Permissive Action Link - AbsoluteAstronomy.com

A Permissive Action Link (PAL) is a security device for nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

s. Its purpose is to prevent unauthorized arming

Arming plug

An arming plug is a small plug that is fitted into flight hardware to enable functions that, for instrument or personnel safety, should not be activated before flight...

or detonation

Nuclear chain reaction

A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

of the nuclear weapon.
The United States Department of Defense

United States Department of Defense

The United States Department of Defense is the U.S...

definition is:
The earliest PALs were little more than locks introduced into the control and firing systems of a nuclear weapon, that would inhibit either the detonation, or the removal of safety features

Nuclear weapon design

Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...

of the weapon. More recent innovations have included encrypted firing parameters, which must be decrypted to properly detonate the warhead, plus anti-tamper systems

Anti-handling device

An anti-handling device is an attachment to or integral part of a landmine or other munition e.g. some fuze types found in air-dropped bombs such as the M83, cluster bombs and sea mines. It is specifically designed to prevent tampering. When the protected device is disturbed it detonates, killing...

which intentionally mis-detonate the weapon, destroying it without giving rise to a useful nuclear explosion.

Background

Permissive Action Links were developed in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

over a gradual process from the first use of atomic weapons to the early 1960s. Importantly, in 1953 the United States Atomic Energy Commission

United States Atomic Energy Commission

The United States Atomic Energy Commission was an agency of the United States government established after World War II by Congress to foster and control the peace time development of atomic science and technology. President Harry S...

and the Department of Defense

United States Department of Defense

The United States Department of Defense is the U.S...

signed the Missiles and Rockets Agreement, which paved the way for the development and implementation of PALs. Certain national laboratories, under the auspices of the AEC, would develop and produce nuclear weapons, while the responsibility for the use and deployment remained with the military. The laboratories were also free to conduct their own research in the field of arms control and security. The thinking behind this was that if the government would ever be interested in such a security device, the research and development of prototypes would already be well advanced. At the beginning of the 1960s, the desire for the usage of such a system grew for both political and technological reasons.

Technologically, the newer, more sophisticated, nuclear weapons were much simpler in their operation and produced en masse. Unlike previous designs, it was not a cumbersome and lengthy preparation to arm these devices, and their numbers increased monthly. As a result, a new form of control was necessary to prevent their unauthorized use. As the Cold War

Cold War

The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...

came to a head in the 1960s, the government felt it best not to leave the use of nuclear weapons in the hands of possibly-renegade generals, including the commander of Strategic Air Command

Strategic Air Command

The Strategic Air Command was both a Major Command of the United States Air Force and a "specified command" of the United States Department of Defense. SAC was the operational establishment in charge of America's land-based strategic bomber aircraft and land-based intercontinental ballistic...

(SAC). Without Permissive Action Links, nuclear weapons were effectively under the independent command of a number of generals, some of whom had a less-than-calm view of the East-West conflict.
In order to protect its NATO allies, the United States had stationed various nuclear weapons overseas; these weapons were thus at least under the partial control of the hosting allied state. This was especially concerning to Congress, as this lack of control was in violation of federal law. Added to this was the fact that some of the allies were considered potentially unstable — particularly West Germany

West Germany

West Germany is the common English, but not official, name for the Federal Republic of Germany or FRG in the period between its creation in May 1949 to German reunification on 3 October 1990....

and Turkey

Turkey

Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

. There was considerable concern that in one of these countries the instructions of the civilian leadership of the host country could overrule the military. In addition, the U.S. realized that in the event of war, parts of West Germany would be overwhelmed early on, and nuclear weapons stationed there could fall into the hands of the Soviets.

For a long time the U.S. military resisted the use of PALs. It feared the loss of independence and feared malfunction, which could put warheads out of action in a time of crisis. But the advantages of PALs outweighed the disadvantages: thanks to the PALs weapons were able to be distributed to a greater extent in Europe, so as to prevent a rapid and selective destruction or conquest by the Soviet bloc, while still retaining U.S. control over the farther-flung weapons.

Development and dissemination

The precursors of Permissive Action Links were simple mechanical combination lock

Combination lock

A combination lock is a type of lock in which a sequence of numbers or symbols is used to open the lock. The sequence may be entered using a single rotating dial which interacts with several discs or cams, by using a set of several rotating discs with inscribed numerals which directly interact with...

s that were set into the control systems of nuclear weapons, such as the Minuteman

LGM-30 Minuteman

The LGM-30 Minuteman is a U.S. nuclear missile, a land-based intercontinental ballistic missile . As of 2010, the version LGM-30G Minuteman-III is the only land-based ICBM in service in the United States...

