During the Cold War, the Royal Observer Corp (ROC) was part of the United Kingdom Warning and Monitoring Organisation (UKWMO). The role of the ROC within UKWMO was to gather data on nuclear explosions nationwide and then track radiation fallout levels.
At its height, the ROC network had a personnel of around 25,000 volunteers but numbers were radically reduced in a reorganisation in 1968. The volunteers maintained a country-wide network of 1500 three-man monitoring posts and 31 Group Controls, one of which was ROC No. 20 Group York (York Cold War Bunker). Each Group Control coordinated data from around 45 monitoring posts.
ROC volunteers maintained the network's systems and structures during peace time and would gather critical data in time of war. After analysis, the data would be circulated via the ROC to Government, the military and civil defence bodies.
Ground Zero Indicator
At each three-man post and Group Control a Ground Zero Indicator, mounted to the bunker roof, would record the bearing and altitude of explosions. In the event of an explosion, volunteers would be required to leave the bunker every twelve hours to replace cassettes in the Ground Zero Indicator. Personal dosimeters would register their accumulated radiation exposure.
Image: The Ground Zero Indictor, also known as a shadowgraph, works like a pin hole camera. Four holes in the outer drum aligned to compass points. Inside the drum, four gridded cassettes hold light sensitive paper. The light emitted from an explosion (range up to around 7 miles) appears as a dark dot on the paper, marking out both its bearing and relation to the horizon. ROC.119
Image: A personal dosimeter. ROC.222.2
By triangulating the results from at least three Ground Zero Indicators at three bunker locations it was possible to provide the coordinates of the bomb and whether it was ground or air burst. Bomb Power Indicators mounted on each bunker recorded the overpressure of the explosion and, by again calculating a series of readings at different posts, the magnitude of the explosion could be calculated
Image: The triangulation calculator correlated data on elevation, range and pressure to give a read out of critical power in megatons and kilotons. Different scales were used for ground and air bursts: ground bursts reduced the destructive power of a bomb as energy was transferred into the ground. However, ground bursts led to more radiative fallout with contaminated particles sucked up into the atmosphere. The destructive power of air bursts was greater but less material became radioactive. That distinction would be critical in measuring and predicting the level and passage of fallout in the ensuing days and weeks. ROC.750
From the late 1960s onwards, AWDREY units (from Automatic Weapon Detection Recognition, Estimation and Yield) were installed at 12 ROC Group Controls, including at York. The technology was designed, built and maintained by the Atomic Weapons Establishment at Aldermaston. For ROC volunteers, AWDREY was always seen as a back-up system to their analogue observation. Ultimately, AWDREY was the precursor to further technological developments which rendered the ROC network obsolete.
Image: A detector head for an AWDREY unit which was mounted on the roof of an ROC Group Control. The device registered the double flash of a detonating nuclear bomb and provided 360 degree coverage at a range of 70 to 100 miles (weather dependent). The signal was fed to the main AWDREY analysis unit within the Control where the gaps in pulses and their strengths was used to calculate the power of the explosion. ROC.1017
The UKWMO was responsible for giving nuclear warnings by a system code-named HANDAL. As part of the system, around 19,000 WB1401 carrier receiver and speaker units were placed around the country at locations including ROC monitoring posts and Group Controls, police stations, hospitals and coastguard stations. This WB1401 unit was located at ROC No 38 Colchester.
Following a nuclear attack, the ROC system would plot radiation fallout levels and provide basic meteorological information. Each post was equipped with a Fixed Survey Meter to record gamma radiation. Information would be communicated across the BT landline system using equipment such as standard commercial headsets, or, where necessary, via VHF radio.
Images: A Fixed Survey Meter for monitoring radiation. The FSM recorded external gamma radiation on an LCD display. The FSM connected to a probe unit which was mounted to the roof of the bunker and protected by a heavy plastic ionising cone. ROC.117.3
Images: Stetomike headset. ROC.319
ROC No. 20 Group York
Due to factors including changes in technology and the international situation, the Royal Observer Corps' civilian volunteers were stood down in 1995. ROC No. 20 Group York was decommissioned in 1991. These photographs were taken when the site closed. The bunker gained Scheduled Monument status and was opened by English Heritage as a visitor attraction, York Cold War Bunker, in 1991.