Ground Zero Indicator
Encyclopedia
The Ground Zero Indicator, known by the acronym GZI was a specially designed shadowgraph
instrument used by the British
Royal Observer Corps
during the Cold War
to locate the Ground Zero
of any nuclear explosion.
s within a white enamelled metal drum, each 'camera' contained a sheet of photosensitive
paper mounted within a clear plastic cassette on which were printed horizontal and vertical calibration lines delineating compass bearing and elevation above the horizon. The bright flash from a nuclear explosion
would burn a mark on one or two of the papers within the drum. The position of the burn spot enabled the bearing and height of the burst to be estimated. With triangulation between neighbouring posts these readings would give an accurate height and position.
The altitude of the explosion was important because a ground or near ground burst would produce radioactive fallout, whereas an air burst would produce only short distance and short lived initial radiations (but no fallout). Once combined with the peak-overpressure readings from post Bomb Power Indicator
readings the power of the burst in megatons could also be calculated by the Triangulation Team in the group control building, using a hand held plastic calculator device.
on a special above ground mount set in the concrete of the entrance hatch. Concentric lugs on the mount ensured that the drum was mounted correctly even in the dark.
The light sensitive photographic paper was unfixed chemically and had to be protected from daylight by being carried in a protective pouch strapped to the observer's chest. The cassettes were always lodged in a specific order so they could be changed in the pitch black with practice. Differing notched cutouts at the bottom of the cassettes ensured they could not be mounted at the incorrect cardinal point.
Daylight inevitably darkened the unfixed papers so routine changes were made at mid day during winter operations and twice daily, at mid day and sunset, during summer months. The sun burned a distinctive suntrail across the papers when there was no cloud cover.
Sixty seconds after any reading on the Bomb Power Indicator
an observer exited the post and changed the GZI cassettes. The exposed papers were returned underground for assessment. The pre-exposed papers had a graticule grid exposed on the papers that showed degrees of bearing. The fireball from any nuclear burst within range would have burned a mark on the paper. The spot size and bearing would be reported to the group control together with an indication that the spot is touching or clear of the horizon, essential for indicating an air or ground burst.
at Cowley
. Delivered sealed, the time of opening was written on the outside of the sealed envelope.
Observers trained blindfolded so that they could exit the post up the vertical steel ladder and complete a GZI cassette change quickly and even in the pitch dark of a winter's night.
Shadowgraph
Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function...
instrument used by the British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
Royal Observer Corps
Royal Observer Corps
The Royal Observer Corps was a civil defence organisation operating in the United Kingdom between 29 October 1925 and 31 December 1995, when the Corps' civilian volunteers were stood down....
during 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...
to locate the Ground Zero
Ground zero
The term ground zero describes the point on the Earth's surface closest to a detonation...
of any nuclear explosion.
Overview
The GZI consisted of four horizontally mounted cardinal compass point pinhole cameraPinhole camera
A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...
s within a white enamelled metal drum, each 'camera' contained a sheet of photosensitive
Photosensitivity
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...
paper mounted within a clear plastic cassette on which were printed horizontal and vertical calibration lines delineating compass bearing and elevation above the horizon. The bright flash from a nuclear explosion
Nuclear explosion
A nuclear explosion occurs as a result of the rapid release of energy from an intentionally high-speed nuclear reaction. The driving reaction may be nuclear fission, nuclear fusion or a multistage cascading combination of the two, though to date all fusion based weapons have used a fission device...
would burn a mark on one or two of the papers within the drum. The position of the burn spot enabled the bearing and height of the burst to be estimated. With triangulation between neighbouring posts these readings would give an accurate height and position.
The altitude of the explosion was important because a ground or near ground burst would produce radioactive fallout, whereas an air burst would produce only short distance and short lived initial radiations (but no fallout). Once combined with the peak-overpressure readings from post Bomb Power Indicator
Bomb Power Indicator
Bomb Power Indicator known by the acronym BPI was a detection instrument, located at the twenty five British Royal Observer Corps controls and nearly 1,000 ROC underground monitoring posts, across the United Kingdom, during the Cold War that would have detected any nuclear explosions and measured...
readings the power of the burst in megatons could also be calculated by the Triangulation Team in the group control building, using a hand held plastic calculator device.
Operations
Normally stored below ground the drum was mounted at the start of exercises or at Transition To WarTransition To War
Transition to war is a North Atlantic Treaty Organization military term referring to a period of international tension during which government and society move to an open war footing...
on a special above ground mount set in the concrete of the entrance hatch. Concentric lugs on the mount ensured that the drum was mounted correctly even in the dark.
The light sensitive photographic paper was unfixed chemically and had to be protected from daylight by being carried in a protective pouch strapped to the observer's chest. The cassettes were always lodged in a specific order so they could be changed in the pitch black with practice. Differing notched cutouts at the bottom of the cassettes ensured they could not be mounted at the incorrect cardinal point.
Daylight inevitably darkened the unfixed papers so routine changes were made at mid day during winter operations and twice daily, at mid day and sunset, during summer months. The sun burned a distinctive suntrail across the papers when there was no cloud cover.
Sixty seconds after any reading on the Bomb Power Indicator
Bomb Power Indicator
Bomb Power Indicator known by the acronym BPI was a detection instrument, located at the twenty five British Royal Observer Corps controls and nearly 1,000 ROC underground monitoring posts, across the United Kingdom, during the Cold War that would have detected any nuclear explosions and measured...
an observer exited the post and changed the GZI cassettes. The exposed papers were returned underground for assessment. The pre-exposed papers had a graticule grid exposed on the papers that showed degrees of bearing. The fireball from any nuclear burst within range would have burned a mark on the paper. The spot size and bearing would be reported to the group control together with an indication that the spot is touching or clear of the horizon, essential for indicating an air or ground burst.
Codeword
Reports following readings on the GZI were prefixed with the words "Nuclear Burst" and the monitoring post's operational identifier e.g. "Nuclear Burst Shrewsbury 56 post - (time) oh six hundred - Bearing zero seven zero - spotsize zero six - touching".Training
For realistic exercise training photographic papers with simulated burst spots were prepared by the Exercise Division of the United Kingdom Warning and Monitoring OrganisationUnited Kingdom Warning and Monitoring Organisation
The United Kingdom Warning and Monitoring Organisation was a British civilian organisation operating between 1957 and 1992 to provide the authorities with data about nuclear explosions and forecasts of likely fallout profiles across the country in the event of war.The UKWMO was established and...
at Cowley
Cowley, Oxford
Cowley in Oxford, England, is a residential and industrial area that forms a small conurbation within greater Oxford. Cowley's neighbours are central Oxford to the northwest, Rose Hill and Blackbird Leys to the south, New Headington to the north and the villages of Horspath and Garsington across...
. Delivered sealed, the time of opening was written on the outside of the sealed envelope.
Observers trained blindfolded so that they could exit the post up the vertical steel ladder and complete a GZI cassette change quickly and even in the pitch dark of a winter's night.