The radars work like finely tuned motion detectors, using radio waves to zero in on movements as slight as human breathing from a distance of more than 50 feet. They can detect whether anyone is inside of a house, where they are and whether they are moving.
Those agencies began deploying the radar systems more than two years ago with little notice to the courts and no public disclosure of when or how they would be used. The technology raises legal and privacy issues because the U.S. Supreme Court has said officers generally cannot use high-tech sensors to tell them about the inside of a person’s house without first obtaining a search warrant.
Agents’ use of the radars was largely unknown until December, when a federal appeals court said officers had used one before they entered a house to arrest a man wanted for violating his parole. The judges expressed alarm, warning that “the government’s warrantless use of such a powerful tool to search inside homes poses grave Fourth Amendment questions.”