How to detect contraband cellphone use in prison cells

It’s virtually impossible to stop cellphone use inside prisons, but a way of pinpointing where a call is coming from could help clamp down on the practice. New Scientist reports.

Cellphone service providers and local networks should block calls that are made inside the jail, according to Robert Johnson, a guard at Lee Correctional Institution in South Carolina. He was shot six times at home, in what he believes was an attack organized by a prisoner.

At the moment there is no way of blocking individual signals. The only option is to jam all signals at one cellphone tower, which would also disrupt calls between guards and by people living in the neighborhood -- including those to the emergency services. Setting up dedicated cellphone towers for the sole use of prison staff is an option but they are expensive -- costing more than $1 million per prison.

Signals in a prison wing are messy to analyze – with radio waves bouncing off walls and stairs. So a team at Intelligent Automation, Inc. (IAI) in Maryland, found a way to analyze the phone signals from immediately outside the prison walls instead, where the signals are unaffected by radio echoes.

By installing four 2-inch antennas at the corners of the building, the team managed to locate a phone in use -- voice, text, or data -- to within 20 inches.

A signal processing computer measures the time it takes for a digital phone signal to reach each antenna. The sub-nanosecond differences allow the software to triangulate, from the best three signals, which cell the phone is in.

Full field tests at a state prison in Virginia, are set for later this year. The research was funded by the U.S. National Institute of Justice.

Some privately run prisons in the U.K. use in-cell payphones instead. Having to wait to use non-private payphones drives illicit cellphone acquisition, they reason.

[Via New Scientist]

Image by Su-May via Flickr

This post was originally published on Smartplanet.com