As the orders go down the chain of command, always subject to the two-man rule, intermediate headquarters, and eventually the nuclear delivery platforms themselves, will receive Emergency Action Messages (EAM) to arm or launch weapons. For most modern weapons, the EAM will also include codes for Permissive Action Links (PAL). At a minimum, a PAL code will actually arm a weapon for release. The circuitry controlling the PAL is deliberately positioned inside the warhead such that it cannot be reached without disabling the weapon, at a minimum, to a level that would require a full factory-level rebuild. There may be separate PAL codes for arming and launch. Some weapons have "dial-a-yield" functions that allow the power of the nuclear explosion to be adjusted from minimum to maximum yield. Most weapons have additional arming circuitry that, even if a valid launch code is entered, will not arm the warhead unless the weapon senses that it has been released on an expected delivery path. For example, the first steps of the final arming process for a ballistic missile depend on physical characteristics of the weapon release, such as the acceleration of a rocket launch, zero-gravity coasting, and various physical aspects of hypersonic reentry into the atmosphere. A gravity bomb dropped from an aircraft will detect the altitude of release and the decreasing altitude as it falls.