Locations of Group Headquarters Bunkers

The map and table show how the twenty five Royal Observer Corp (ROC) Group Headquarter covered the UK. Five of the groups shown in bold letters acted as Sector Headquarters too they are shown as lighter coloured dots on the map.

The building were purpose built two storey designs and usually semi-sunken to give protection against blast and enhanced fallout protection.

Since the stand down of the UKWMO in 1992 the ROC Group bunkers have been sold off. Many sites have been demolished for development. Number 20 Group headquarters at York has been saved by English Heritage, restored and opened up to the public as a Museum. The current state of all the other Group Headquarters bunkers may be found on the Subterranea Britannica web site.

 

The Function of Group HQ

Each Group HQ had twenty to thirty ROC posts within its control. Each collecting data from their Ground Zero (GZ) camera, Bomb Power Indicator and Radiac Survey Meter and forwarding it to Group for processing. Bomb detonation locations and estimated yield were calculated. Using this information and the prevailing weather conditions the path of any fallout would be predicted. Radiation readings taken by posts would be used to monitor the path of fallout to confirm the predictions were correct or make adjustments.

Triangulation of the Bomb Ground Zero

The point under the bomb was known as Ground Zero. A term that has become well used since September the Eleventh 2001. Posts send in details of bomb detonations recorded on their GZ camera as a bearing to the fireball. Then Group could use a minimum of two of these bearing to work out exactly where the bomb had detonated by triangulation.

Triangulation requires a map of the area with the ROC posts marked on it. In this example some of the posts in the York Group. If 22 Post determined the bearing to be 159°, using North as a reference the plotter could draw a line from the post at that angle. The detonation could have occurred anywhere along the line. A second bearing of 124° from 35 Post is plotted on the map and where the lines cross is the location of Ground Zero. This is called triangulation because it creates a triangle between 22 Post, 35 Post and GZ.

Only two posts needed to have witnessed the fireball for a triangulation to determine the GZ, but a third reading of 107° from 50 Post confirms the target as Hull Docks. More points give a more accurate result. If a multi warhead weapon was used then more than one fireball may recorded on the GZ camera. Plotting a line for each fireball from three or more posts would still allow the ground zero for each warhead to be determined.

In the 21st Century this may seem primitive, however it was very quick and effective. The UKWMO was set up more than 10 years before pocket calculators came onto the market. A pencil and ruler was not affected by EMP, did not require batteries and is certain less prone to operator error.

Bomb Burst Height

Once the ground zero had been triangulated the height of the detonation could be calculated using the GZ Camera's elevation data and spot size. Nuclear bombs may have been detonated at different heights to suit the type of target. If the fireball does not touch the ground it is known as an air burst. If it touches the ground it is a ground burst. Detonation height is very important in predicting fallout and damage.

Air bursts create less fallout but a greater circle of damage than ground burst. A ground burst causes a large crater but a smaller circle of damage but as the ground material and debris are sucked up into the mushroom cloud there is fallout.

This table shows the height threshold for different bomb powers. Below the threshold there will be contamination as the fireball touches the ground. Above this threshold the weapon is regarded as air burst and creates a lesser amount of fallout.

The Hiroshima and Nagasaki bombs had a power of about 20 Kilotons, a 10 Megaton bomb is 500 times more powerful than these, but few targets would warrant a bomb so large. On 30th October 1961 the Soviet Union exploded a 50MT bomb, but it never went into service. A full description is given in Wikipedia search for 'Tsar Bomba'.

Compared with nature these sorts of powers are quite trivial. Apparently the 2004 Boxing Day Tsunami earthquake released 9,500,000 MT of energy.

AWDREY

Atomic Weapon Detection Recognition and Estimation of Yield (AWDREY) System was designed and built by the Atomic Weapon Research Establishment at Aldermaston and went into service in 1968 at 13 of the 25 Royal Observer Corp (ROC) Group controls as an aid to determine the size of the bomb.

