imagery intelligence (imint) taken by satellites (e.g., us corona, kh-5, etc.) covert high-altitude reconnaissance aircraft (e.g., lockheed u-2) and drones/unmanned aerial vehicles (e.g., global hawk), and sensor-carrying aircraft allowed by treaty (e.g., oc-135b open skies), is a fundamental method of verification. specific "protocols" spelling out the details of treaty implementation may require cooperation with imint, such as opening the doors of missile silos at agreed-to times, or making modifications to aircraft capable of nuclear weapons delivery, such that these aircraft can be identified in photographs.