ICBM. There they could perform different functions: some blocked the cavity through which the nuclear materials were shot

Gun-type fission weapon

Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of sub-critical material into another...

to create a reaction; other locks blocked circuits; and some simply prevented access to the control panel. For testing, some of these mechanisms were installed during 1959 in weapons stationed in Europe.

The work on PAL prototypes remained at low levels until 1960. Sandia National Laboratories

Sandia National Laboratories

The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....

(SNL) successfully created a number of new combination locks that were adaptable to different types of weapons. In the spring of 1961, there were a series of hearings in Congress, where Sandia presented the prototype of a special electro-mechanical lock, which was then known still as a "Proscribed Action Link." The military leadership, however, soon realized that this term had negative connotations for the use of weapons by the officer corps (proscribed = "prohibited"), and changed the meaning of PAL to "Permissive Action Link" (permissive = "allowing / tolerating").
In June 1962, President John F. Kennedy

John F. Kennedy

John Fitzgerald "Jack" Kennedy , often referred to by his initials JFK, was the 35th President of the United States, serving from 1961 until his assassination in 1963....

signed the National Security Action Memorandum

Presidential directive

Presidential Directives, better known as Presidential Decision Directives or PDD are a form of an executive order issued by the President of the United States with the advice and consent of the National Security Council...

160 (NSAM 160)(see image). It was a presidential directive that ordered the installation of PALs in all U.S. nuclear weapons in Europe. All other U.S. nuclear weapons were excluded at that time. The conversion was completed September 1962 and cost $23 million ($ in dollars).

For the Minuteman ICBM force, the US Air Force's Strategic Air Command

Strategic Air Command

worried that in times of need the codes would not be available, so they quietly decided to set them to 00000000. The missile launch checklist

Checklist

A checklist is a type of informational job aid used to reduce failure by compensating for potential limits of human memory and attention. It helps to ensure consistency and completeness in carrying out a task...

s included an item confirming this combination until 1977.

The complete conversion to PAL systems was relatively slow. In 1974, U.S. Defense Secretary James Schlesinger found that a variety of tactical nuclear weapons were still not fitted with Permissive Action Links, even though the technology had been available for some time. It took another two years until all the tactical nuclear weapons were fully equipped with PALs. In 1981, almost 20 years after the invention of PALs, just over half of U.S. nuclear weapons were still only equipped with mechanical locks. It took until 1987 until these were completely replaced.

Modernization and the present

Over the years the Permissive Action Links have been continuously maintained and upgraded. In 2002, PALs on older B61 nuclear bomb

B61 nuclear bomb

The B61 nuclear bomb is the primary thermonuclear weapon in the U.S. Enduring Stockpile following the end of the Cold War. It is an intermediate yield strategic and tactical nuclear weapon featuring a two-stage radiation implosion design....

s were replaced and upgraded with new systems to improve reliability and security, as a part of extending the weapons' service life to at least 2025.

Code Management System

1995 saw the development of the Code Management System (CMS). The CMS has simplified the control and logistics for staff and improved the flexibility and speed in deploying and arming weapons. New codes can be used to recode, lock, and manage the weapons, while the secrecy and validity of the possible launch orders is still ensured. In total, CMS consists of fourteen custom products (nine software and five hardware products). The software products were developed by Sandia National Laboratories while the hardware was created by the National Nuclear Security Administration

National Nuclear Security Administration

The United States National Nuclear Security Administration is part of the United States Department of Energy. It works to improve national security through the military application of nuclear energy...

.

The CMS was fully operational for the first time in November 2001. A part of the system, a special cryptographic processor fitted in to the weapons in 1997, had a potential Year 2000 problem

Year 2000 problem

The Year 2000 problem was a problem for both digital and non-digital documentation and data storage situations which resulted from the practice of abbreviating a four-digit year to two digits.In computer programs, the practice of representing the year with two...

. By the spring of 2004, all PAL systems were equipped with the Code Management System. It is thus currently the general foundation for future hardware and software improvements to Permissive Action Links.

