All national commercial mobile service providers (CMSPs) in the U.S. do their best to approximate National Weather Service warning polygons when relaying tornado warnings through the wireless emergency alert (WEA) system, according to an executive of CTIA, a wireless industry trade group. The result is geographic targeting that is more granular that the county-wide targeting of NOAA Weather Radio. And the industry is considering proposals to further improve the geographic targeting of WEA.

Information that CTIA assistant vice president for regulatory affairs Brian Josef provided to “W9LW’s Ramblings” contradicts a graphic that appeared on Twitter April 6 (below).

@spann instead of shrinking the size of the warning area w/ the polygon, it's now even larger w/ WEA! pic.twitter.com/TnxoEyFvi4

— Dan Skoff (@weatherdan) April 6, 2016

The graphic in the tweet originally appeared in a 2013 blog by WeatherCall, a company whose sales pitch includes pointing out shortcomings of WEA and other warning modalities. The tweet and blog post claim that wireless emergency alerts get sent by every CMSP tower in every county covered by a warning polygon, thus providing irrelevant warnings to more people than would receive the warning via NOAA Weather Radio.

This might have been true when WEA was first implemented in 2012, the CTIA’s Josef told this blog. And to this day, federal regulations for WEA continue to permit the practice of alerting entire counties. But shortly after WEA was implemented, carriers found ways to improve the granularity of alerts. Today, every national carrier geographically targets tornado warnings based on NWS polygons, Josef said.

The system remains less than perfect, however. The most common way carriers target the reception of WEA messages is by broadcasting them only from the towers that are physically located within the warning polygon, according to a 2015 report by the Johns Hopkins University Applied Physics Laboratory.

In the graphic example below, black hexagons represent the coverage areas of one carrier’s cell towers (in reality, coverage areas are not hexagonal) and the black dots in the center of each hexagon represent the towers themselves (in reality, towers are not usually spaced so evenly). The red polygon represents the boundary of an example tornado warning. Green hexagons represent the areas that would receive that warning via WEA, if the carrier sends it only via towers that are located within the polygon.

As you can see in the graphic above, people in the white areas inside the red polygon will not receive the warning, even though the warning includes their locations. People in green areas outside the red polygon will receive the warning, even though the warning does not include their locations.

“There’s always going to be some overshoot, there’s going to be some undershoot, but they’re trying to employ the techniques that best approximate that alert area,” Josef said of the mobile phone carriers.

Researchers at Johns Hopkins University suggest a new, more precise geo-targeting method for wireless emergency alerts in a report published last June that the U.S. Department of Homeland Security commissioned. It proposes arbitrary-size, location-aware targeting (ASLAT), through which carriers would broadcast an alert over an area larger than the warning polygon and individual mobile devices would determine whether to display the warning, based on each device’s own calculation of its location.

ASLAT would require some changes to existing WEA standards, cellular network functionality and mobile device behavior. Josef stressed the importance of assuring that any new targeting technique not increase data congestion on cell sites at a time when weather conditions would already increase device usage.

“The carriers are constantly looking at ways to further refine and enhance geo-targeting techniques,” Josef said. “We also want to make sure we don’t endanger what has been a successful service.”