A sensor head consisting of two banks of 5 solar cells is mounted on the roof of the bunker and connected a main unit in the operations room. The electromagnetic and optical sensors were capable of detecting a nuclear burst over a 100km range. The circuitry was designed to detect the characteristics of a nuclear bomb and record the time, direction and intensity of the flash and electromagnetic pulse allowing the size of the bomb (yield) to be estimated. The chart shows the signal characteristics of a nuclear weapon and the timing of the logic gates. Apparently it incorrectly recorded lightning flashes from thunderstorms as detonations.

Bomb Size

Once the location of Ground Zero has been triangulated the distance from each post is known. The peak overpressure reading measure on the Post's bomb power indicator can now be used to estimate the yield of the bomb. Before the introduction of AWDREY this was the only means of estimating the yield.

This is the plastic calculator used to work out the yield size of the nuclear detonation. The rings are rotated to align the reported overpressure from the BPI, spot size from the GZI, range and elevation, as reported by the monitoring posts.

The user instructions are printed on the inner dial, these can been clearly seen in the enlarged photograph. 'AB' refers to Air Burst and 'GB' to Ground Burst, variations in settings are needed to obtain the correct estimation for the different types of detonation.

Message Format

Chris Clarke kindly sent me this information about the wording of messages to the control room.

A reading on AWDREY would be announced as "TOCSIN BANG - MAI(for Maidstone) - 10:45 (hours) - 1 MEGATON". The codeword for the ROC Posts announcing a Bomb Power Indicator reading would be "TOCSIN 22 POST - (time)OH NINE FORTY FIVE - PRESSURE TWO POINT ONE". The code word for posts making reports following the Ground Zero Indicator reading would be "NUCLEAR BURST 22 POST - (time)OH NINE FORTY FIVE-BEARING ZERO EIGHT FIVE - SPOT SIZE ELEVEN - TOUCHING".

My own research on 'Wiktionary' below is interesting bearing in mind the motto of U.K.W.M.O. is 'Sound an Alarm'

From Old French toquesain (modern tocsin), from Provençal tocasenh, from tocar ‘strike, touch’ + senh ‘bell’.
NOUN:2) A bell used to sound an alarm.

Bomb Designation

Each bomb is allocated a designation letter which will be used nationally to refer to the consequences of the detonation. The first bomb in York would be 'YORA', second 'YORB'. The first in Lincoln Group would be 'LINA'. The time of detonation, location, burst type (Ground/Air) and yield would be recorded for future reference.

Plotting Fallout

Using the radiation readings supplied by the ROC posts, the known wind direction and speed enabled the fallout to be plotted. This would be updated as each set of readings was received. A plastic panel overlaid with a map was used to display the track. A soft wax pencil, which could be wiped off, was used on the rear of the panel.

Here are the display panels at York Group HQ. In the picture below from the UKWMO booklet shows the display panel being marked up from the reverse side where the fallout path was plotted. Red mushrooms indicate ground bursts that produce fallout and Green mushrooms for air bursts that produce little or none.

UKWMO Warning Team

The responsibility for all threat assessments and their communication to the public resided with the UKWMO, the ROC played no part in this other than in a clerical capacity. Within the Group, the UKWMO warning team would attempt to predict the cone of fallout and the arrival time and plot this on a chart. Each Group Headquarters area was subdivided into Warning Districts. If fallout were imminent in a district a 'Fallout Warning BLACK' would be issued via the Carrier Control Point (CCP) for the affected district(s). Eventually the 'Attack Message White' or 'All Clear' would be issued via the CCP when fallout levels had decayed to a safe level.

Fallout Prediction - Warning Districts

This portion of the plotting screen, shows part of the Horsham Group Control with the warning districts marked in Red. The little numbered dots are the ROC monitoring posts and the grid is the OS Map squares. The green trumpet shaped area is the fallout from a ground burst bomb designated 'HORA' in this exercise.