Features

Permissive Action Links are powered by low-maintenance radioisotope generator

Atomic battery

The terms atomic battery, nuclear battery, tritium battery and radioisotope generator are used to describe a device which uses the emissions from a radioactive isotope to generate electricity. Like nuclear reactors they generate electricity from atomic energy, but differ in that they do not use a...

s. Instead of a conventional battery, these generators produce electricity based on the heat evolved from the radioactive decay of plutonium-238

Plutonium-238

-External links:**...

. Although half-life of ²³⁸Pu is 87.7 years, these generators have shorter lifespans. This is due to the pressurization of the generator from helium produced by the alpha decay

Alpha decay

Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...

of the plutonium fuel.

PALs are also linked directly or indirectly with a number of security measures, which together form a comprehensive security package. In general, elements of PAL systems are located deep within the nuclear device. This makes it almost impossible to bypass the system.
PAL devices have been installed on all or nearly all nuclear devices in the US arsenal, including the Minuteman

LGM-30 Minuteman

ICBM, MGM-13A Mace Tactical Ballistic Missile

Tactical ballistic missile

A tactical ballistic missile is a ballistic missile designed for short-range battlefield use. Typically, range is less than . Tactical ballistic missiles are usually mobile to ensure survivability and quick deployment, as well as carrying a variety of warheads to target enemy facilities, assembly...

, CGM-13B Mace Tactical Ballistic Missile

Tactical ballistic missile

, Sergeant

MGM-29 Sergeant

The MGM-29 Sergeant was an American short-range, solid fuel, surface-to-surface missile developed at the Jet Propulsion Laboratory. Activated by the US Army in 1962 to replace the MGM-5 Corporal it was deployed overseas by 1963, carrying the W52 nuclear warhead or alternatively one of high explosives...

, Pershing, and WAC Corporal

Wac Corporal

The WAC or WAC Corporal was the first sounding rocket developed in the United States. Begun as a spinoff of the Corporal program, the WAC was a "little sister" to the larger Corporal. It was designed and built jointly by the Douglas Aircraft Company and the Guggenheim Aeronautical Laboratory.The...

missiles as well as the Nike-Hercules

Nike-Hercules Missile

The MIM-14 Nike-Hercules , was a solid fuel propelled two-stage surface-to-air missile, used by US and NATO armed forces for high- and medium-altitude air defense...

, Honest John

MGR-1 Honest John

The MGR-1 Honest John rocket was the first nuclear-capable surface-to-surface rocket in the US arsenal.The first nuclear-authorized guided missile was the MGM-5 Corporal. Designated Artillery Rocket XM31, the first such rocket was tested 29 June 1951 and the first production rounds were delivered...

rockets, the Davy Crockett

Davy Crockett (nuclear device)

The M-28 or M-29 Davy Crockett Weapon System was a tactical nuclear recoilless gun for firing the M388 nuclear projectile that was deployed by the United States during the Cold War...

system, and artillery-launched howitzer rounds

Nuclear artillery

Nuclear artillery is a subset of limited-yield tactical nuclear weapons, in particular those weapons that are launched from the ground at battlefield targets...

Two-man rule

Modern PALs use the two-man rule

Two-man rule

The two-man rule is a control mechanism designed to achieve a high level of security for especially critical material or operations. Under this rule all access and actions requires the presence of two authorized people at all times.-Nuclear weapons:...

, which is designed to prevent accident

Accident

An accident or mishap is an unforeseen and unplanned event or circumstance, often with lack of intention or necessity. It implies a generally negative outcome which may have been avoided or prevented had circumstances leading up to the accident been recognized, and acted upon, prior to its...

al or malicious launch

Rocket launch

A rocket launch is the takeoff phase of the flight of a rocket. Launches for orbital spaceflights, or launches into interplanetary space, are usually from a fixed location on the ground, but may also be from a floating platform such as the San Marco platform, or the Sea Launch launch...

of nuclear weapon

Nuclear weapon

s by a single individual.

For example, on a ballistic missile submarine

Ballistic missile submarine

A ballistic missile submarine is a submarine equipped to launch ballistic missiles .-Description:Ballistic missile submarines are larger than any other type of submarine, in order to accommodate SLBMs such as the Russian R-29 or the American Trident...

(SSBN), both the commanding officer

Commanding officer

The commanding officer is the officer in command of a military unit. Typically, the commanding officer has ultimate authority over the unit, and is usually given wide latitude to run the unit as he sees fit, within the bounds of military law...

(CO) and executive officer

Executive officer

An executive officer is generally a person responsible for running an organization, although the exact nature of the role varies depending on the organization.-Administrative law:...

