A cellphone "sniffer" could hunt down and pinpoint illicit cellphones in prisons, often used to conduct criminal activity, its U.S. developers says. While it is virtually impossible to stop cellphones being used inside prisons, a way of pinpointing exactly where a call is coming from could help clamp down on the practice, researchers at Intelligent Automation Inc. in Maryland said. There is no way currently to block individual signals, so the only option would be to jam all signals at one cellphone tower, which would disrupt calls by people living in the neighborhood of the prison. Instead, IAI's system would analyze phone signals from immediately outside the prison walls to triangulate and pinpoint their origin. IAI engineers said by installing four small antennas at the corners of a prison building, they managed to locate an in-use cellphone within about 2 feet, NewScientist.com reported. A signal processing computer that measures the time it takes for a digital phone signal to reach each antenna allows the software to triangulate, from the best three signals, which cell the phone is in, engineers said. "We can detect phone activity, whether it is voice, text or data, in the monitored area, and map the cellphone location," IAI's Eric van Doorn said. A full field test of the system will be conducted at a state prison in Lawrenceville, Va., this year, he said.