How Britain's Emergency Alerts Work

On April 23rd, 2023, the UK Government tested its new Emergency Alert System. This is intended to warn people about an immediate risk to life nearby. Every phone that was on should have gotten the notification unless it was disabled by the user. As you can see by google trends, this alert created interest into these alerts as there was a sharp spike of google searches for “Emergency Alerts” just after the alert went off.

Here is the alert that the welsh public recieved

How do these systems work?

According to Gov.uk when there is an emergency alert, mobile phone masts broadcast an alert. All phones that are connected to the towers will get the alert. They also say that the phone doesn’t need to be connected to mobile data to get the alert. These alert services use the CAP protocol.

What is CAP?

CAP or the Common Alerting Protocol is an XML based data format that is used to exchange public warnings for emergencies between different alerting technologies. Cap alerts can be sent over many different systems such as Cell Broadcasts. For the test the UK used a Cell broadcast to send out the alert for everyone.

How does a Cell Broadcast work?

A Cell Broadcast or CB for short is a method to send messages to many mobile phone users in a defined area at the same time. A cell broadcast is a one-to-many messaging service as there is one tower sending a message to many different clients. These cell broadcasts are geo-targeted and geo-fenced meaning they will only work for people in a defined virtual perimeter. The latest generation can send an alert to a whole network in less than 10 seconds.

What is needed to receive the alert?

The government state that to get an alert, all you need is a phone that is connected to either a 4G or 5G network. They specifically state that being connected to 2G or 3G will mean you won’t get the alert. This could be a problem as the UK only has 4G signal in about 92% of its land.

I can’t find any explanation on what would happen if you didn’t have a sim card. However, the government claims that your phone doesn’t have to be connected to mobile data or wifi to get this alert. This could suggest that the system works like a the emergency calls system where your emergency call can be made using a tower belonging to a network that isn’t your own. However, in this case it would be the alert reaches you by being broadcasted from a different tower than one belonging to your own network but still being received.

That's great but...

Before the alert there were many posts on the internet on how to disable alerts like this. This could do more harm than good as many people could have disabled these alerts and many not enable them. In the future this could be dangerous as they may not get a life saving alert.