U.K.W.M.O. Group and Sector Headquarters

This topic gives a closer look at the functions of the Group and Sector in predicting and monitoring the fallout situation. It should be read in conjunction with the overview of the Royal Observer Corps in the Cold War period.

Locations of Group Headquarters Bunkers

Map of UKWMO Groups

Map of UKWMO Group Headquarters

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.

York UKWMO Group HQ

ROC Museum York (former UKWMO Group Headquarters)

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

The Function of ROC Group HQ

Each Group HQ had twenty to thirty ROC monitoring posts within its control. Each collecting data from their Ground Zero Indicator (GZI) camera, Bomb Power Indicator and Radiac Survey Meter and forwarding it to Group for processing. Bomb detonation locations and estimated yield were plotted. 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 to them.

Information Gathered by Monitoring Posts

Whenever a post detected a bomb pressure reading in excess of 2 kPa, the BPI reading would be immediately passed to group as a 'TOCSIN' message. The definition of 'TOCSIN' 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.

After a one minute delay after the last (if more than one) BPI reading the GZI cassette papers would be retrieved and the details of the spot(s) passed to control as a 'NUCLEAR BURST' message. The first sign of fallout would be passed as a 'FIRST FALLOUT' message. After this, radiation dose rates would be collected at 10 minute intervals.

The Group operating procedures detailed how these various messages are processed onto forms, the forms used to create plotting information locally and shared with other UKWMO Group and Sector controls. The local data combined with that received from other parts of the UKWMO would be shared with user services such as Local Authority Emergency Centres, Regional Government Headquarters, Armed Forces Headquarters and Nuclear Reporting Cells.

Triangulation of the Bomb Ground Zero

The point under a nuclear bomb detonation is known as its Ground Zero (GZ). A term that has become well used in public since the World trade centre attack of September the Eleventh 2001. ROC Posts send in details of bomb detonations recorded by their GZI camera as a bearing to the nuclear fireball. Group would use a minimum of two of these bearings to triangulate the ground-zero of the bomb.

Triangulation of Ground Zero

Triangulation of Ground Zero

Triangulation requires a map of the area with the ROC posts marked on it. In this example we use 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 ground zero.

Only two posts needed to have witnessed the fireball for a triangulation to determine the ground zero, but a third reading of 107° from 50 Post confirms the target as Hull Docks. The more posts that observe the fireball the greater the accuracy of the ground zero location. If a multi warhead weapon was used then more than one fireball may recorded on the GZI camera. Plotting a line for each fireball from three or more posts will 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 is not affected by Electro Magnetic Pulse, does not require batteries and most certainly less prone to operator error.

Determining Bomb Burst Height

Nuclear weapons would have been detonated at different heights to suit the type of target. Detonation height is very important to the UKWMO 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 get sucked up into the mushroom cloud there would be a lot of radioactive fallout.

Once the ROC Group control had determined the ground zero by triangulation, the height of the detonation could be calculated using the GZI Camera's elevation data and spot size. 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.

Bomb PowerThreshold
20 Kilotons600
½ Megaton2,200
1 Megatons2,900
5 Megatons5,400
10 Megatons7,200

This table shows the height threshold for different nuclear bomb yields expressed in tons of TNT. Below this threshold height the fireball touches the ground. Anything above this height the weapon is regarded as air burst.

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 bomb powers are quite trivial, apparently the 2004 Boxing Day Tsunami earthquake released the equivalent 9,500,000 Megaton of TNT in energy.


Awdrey Display Unit

AWDREY Nuclear Burst Detector Display Unit

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. Similar in design to a 'Bhangmeter' it uses an observed flash signature associated with nuclear weapons detonated below 30 km. This signature consists of a short and intense flash lasting around 1 millisecond, followed by a second much more prolonged and less intense emission of light taking a fraction of a second to several seconds to build up. This signature, with a double intensity maximum, is characteristic of atmospheric nuclear explosions and is the result of the Earth atmosphere becoming opaque to visible light and transparent again as the explosion's shock wave travels through it.

Awdrey Logic

Diagram of the logic used by AWDREY unit Click to Zoom

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 characteristic double flash 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.

Readings from the AWDREY would be entered on an 'Awdrey Data' form 'AA' and verbally announced in the group control, using the format 'Codewords' + Group Name + 6-figure time as indicated by AWDREY. For example 'TOCSIN BANG MAIDSTONE EIGHTEEN TWENTY FOUR OH EIGHT'

Nuclear Bomb Size


Bomb Size Calculator Click to Zoom

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 (BPI) can now be used to estimate the yield of the bomb. Before the introduction of AWDREY this was the only means of estimating the bomb's 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. These instruments are described on the 'ROC Observer Posts' page on the top navigation tabs.

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

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 (YOR) Group would be 'YORA', second 'YORB'. The first in Lincoln (LIN) Group would be 'LINA', second 'LINB'. The time of detonation, location, burst type (Ground/Air) and yield are recorded for future reference.

Plotting Fallout

In Group and Sector controls, an illuminated plastic panel overlaid with a map was used to display the predicted and confirmed track of any radioactive fallout. A soft wax pencil, which could be wiped off, is used to draw on the rear of the panel. Once the yield of a ground burst weapon is determined, the known wind speed and direction are used to plot a predicted fallout path. The measured radiation readings supplied by the ROC posts are then used to confirm and refine the predicted path.

