The Security Technologies Group (STG) develops the metrology for through-barrier sensing and imaging technologies. These nonmetal barriers may be ordered/structured, such as building walls, barricades, bridges, etc., or disordered, such as hedges and rubble. Because the purpose is to sense or image through these barriers, the frequencies (or equivalently, the wavelengths) of the electromagnetic energy used in these imaging systems is relatively low, typically between 100 MHz and 5 GHz (or between 3 m and 0.06 m). The lower frequencies typically have greater penetration through nonmetal barriers than the higher frequencies, but their spatial resolution is lower, which would affect image resolution.