The fallout progress is shown for various times in the fallout zone. Group HQ would alert the Carrier Control Points in the areas affected and ask them to issue Black Warnings for the warning districts likely to expect fallout within the hour.

Monitored Radiation Levels

At Group HQ the private circuits from the post clusters TeleTalk connected to the tote board positions. Every five minutes all the posts would pass their radiation readings to Group. The tote boards are double sided so the operator could update one side of the tote board then rotate it so the information was available for all to see whilst working on the next five minute periods readings.

The actual level of fallout reported by the monitoring posts would be used to confirm and update the predictions created by the warning team.

Here is the view of the tote boards as seen from the control room balcony. Along the top of the tote boards is the time for the reading. Each board display two clusters worth of information arranged in numerical order with the master post at the top of each column. The readings taken at this bunker are shown as Post 99. The data is accessible to anyone who needs it, for example updating the plotted fallout cone on the display panel.

Sharing of Data

The bomb data would be shared amongst adjacent Groups in order to create a national picture. Using a network of telegraph links between Group headquarters bomb data could be exchanged to create a national picture in each Group. Groups also had telegraph and speech links so they could send bomb data to Regional Government Headquarters to update a similar set of display there.

 

ROC Sector Headquarters Function

The Sector Headquarter were originally located at RAF Sector Operations Centres but were moved to be co-located with a Group Headquarter within the Sector. Sector Controls had some additional roles to perform. The Preston Sector had a set of equipment so they could issue the National Attack Warning via the HANDEL system should the UKWMO people at Strike Command be unable to do so.

Nuclear fallout as we know from the accident at Chernobyl does not respect national boundaries. Bombs aimed at targets in the UK could create fallout in Europe and vice versa. The UKWMO had Liaison Officers (LO) who were a small band of volunteers based in adjacent NATO neighbours to UK, two in each, 12 in all. There were LOs in Belgium, Norway, Denmark, Netherlands and France (two centres). Each NATO HQ was plugged into an adjacent Sector Control in UK. Belgium and France linked to Southern Sector in Horsham while the Midland Sector at Lincoln linked with Denmark and Netherlands.

Sectors had communication links from the Meteorological Office to receive weather forecasts. Should the Central Office be disabled there were direct links to selected Upper Air observation Stations too. Combined with meteorological data from ROC posts a weather forecast could be produced at Sector.

Selected Groups or Sectors had links to RAF Radar stations. During a visit to the Neatishead Radar Museum I noticed a link to Fiskerton which was the location of the ROC Midland Sector Headquarters at Lincoln.

 

Group and Sector Communications Overview

Each Group HQ had a Communications Centre for transferring messages to and from other Group HQ's. These are hard copy printed messages. The centre allowed a single message to be broadcast to a number of other locations. There was a link to the Regional Government Headquarters (RGHQ) and in the 1980’s outgoing links were added to County Council Controls.

Each Group HQ bunker had a telephone switchboard with lines into the public telephone network and many extensions in offices and the operations room. In addition to the public telephone lines there were two dedicated private circuits (leased lines) to the adjacent Group HQ. One private circuit was a landline and the other a radio link. These allowed officers at one Group to speak to their counterparts in an adjacent Group. Group also had a landline and radio link to the RGHQ

Each Group HQ area was subdivided into Warning Districts with a Carrier Control Point (CCP) located in the district police station. Group HQ had private speech circuits to the CCP. The CCP would issue fallout warnings and the all clear message to the public in the district via the Carrier Receivers.

A Carrier Receiver was provided at Group so it would be aware of an imminent attack. This was connected to the CCP in whose area the Group bunker was physically located and was only one of many CCPs within the Group’s territory.

1981 saw the first results of a Home Defence Review that had recognised the need to improve and update the communications and equipment in use in the UKWMO. Before then many of the important circuits to Posts and CCPs used Emergency Circuits which were used for normal telephone traffic and had to be switched over in an emergency or for an exercise. These emergency circuits were upgraded to permanently connected private circuits.