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Geographic Accuracy of Wireless Emergency Alerts (WEAs)

When the WEA program launched, participating wireless providers were generally required to send alerts to a geographic area no larger than the county or counties affected by the emergency. As of November 2017, however, all participating wireless providers were required to transmit alerts to a geographic area that best approximates the area the alert originator specifies, even if it is smaller than a county.

In addition, beginning Dec. 18, 2019, participating wireless providers were required to improve geo-targeting of alerts even further.

How Does it Work?

When the Alert Originator initiates an alert with a delivery area that is specified by using a polygon or circle, wireless providers must “match” the specified target area and deliver alerts to this area with no more than a one-tenth of a mile overshoot.

The wireless industry determined that the only way to deliver that level of precision was to use device-based geo-targeting, which uses location processing in the mobile phone itself. The handset receives the alert including the polygon, then the phone uses its GPS-assisted location to determine whether it is inside or outside the polygon.

Inside vs. Outside the Alert Area

Enhanced geo-targeting is also designed to deliver alerts to 100 percent of WEA-capable mobile phones inside the targeted area in addition to not alerting mobile phones that are more than one-tenth of a mile (528 feet) outside the targeted alert area.

Wireless providers send the geographic information of the alert target area with the WEA message that is broadcast from cell sites near the targeted alert area. Upgraded mobile phones that receive a WEA with geographic information then use the device location functions to determine whether the mobile phone is inside or outside of the targeted alert area.

Upgraded WEA-capable mobile phones and devices will only display an alert if they are inside the targeted alert area.

Standard WEA-capable mobile phones and devices will display any alert received even if the mobile phone or device is outside of the targeted alert area. 

Because mobile phones inside a targeted alert area may be connected to a cell site that is outside of the alert area, wireless service providers broadcast the WEA message from cell sites that are outside but near the targeted alert area. Although this helps to maximize delivery of WEA to mobile phones inside the targeted alert area, it can result in alerting mobile phones outside the targeted alert area.

Continuous Improvement

This technology is evolving and not all handsets currently support device based geo-fencing. In time, all devices will support it either through a software upgrade or out of the box. Therefore, alerts will overreach until technology catches up to the FCC requirement. 

Wireless providers will continue to deliver the alert to an area that best approximates the target area in instances where they are technically incapable of matching it, such as for mobile phones or devices without location services enabled or when part of the target area is outside of a wireless provider’s coverage area.

Last updated August 5, 2020