(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 ICBMs, the set of keys

Key (lock)

A key is an instrument that is used to operate a lock. A typical key consists of two parts: the blade, which slides into the keyway of the lock and distinguishes between different keys, and the bow, which is left protruding so that torque can be applied by the user. The blade is usually intended to...

is distributed among the key personnel on the submarine and are kept in safes (each of these crew members has access only to his keys), some of which are locked by combination lock

Combination lock

s. Nobody on board has the combination to open these safes - the unlock key comes as a part of the launch order from the higher authority.

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 which holds a code). The Sealed Authenticators are stored in a safe which has two separate lock

Lock (device)

A lock is a mechanical or electronic fastening device that is released by a physical object or secret information , or combination of more than one of these....

s so a single crew member cannot open the safe alone. Both crew members must simultaneously turn the 4 launch keys. An additional safeguard is provided by requiring the crew in another Launch Control Center

Launch control center (ICBM)

A launch control center , in the United States, is the main control facility for intercontinental ballistic missiles . A launch control center monitors and controls missile launch facilities. From a launch control center, the missile combat crew can monitor the complex, launch the missile, or relax...

to do the same for the missiles to be launched.

The two-man rule also applies when the President of the United States

President of the United States

The President of the United States of America is the head of state and head of government of the United States. The president leads the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces....

orders the launch of nuclear weapons. Although only the President can order the launch, the order must be confirmed by the Secretary of Defense.

Stronglinks and weaklinks

Another part of the PAL design is the inclusion of "stronglinks" and "weaklinks." These words, which come from the proverb "A chain is only as strong as its weakest link" ensure resilience to accidental activation through damage. The stronglinks include the ruggedization of some components and inclusion of insensitive munitions

Insensitive munitions

Insensitive munitions are munitions that are chemically stable enough to withstand mechanical shocks, fire, and impact by shrapnel, but that can still explode as intended to destroy their targets.-Description:...

so that they will not be circumvented by fire, vibration, or magnetic fields, leaving the PAL vulnerable to bypass after such damage. On the other hand, 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 fails safe.

Critical signal detection

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.

Parameter recognition

A further safeguard is the Environmental Sensing Device (ESD). It determines through environmental sensors whether the weapon is operating under expected parameters. For example, on an ICBM, a nuclear warhead would first be exposed to a strong acceleration, and then a period of free fall. The ESD determines the external parameters such as acceleration curve, temperature and pressure and only activates the weapon when these external effects occur in the correct order and must fall within specific parameters. So, should unauthorized personnel succeed in stealing a weapon, they could not activate it as long as a functional launch mechanism was not also stolen along with it. Of course, without the PAL codes they would still not succeed.

Misfire

The conventional explosives needed to start the chain reaction are tailored to the characteristics of the fissile material in the core of the weapon. If the detonation does not occur exactly as planned, such as in the case of a misfire, a nuclear reaction is unlikely to occur - the explosion will be no greater than the amount of conventional explosive (although radioactive material—the unreacted nuclear fuel—may be dispersed). Computer simulations have assisted in calculating the likelihood of a chain reaction still occurring in a misfire. The odds of a nuclear chain reaction after a misfire of the conventional explosives is one in one million. The likelihood of a full-yield nuclear explosion is even lower, at one in one billion.

Limited attempts

The weapon renders itself inoperable if an activation code is entered incorrectly multiple times. This precludes attempts to bypass the PAL codes through trial and error. The weapon must be returned to a maintenance facility in order to work again.

PAL versions

Over the years the design and feature set of PALs has increased, as has the length of the access code. US-manufactured PALs are divided into five categories; however, the earliest PALs were never assigned a category letter.

CLASS	\| Code Length	\| Description
		The first PALs were combination locks with a three-number sequence. Later versions used four numbers, so that the access code could be divided between two people, each of whom would only know half of the sequence.
		Class A PALs were electromechanical switches designed for ballistic missiles. The four-digit code was entered into the weapon using a portable electronic device.
		Category B PALs were essentially identical in function to Category A, but designed with newer technology. Additionally, they could be activated via a wired remote, and were thus used on weapons launched by aircraft.
		These PALs featured a 6-digit switch, and allowed for only limited code attempts before lockout. Such behavior was pioneered in some late model Category B PALs.
		Permissive Action Links in Category D had all the features of the previous generation, but also allowed for the input of multiple types of codes, including ones that could set the device to a training mode, or disable the weapon entirely.
		Category F PALs expand the code length to 12 digits, and disable the weapon in addition to lockout after a series of failed code entry attempts. They also include the ability to control the magnitude of the nuclear reaction (the so-called Dial-A-Yield feature) and an emergency stop.

