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Through-the-wall surveillance (TWS) technology detects motion through interior or exterior building walls. It can penetrate brick, reinforced concrete, concrete block, sheetrock, wood, plaster, fiberglass and common building materials, but not solid metal (e.g., it can "look" through rebar reinforced concrete but not a solid metal wall). It can be used, for example, to locate and track individuals inside a building during hostage rescue operations.
Some TWS devices must be placed next to the structure; others can operate at a distance from it, enabling building searches from a vehicle or other safe cover.
Refinements in technology and policy have overcome many barriers posed by earlier versions, which were often costly, had to be placed near or directly up against the wall, or were approved for use by the military but not law enforcement.
The NIJ-funded Sensor, Surveillance and Biometric Technologies Center of Excellence has published several reports on TWS technology:
- Through-the-Wall Sensors for Law Enforcement: Market Survey
- Through-the-Wall Sensors for Law Enforcement: Best Practices
- Through-the-Wall Sensors for Law Enforcement: Test and Evaluation
- Through-the-Wall Sensors for Law Enforcement: Use Case Scenarios