GPS signals are easily jammed; the sharpest-eyed spy satellites have trouble seeing through clouds; and communications among myriad military forces and systems frequently clog the airwaves.

• GPS信号很容易受干扰，即使是最敏锐的间谍卫星也难以透过云层寻找目标，而且无数军队和系统之间的通讯常常阻塞信号。