page_title: Operational instruments of the Royal Observer Corps
section_title: Initial detection of nuclear bursts on the UK
hierarchical_section_title: Operational instruments of the Royal Observer Corps / Initial detection of nuclear bursts on the UK
caption_attribution_description: Photo showing the ROC Bomb Power Indicator.
context_page_description: The Royal Observer Corps was a civil defence organisation operating in the United Kingdom between October 1925 and 31 December 1995, when the Corps' civilian volunteers were stood down. Composed mainly of civilian spare-time volunteers, ROC personnel wore a Royal Air Force style uniform and latterly came under the administrative control of RAF Strike Command and the operational control of the Home Office. Civilian volunteers were trained and administered by a small cadre of professional full-time officers under the command of the Commandant Royal Observer Corps; a serving RAF Air Commodore.
  This sub article lists and describes the instruments used by the ROC in their nuclear detection and reporting role during the Cold War period.
context_section_description: Atomic Weapons Detection Recognition and Estimation of Yield known as AWDREY was a desk mounted automatic instrument, located at controls, that detected nuclear explosions and indicated the estimated size in megatons. Operating by measuring the intense flashes emitted by a nuclear explosion, together with a unit known as DIADEM which measured Electromagnetic Pulse (EMP), the instruments were tested daily by wholetime ROC officers and regularly reacted to the EMP from lightning strikes during thunderstorms. AWDREY was designed and built by the Atomic Weapons Establishment at Aldermaston and tested for performance and accuracy on real nuclear explosions at the 1957 Kiritimati (or Christmas Island) nuclear bomb test (after being mounted on board a ship). Reports following a reading on AWDREY were prefixed with the codewords "Tocsin Bang". 
  The Bomb Power Indicator or BPI consisted of a peak overpressure gauge with a dial that would register when the pressure wave from a nuclear explosion passed over the post. When related to the distance of the explosion from the post this pressure would indicate the power of the explosion. Reports following a reading on the BPI were preceded by the codeword "Tocsin". 
  The Ground Zero Indicator, or GZI or shadowgraph, consisted of four horizontally mounted cardinal compass point pinhole cameras within a metal drum, each 'camera' contained a sheet of photosensitive paper on which were printed horizontal and vertical calibration lines. The flash from a nuclear explosion would produce a mark on one or two of the papers within the drum. The position of the mark 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).
caption: Bomb Power Indicator with ‘Kilopascal’ overlay. This was added in the 1970s to cover the original P.S.I  scale which had been superseded.
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