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Mysteries of Wireless Emergency Alerts Revealed

A new report explains in detail how Wireless Emergency Alerts accomplish geo-targeting. The report could help practitioners understand overshooting or undershooting of alerts via WEA.

Hear Ye, Hear Ye. I’m about to reveal one of the great mysteries of alerting. I’m going to disclose how geo-targeting of Wireless Emergency Alerts (WEA) really works. (OK, I embellish a bit.)

Even with success stories of lives saved through weather alerts and children found through AMBER Alerts, there still seems to be hesitation about WEA among local practitioners. We think some local public safety officials are reluctant to use WEA for “imminent threats” because they don’t understand how it delivers alerts to targeted geographic areas. No wonder. As it turns out, it's complicated and not an exact science.

At one point, I thought the cell carriers didn’t want us to understand it so they could protect proprietary information. I was wrong. In fact, there’s an organization made up of the carriers and others in the information and communications technology sector that recently published an explanation of how WEA geo-targeting works. In a 56-page document found here, the Alliance for Telecommunications Solutions (ATIS) lays it out thoroughly.  

At the heart is the Common Alerting Protocol (CAP). The standard means everyone is essentially speaking the same language, but it doesn’t mean they’re doing the same thing with what they understand.  

First, carriers are “broadcasting” alerts. They are sending them into the airwaves to be received by properly tuned cell devices. Stating the obvious here, but airwaves aren’t always predictable; they are influenced by topography and atmospheric conditions.  

Second, even though alert originators use a relatively precise method to designate where they would like to deliver alerts, the carriers’ distribution systems aren't quite so precise.  

Here’s how it works: The carriers refer to an alert originator’s desired area as an “alert area” and the area they actually broadcast the alerts as the, well, “broadcast area.” (The simplicity stops at the names.) An alert area is determined by the same type of GIS polygon that public safety practitioners commonly use for other types of notifications, analysis and dispatching. It’s pretty straightforward.

The broadcast area, however, is more complicated. It’s based on a labyrinth of cell towers and a honeycomb of tower signal “sectors.” Then, other factors enter the picture like RF engineering, traffic load distribution, and even local municipal policy. The area covered by the sectors may not, probably won’t, correspond precisely to the alert area polygon. This means any WEA alert will likely overshoot or undershoot the desired alert area. Overshooting occurs when towers are within the polygon, but their coverage sectors span beyond. Undershooting occurs when the towers within the polygon do not cover the full area of the polygon.

The figure below illustrates an example of overshooting and undershooting.  

The figure was reproduced from ATIS-0700027, Feasibility Study for WEA Cell Broadcast Geo-Targeting, with permission from ATIS. A copy of the full report can be obtained here

Add to the complexity is the fact that factors that determine the broadcast area are not the same for all carriers. So, even using the same polygon from an alert originator, the broadcast area for different carriers will likely look different.

WEA will probably be tweaked. In fact, that’s the purpose of the ATIS report and the FCC’s solicitation of recommendations from a working group called CSRIC (CIZZ-rick), the Communications Security, Reliability and Interoperability Council. Even with WEA tweaking, overshooting and undershooting will remain a reality. (Full disclosure, I’m a member of a CSRIC working group, focused on complementary alerting strategies to WEA and EAS.)


So as a practitioner planning to issue an alert, you’re faced with deciding whether to risk a bit of undershooting/overshooting and delivery variance among carriers. Or do you risk missing an opportunity to alert folks through one of the most accessible means of alerting we’ve seen so far, an individual’s personal device, even if you under-alert or over-alert a bit?  

As you can probably tell through wording of the question, and if you’ve read any of my other stuff, you know my opinion. (See short video here.) When there’s a serious event, you have to accept realities that no single alerting channel is perfect. It takes a combination of alerting channels, working together, to be effective. In today’s world, it’s difficult to predict how to best reach people when there are so many communication options available. And, we know from social science study that people need to get alerts from at least two sources before they will actually take action. (Sad, but true.)

I think many public safety officials are missing an opportunity to use WEA for imminent threats. I hope it’s because of a lack of understanding of how it works, not concerns about an ability to precisely geo-target. WEA may not represent exact geo-targeting, but as the diagrams and ATIS report show, it gets close. The risk/return argument is strong.

Rick Wimberly is a contributing writer for Emergency Management magazine.