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While the mass use of strategic nuclear weapons is the ultimate terror of modern warfare, it represents the final stage of conflict escalation on the world stage. A more immediate threat comes from tactical nuclear weapons. Tactical nuclear weapons are generally considered low yield, short-ranged weapons designed for the use at the theater-level, alongside conventional forces. Both the US and Russia define a tactical nuclear weapon by it’s operational range.

Their battlefield centric missions and perception as being less destructive encourage their forward-basing and can make the decision to use tactical nuclear weapons psychologically and operationally easier, potentially pushing a conflict into the realm of strategic nuclear escalation. Surveillance systems designed to detect these detonations must home in on the telling characteristics of a nuclear weapon.

BHANGMETER
As the very first nuclear weapon detonated, it was observed by both cameras and other optical instrumentation, that a peculiar double-peaked illumination curve of light was emitted from the bomb. It was soon determined from analyzing the fireball expansion phenomenon of the detonation, that two -millisecond range peaks of light were separated by a period of minimum intensity lasting from a fraction of a second to a few seconds, that corresponded to an atmospheric shockwave break away from the expanding front of the fireball. It took for the shockwave front to transition from opaque to transparent was directly correlated to the weapons yield.

FIRST METERS
In 1948, during the third series of American nuclear testing, called Operation Sandstone, the first purpose-built proof-of-concept device for specifically detecting nuclear detonations would be tested. While this device was simple and devised on site, it provided a measurement of light intensity over time using a photocell coupled to a cheap oscilloscope. During a meeting with the project group, Reines would coin the term Bhangmeter for the device.

A calibration curve was developed from the average of these measurement devices and the testing weapon’s yield. From this data, the bhangmeter was able to optically determine a nuclear weapon’s yield to within 15%. Though blue light was used to produce this initial calibration data due to its higher contrast within the detonation, it was soon discovered that changing the observed spectrum of visible light also modified the amount of time it took for the light intently to start its initial drop off. During further tests it was also realized that the altitude of a bomb’s detonation could also be determined from analyzing the time-to-minimum light intensity as the duration of the initial fireball expansion was largely influenced by the effect the ground had on its shape.

ADOPTION
These aviation compatible, AC powered systems were specifically designed and deployed to monitor the Soviet test of Tsar Bomba, the largest nuclear weapon ever detonated. Around the same time, the first large scale nuclear detonation network would be deployed by the US and the UK. Linked by Western Union’s telegraph and telephone lines, the system was designed to report the confirmation of a nuclear double-flash before the sensors were destroyed by the detonation. The Bomb Alarm Display System was in use from 1961 to 1967 and while it offered adequate surveillance for the onset of nuclear war, the emergence of the Partial Nuclear Test Ban Treaty in 1963 now warranted the ability to monitor atmospheric nuclear testing at the global level.

SATELITES
The solution to the challenge of this new scope of nuclear detection came with Project Vela, a group of satellites developed specifically for monitoring test ban compliance. They could determine the location of a nuclear explosion to within about 3,000 miles, exceeding the positional and yield accuracy of the original system.

GPS
As the Vela program was being phased out in the mid 1980s, the task of specifically detecting nuclear detonations would become a part of the new global position system. Known as the GPS Nuclear Detonation Detection System, this capability took advantage of the extensive coverage of earth's surface offered by the constellation.

These bursts propagate from a nuclear detonation in a spherical shell and by measuring their intensity against the accurate timing information of 4 or more satellites of the GPS constellation, these time differences of arrival can be used to calculate the position of the x-ray burst source. Each of the GPS satellites are equipped with a specialized antenna and support system to both detect and measure these EMP incidences. The Bhangmeters that complement the other sensors on the GPS constellation are the most sophisticated satellite based system to date.

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