Usage of PALs by other states

The increase in the number of nuclear-armed states was a similar cause for concern for the United States government for reasons similar to the original impetus for PALs. Thus, since the 1960s, the US has offered its own PAL technologies to other nuclear powers. The US considered this a necessary step: if the technology were kept secret, it would only be half as effective as possible, since the other power in a conflict might not have such safety measures.

In the early 1970s, France was an early recipient of United States assistance on this critical element of nuclear security. The Nuclear Non-Proliferation Treaty

Nuclear Non-Proliferation Treaty

The Treaty on the Non-Proliferation of Nuclear Weapons, commonly known as the Non-Proliferation Treaty or NPT, is a landmark international treaty whose objective is to prevent the spread of nuclear weapons and weapons technology, to promote cooperation in the peaceful uses of nuclear energy and to...

(NPT) went into effect in 1970 and precluded treaty members (including the US) from directly disseminating technology related to nuclear weapons development or enhancement. In order to get around this prohibition, the US developed a legal trick: "negative guidance". French nuclear scientists would regularly brief US scientists on French developments in the field of PALs, and the US scientists would tell their French counterparts whether they were on the right track. In 1971, the US also offered its technology to the Soviet Union, which developed a similar system.

In the early 1990s the People's Republic of China

People's Republic of China

China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...

requested information to develop its own PALs. The Clinton

Bill Clinton

William Jefferson "Bill" Clinton is an American politician who served as the 42nd President of the United States from 1993 to 2001. Inaugurated at age 46, he was the third-youngest president. He took office at the end of the Cold War, and was the first president of the baby boomer generation...

administration believed that to do so would give too much information to the Chinese about American weapon design, and therefore, refused the request.

In 2007 the British Government revealed that its nuclear weapons were not equipped with Permissive Action Links. Instead, the UK's nuclear bombs to be dropped by aircraft were armed by just inserting a key into a simple lock similar to those used to protect bicycles from theft. The UK withdrew all air-launched bombs in 1998. The current UK Trident warheads can also be launched by a submarine commander with the support of his crew without any code being transmitted from the chain of command.

Detailed information about PAL systems design and their use is classified, although these mechanisms have been offered to Pakistan

Pakistan

Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...

for protection of their nuclear weapons. In the end, the US decided that it could not do so for legal reasons; the Pakistanis were also concerned that such technology would be sabotaged by a "kill-switch" that the US could operate. However, many experts in the field of nuclear technology in the US government supported the publication of the PAL system because they considered Pakistan's arsenal as the world's most vulnerable to abuse by terrorist groups.
In November 2007, the New York Times revealed that the US has invested 100 million USD since 2001 in a secret program to protect Pakistan's nuclear arsenal. Instead of transferring PAL technology, the US provided helicopters, night vision and nuclear detection devices, as well as training to Pakistani personnel in order to prevent the theft or misuse of Pakistan's nuclear material, warheads, and laboratories.

External links

The JCAE and the Development of the Permissive Action Link
Keeping Presidents in the Nuclear Dark
Security Engineering, Chapter 11: Nuclear Command and Control, Ross Anderson, ISBN 0-471-38922-6
http://www.bbc.co.uk/mediaselector/check/player/nol/newsid_7090000/newsid_7099100?redirect=7099125.stm&news=1&nbram=1&nbwm=1&bbram=1&bbwm=1The Dr. Strangelove scenario - a BBC Newsnight
Newsnight
Newsnight is a BBC Television current affairs programme noted for its in-depth analysis and often robust cross-examination of senior politicians. Jeremy Paxman has been its main presenter for over two decades....

report featuring the lack of security features on the WE.177
WE.177
WE.177 was the last air-delivered tactical nuclear weapon of the British Armed Forces. There were three versions; WE.177A was a boosted fission weapon, while WE.177B and WE.177C were thermonuclear weapons...

] (streaming video)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Background

Development and dissemination

Modernization and the present

Features

Two-man rule

Stronglinks and weaklinks

Critical signal detection

Parameter recognition

Misfire

Limited attempts

PAL versions

Usage of PALs by other states

See also

External links