Dozens of law enforcement agencies are using radar that can detect humans moving in their homes, without the need to gain entry according to a USA Today report that has reawakened the debate about the crossover of technology and the law.

The newspaper says at least 50 agencies, including the FBI and US Marshals Service, are using such radar devices.

The main reason for using such devices is said to be to protect officer safety when preparing to storm a building, giving them an insight into the number of people inside and their location.

However, the USA Today investigation was sparked by a court case last month involving officers using a radar device as part of an operation to arrest a parole violator.

In a judgment written in surprisingly clear and everyday language, appeal court judges said the other facts of the case meant that the radar use didn’t have to be a deciding issue, despite being raised by the violator. However, they warned that using such devices could raise two, somewhat paradoxical, issues in future cases:

• using radar to track movements in a home could violate the 4th amendment, which bars unreasonable searches; but
• if using radar was considered legitimate, it could be classed as an initial “protective sweep”. That could override the usual legal position by which officers entering a property to arrest a suspect are allowed to search for weapons, even if the alleged offense isn’t weapons-related.

A 2001 Supreme Court ruling classed thermal image scanning from outside a property as a search that thus requires a warrant. That ruling noted the same should apply to radar systems, but it doesn’t yet appear anyone has successfully persuaded a court to uphold that principle.