call and make it available to the PSAP receiving the call. AT&T does this, and our Location Success Rate (LSR) is one of the highest in the industry, exceeding 98 % nationwide.
• At the moment a call is set up and routed, no service provider has the caller’s location estimate. This may take as long as 30 seconds to compute. An Automatic Location Information (ALI) record is staged for retrieval that provides basic location information, specifically, the sector and site serving the call. When the location estimate is ready, the ALI record is updated with the additional location information.
• If the PSAP answers the call before the ALI record is updated, it does not receive the additional location information. How soon the ALI record is updated depends on the location determination technology used by the service provider and other factors. AT&T has used a network based solution on its 2G network. This can be fast (as little as seven seconds but may take longer), but not as precise as some other technologies. On its 3G and 4G networks, AT&T uses AGPS (Assisted Global Positioning System) technology, which is much more accurate. However, it can take up to 30 seconds for this technology to yield a location estimate.
• If the call is answered before the update of the ALI record, the PSAP must refresh the ALI record to retrieve the additional information. The refreshing of the ALI record is a simple process, initiated by the PSAP operator or in some cases by an automated process. It is typically called a “rebid.”
• AT&T began to make wide use of AGPS technology on our 3G and 4G networks, around 2011. The number of calls requiring a rebid increased because of the additional computational time required to generate a location estimate. A number of PSAPs had gotten out of the habit of rebidding for location information. There was a perception that AT&T was not delivering Phase II information, but in fact, many PSAPs were failing to initiate a rebid to retrieve the additional information.
• This is an education and training issue, but does not represent a regulatory issue. It grew out of the fact that for several years PSAPs did not need to rebid, since the older 2G technology was quicker (although much less accurate). PSAPs which have a procedure (either manual or automatic) of rebidding at 30 seconds into the call will receive location information. AT&T has always recommended a 30 second rebid interval in guidance to public safety.
• In August, 2013, several California PSAPs wrote to the FCC asserting that wireless service providers, including AT&T, were not providing Phase II information on all wireless 9-1-1 calls. Meetings between the wireless industry and the PSAPs indicated that rebid procedure was the probable issue. The FCC reviewed information from AT&T and took no action.