IMSI Catchers

ID card

What is an IMSI catcher?

An IMSI catcher is an intrusive piece of technology that can be used to locate and track all mobile phones that are switched on in a certain area. 

An IMSI catcher does this by ‘pretending’ to be a mobile phone tower - tricking your phone into connecting to the IMSI-catcher, and then revealing your personal details without your knowledge.

IMSI catchers are indiscriminate surveillance tools that could be used to track who attends a political demonstration or a public event like a football match. They can even be used to monitor your calls and edit your messages – and you wouldn’t even know it was happening.

Who is using IMSI catchers?

The police “neither confirm nor deny” that they use IMSI catchers. This means that the police are refusing to explain to the public how they could use these invasive devices to secretly access data from our mobile phones.

However, documents obtained by Bristol Cable show that the following police forces have either purchased the technology or allocated money to do so:

  • Avon & Somerset Police
  • Essex Police
  • Kent Police
  • The Metropolitan Police
  • South Yorkshire Police
  • Staffordshire Police
  • Warwickshire Police
  • West Mercia Police
  • West Midlands Police

How does an IMSI catcher find out my identity? 

“IMSI” stands for “International Mobile Subscriber Identity”, a number unique to your SIM card. Once your phone is tricked into connecting to an IMSI catcher, it reveals this unique number. Once the police have your IMSI, they can easily determine your identity.

How does an IMSI catcher find out my location?

Once your phone has been tricked into revealing its IMSI, the IMSI catcher can determine your phone’s general location by measuring the strength of the signal from the phone. Measuring the strength of the signal from different locations permits an ever-more precise determination of the phone’s location.

Can an IMSI catcher snoop on my calls and text messages?

Yes. Some IMSI-catchers can ‘intercept’ your text messages, calls and internet traffic. This means others can read or listen to your personal communications. Some IMSI catchers can even re-route or edit communications and data sent to and from your phone.

Some IMSI catchers can also block service so you can no longer use your phone to make or receive calls and text messages – even for emergency calls.

What about my rights?

IMSI catchers are an enormous invasion of your right to privacy – they trick all mobile phones within a certain area into giving up information about their owner – even during what the police might describe as ‘targeted’ operations. These devices will hoover up all this personal data, yet the police refuse to disclose what they do with this data, how long they keep it or who they might share it with.

Where an IMSI catcher actually intercepts calls, text messages or internet traffic from your phone, this represents an even more extraordinary invasion of your privacy.

By combining your phone data with other information, the government can not only identify you but also start to track you and build up a profile about you.

And it’s not just your right to privacy that’s under threat.

IMSI catchers can also interfere with your right to free expression, free assembly and free association. For example, the government can use IMSI catchers at political gatherings and protests to identify and collect the personal data of those in attendance.