All four nationwide wireless carriers yesterday submitted a joint filing to the FCC that outlines changes to their previous proposal on location accuracy for indoor 911 calls, including new performance metrics, greater focus on the vertical Z-axis coordinate and a willingness to work on privacy issues associated with a proposed new database of Wi-Fi and Bluetooth beacon infrastructure.

Submitted eight days before FCC commissioners are scheduled to vote on new 911 location-accuracy rules, the new proposal was signed by representatives of AT&T, Verizon, Sprint and T-Mobile—the carriers that announced an initial location-accuracy roadmap agreement with the Association of Public-Safety Communications Officials (APCO) and the National Emergency Number Association (NENA) in November.

Public-safety officials widely applauded the initial agreement’s focus on delivering a dispatchable-location to public-safety answering points (PSAPs) when a wireless 911 call is made indoors, which is a growing trend for emergency calls. However, critics of the agreement argued that the carrier roadmap did not include enough accountability to require carriers to deliver dispatchable-location fixes for indoor calls and that there was no commitments to supply a Z-axis vertical coordinate for 911 calls made from a multi-story building.

With this in mind, the four carriers proposed an “amended roadmap” that included three new commitments, according to an ex parte filing submitted yesterday to the FCC:

“(1) Adopts new, quantifiable indoor-specific metrics to assure widespread wireless 911 indoor-positioning fixes, including vertical-location fixes;

“(2) Expands the Roadmap’s performance metrics to apply to all 911 calls, not just 911 calls that use Voice-over-LTE (VoLTE) technology; and

“(3) Commits to a National Emergency Address Database [NEAD] privacy and security plan to be developed and transmitted to the FCC,” according to the filing.

In the filing, the carriers’ representatives assert that the amended roadmap provides “performance metrics superior to those in the draft order on circulation [at the FCC].” With this in mind, the carriers’ new proposal is designed “to replace, not supplement” the rules in the FCC’s draft order, according to the filing.

“The amended roadmap commits carriers to widespread implementation of solutions that either provide a dispatchable location or a Z-axis component, or both, to assure the availability of accurate horizontal and vertical location information for indoor calls,” the filing states. “With these commitments, there can be no doubt the roadmap provides clear targets and accountability for indoor location through aggressive performance metrics verified by live call data and an open and transparent testbed.”

Under the new roadmap proposal, the carriers “commit to implement either a dispatchable location or Z-axis location solution in the 50 most populous Cellular Market Areas (CMAs),” the filing states. A carrier would be deemed to have implemented a dispatchable-location solution if the number of “location reference points”—notably, Wi-Fi access points or a low-energy Bluetooth beacons—is greater than the population of the CMA divided by 4, based on the notion that a single reference point can serve multiple users.