Characterizing a cargo container and its contents. A system comprising an emitter to emit radio frequency (RF) energy, a receiver to receive an RF response related to the energy, and a processor operative to compare a plurality of RF responses. A method for characterizing a cargo container and its contents, the method including providing a system as described above, via the system: emitting a RF energy within a container, receiving an RF response of the container and the container contents, emitting a second RF energy within a container, receiving a second RF response of the container and the container contents, and comparing the second RF response with the first RF response.