Editing Permissive action link (section)

=== Environmental sensing device ===
An environmental sensing device (ESD) determines through environmental sensors whether the weapon is operating in its combat environment. For example, on an ICBM, a nuclear warhead would first be exposed to a strong acceleration, then a period of free fall and then further acceleration as the warhead reenters the atmosphere. The ESD determines the external parameters such as acceleration curve, temperature and pressure, and only arms the weapon when these environments are sensed in the correct order.<ref name=b61history>{{cite report |date=August 1971 |title=History of the TX-61 Bomb |url=https://osf.io/bfjrc/ |publisher=Sandia National Laboratories |number=SC-M-71-0339 |access-date=May 9, 2021 |archive-date=March 30, 2021 |archive-url=https://web.archive.org/web/20210330090725/https://osf.io/bfjrc/ |url-status=live }}</ref>

ESDs are not exclusive to weapons equipped with PALs and some weapons, such as the [[W25 (nuclear warhead)|W25]], also had ESDs despite not being fitted with PALs.<ref name=w25history>{{cite report |date=August 1967 |title=History of the Mk 25 Warhead |publisher=Sandia National Laboratories |number=SC-M-67-663}}</ref>