Editing Permissive action link (section)

== Features ==
Elements of PAL systems are located deep within the nuclear device. The design and construction attempt to create a [[Black box|black box system]] so as to limit information leakage. PALs are also linked directly or indirectly with a number of other security measures, which together form a comprehensive security package. To prevent exploitation and sniffing via power line attacks permissive action links are powered by low-maintenance [[atomic battery|radioisotope generators]]. Instead of conventional batteries, these generators produce electricity using the heat from the radioactive decay of [[plutonium-238]]. Although the half-life of <sup>238</sup>Pu is 87.7 years, these generators' lifespan is shorter than that; the [[alpha decay]] of the plutonium produces helium, causing the pressure inside the generator to increase.<ref>[http://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/94-95winter/surveillance.html Milliwatt Surveillance Program Ensures RTG Safety and Reliability] {{Webarchive|url=https://web.archive.org/web/20110307015400/http://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/94-95winter/surveillance.html|date=March 7, 2011}}. In: ''The Actinide Research Quarterly'', Winter 1994. accessed February 4, 2009.</ref>

{{quote|"Bypassing a PAL should be, as one weapons designer graphically put it, about as complex as performing a [[tonsillectomy]] while entering the patient from the wrong end."|sign=[[Peter Zimmerman|Peter D. Zimmerman]], nuclear physicist and weapons inspector<ref>Dan Caldwell, Peter D. Zimmerman: ''Reducing the Risk of Nuclear War with Permissive Action Links.'' In: Barry M. Blechman, David K. Boren (Eds.): ''Technology and the Limitation of International Conflict.'' Johns Hopkins Foreign Policy Institute, Washington, D.C. 2000, {{ISBN|978-0-941700-42-9}}.</ref>}}

PAL devices have been installed on all nuclear devices in the US arsenal. The US Navy was last to receive them, with all weapons fitted with PALs by 1996 or 1997.<ref>{{cite book |last=Busch |first=Nathan E |title=No End in Sight: The Continuing Menace of Nuclear Proliferation |publisher=University Press of Kentucky |page=48 |isbn=9780813126760}}</ref>

=== Two-man rule ===
{{main|Two-man rule}}

[[File:SAS Container.png|thumbnail|right|These two locks are part of the implementation of the two-man-rule in a Minuteman ICBM launch control capsule]]

Modern PALs use the [[two-man rule]], which is designed to prevent accidental or malicious [[rocket launch|launch]] of [[nuclear weapon]]s by a single individual.{{fact|date=May 2023}}

For example, on a [[ballistic missile submarine]] (SSBN), both the [[commanding officer]] (CO) and [[executive officer]] (XO) must agree that the order to launch is valid, and then mutually authorize the launch with their operations personnel. Instead of another party confirming a missile launch as in the case of land-based [[ICBM]]s, the set of [[key (lock)|keys]] is distributed among the key personnel on the submarine and kept in safes (each of these crew members has access only to his keys), some of which are locked by [[combination lock]]s. Nobody onboard has the combination to open these safes; the unlock key comes as a part of the launch order from the higher authority.<ref>{{cite news |url=http://www.time.com/time/magazine/article/0,9171,101361-1,00.html |archiveurl=https://web.archive.org/web/20091008111941/http://www.time.com/time/magazine/article/0,9171,101361-1,00.html |archivedate=2009-10-08 |first=Douglas C. |last=Waller |title=Practicing For Doomsday}}</ref>

In the case of [[Minuteman missile]] launch crews, both operators must agree that the launch order is valid by comparing the order's authorization code against a code from a "sealed authenticator" (a special sealed envelope that holds a code). The sealed authenticators are stored in a safe that has two separate [[Lock (security device)|locks]] so that a single crew member cannot open the safe alone. Both crew members must simultaneously turn the four launch keys. An additional safeguard is provided by requiring the crew in another [[Missile launch control center|launch control center]] to do the same for the missiles to be launched.

=== Stronglinks and weaklinks ===
[[File:Permissive Action Link Schematic Composite en.png|thumbnail|right|Simplified illustration of some nuclear weapon safety mechanisms]]
Another part of the PAL design is the inclusion of [[Strong link weak link|"stronglinks" and "weaklinks"]]. These ensure resilience to accidental activation through damage. The stronglinks include an increased ruggedness of some components and the inclusion of [[insensitive munitions]] so that they will not be circumvented by fire, vibration, or magnetic fields, leaving the PAL vulnerable to bypass after such damage. Also, activation-critical electronics within the weapon, such as capacitors, are selected so that they will fail before the safety device in the event of damage, ensuring that the weapon [[failsafe|fails safe]].<ref>David W. Plummer, William H. Greenwood: ''History of Nuclear Weapon Safety Devices''. Sandia National Laboratories, Albuquerque 1998. Presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, July 1998. ([http://www.osti.gov/bridge/servlets/purl/671923-JYRvMV/webviewable/671923.pdf PDF; 1,3&nbsp;MB] {{Webarchive|url=https://web.archive.org/web/20220617075838/http://www.osti.gov/bridge/purl.cover.jsp?p%2F671923-JYRvMV%2Fwebviewable%2F671923.pdf |date=June 17, 2022 }}, accessed September 23, 2010).</ref>

=== Critical signal detection ===
[[File:B-61 bomb (DOE).jpg|thumbnail|right|A [[B61 nuclear bomb|B-61]] bomb contains 5,919 parts, including its PAL]]

Nuclear weapons will only respond to a specific arming signal. This is passed to the weapon by a unique signal generator located outside the weapon. This output is specific and well-defined, precluding approximation, emulation, noise, or interference from being accepted as a [[false positive]].<ref name="Peacetime Operations">Donald R. Cotter: "Peacetime Operations: Safety and Security." In: Ashton B. Carter, John D. Steinbruner, Charles A. Zraket (Eds.): ''Managing Nuclear Operations''. Brookings Institution Press, Washington, D.C. 1987, {{ISBN|978-0-8157-1313-5}}.</ref>

=== 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>

=== Limited retry and non-violent disablement ===
Modern PALs are believed to feature a limited number of code reentries before the weapon locks out, requiring that the weapon be returned to [[Pantex]] for rebuilding. This system may also include a non-violent disablement system, where some of the weapon's internal components are destroyed to hamper use. This system may be part of the ordinary limited retry lockout system, or may be a feature that can be enabled if the local situation calls for it. The non-violent disablement system may also be part of the weapon's anti-intrusion system, designed to activate if someone tries to enter one of the weapon's exclusion regions such as for the purpose of circumventing the weapon's PAL.<ref>{{cite web | first = Carey | last = Sublette | url = https://nuclearweaponarchive.org/Usa/Weapons/Pal.html | title = Principles of Nuclear Weapons Security and Safety | publisher = Nuclear Weapon Archive | date = 1 October 1997 | access-date = 2021-05-09 }}</ref>