Display Screens

ROC Group HQ Display Screens

Here are the display panels at York Group HQ. The picture below from the UKWMO booklet shows the display panel being marked up from the reverse side with a plot of the fallout path. Red mushrooms indicate ground bursts that produce fallout and Green mushrooms for air bursts producing little or none.

Plotting on Screen

Plotting Fallout on Map of UK

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.

Feedback - extract of an email from a former member of a UKWMO Warning Team

We (the Warning team members) were indeed a mixed bunch of scientists, engineers, ex military and lawyers, whose rather awesome job as volunteers (especially looking back on it) was to take the data generated by the ROC posts and staff around us, and turn it into real intelligence to predict what was going on 'out there' in terms of threat of fallout so that we could warn the public quickly and as accurately as possible. Our tool were weather forecasts, the bomb size, direction and type data from AWDREY, and predicted fallout patterns. As civilians, the warning team did not have to work under the same strictures as our uniformed colleagues, and we could see the farcical nature of some of our work. So the warning teams did what they could to try and practice warning the public – the whole point of the ROC network, whilst trying not to think of their families whom, one supposed, were being left to fend for themselves 'up top'. The debate on what would actually happen in the event never really developed, it was far too contentious.

Fallout Prediction - Warning Districts

Horsham Warning Districts

Horsham Warning Districts Map

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' and marked with a Red mushroom 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

Operating the Tote Boards

Operating the Tote Boards Click to Zoom

The private circuits from the monitoring post cluster's TeleTalk are terminated on the Post Display Plotter (PDP) desks who operate the tote board displays. Every even five minutes the PDP requests all the posts to pass their radiation readings. The tote boards are double sided so the PDP 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 set of readings.

Control Room view of Tote

Control Room view of Tote

Here is the view of the tote boards as seen from the control room balcony. The time for the reading is written along the top of a tote board. Each board displays 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. Each cluster is shown as three columns, the first is a code, the second the post number and the last the data. The code 'FF' with the time as data, indicates the time of arrival of the first fallout. The code 'CC' with a number as data shows the current radiation level at that post. Decimal points are shows as '3x6' for '3.6' otherwise the number is the reading in centi-greys. Other codes are 'PP' for post out of contact, replaced with 'QQ' for the first reading when it is back in contact. 'US' indicates the lack of radiation reading due to a faulty field survey meter. The data is visible to everyone in the control room to use for their task.

Sharing of Data

The bomb and fallout data was shared amongst adjacent UKWMO Groups and Sectors in order to build up a national picture at each location. The UKWMO's function is to warn the general public but also very importantly, to share their data with other bodies. Fallout and damage predictions are shared with local and regional government to enable them to plan their relief work. Data was also shared with the armed services so they would know if it were safe to use their facilities or move around the country.

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 the Air Defence Operations Centre at Strike Command be unable to do so.

As we know from the accident at Chernobyl, nuclear fallout does not respect national boundaries so 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 Met Office be disabled there were direct links to selected upper air observation stations too. Combined with meteorological data from ROC posts a basic 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

Map of Comms Links

ROC Group HQ Comms Links

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. In addition to the links to adjacent groups there was a link to the Regional Government Headquarters (RGHQ) and in the 1980’s outgoing links were added to County Council Controls.

A Carrier Receiver was provided at Group so it could receive the national attack warning would be aware of an imminent attack. This was connected to the CCP serving the area where the Group bunker was physically located.

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 was a network of dedicated private circuits (leased lines) and radio links to the adjacent Group HQ and RGHQ. This network carried both speech and teleprinter signals.

Warning Switchboard

Warning Switchboard at Group HQ

A special switchboard 'Warning Switchboard' was at the centre of another network of private circuits radiating out to the black telephone on the Carrier Control Point (CCP) located in major police stations in the group's area. Warning messages could be sent to the CCP requesting them to broadcast fallout 'BLACK' or all clear 'WHITE' messages over the carrier system to the various warning districts.

As a contingency against the national attack warning system failing, Group operating procedures included passing the 'RED' message to CCP's if any bomb detonations anywhere in the country had been detected before a Red warning had been received from the Group's own carrier receiver.

Radio Room at Closure in 1991

radio room

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.

In this photograph of a Group Control's radio room, three racks can be seen. On the left, the message switch with the No Smoking sign stuck on. In the middle a smaller rack with two shelves, this is the RN4 radio to Monitoring Posts. On the right, the cream and orange multiplex rack for RN2 for speech circuits to other ROC Groups and RGHQ.

General Coverage Receiver

Eddystone RX

To enable group controls to listen to radio broadcasts from the UK and abroad, they were equipped with a general coverage receiver. This receiver tuned AM Long and Medium Wave, FM 88-108 MHz and AM short wave 1.5-22 MHz. But it didn't provide any SSB or CW reception.

Jim Went kindly contributed this photo taken on the last night the Shrewsbury control was manned, noting "The photograph of the logistics room (formerly known as the officers' room) with the Eddystone receiver on the table (I think it's an EB35 Mk.3) was taken by a fellow member of my crew who died over 20 years ago. I'm sorry it doesn't show the radio in detail."

Communications are so vital to the operation of the UKWMO, they are described in their own topic if you wish to read more. Top of page, Menu Tab: Communication / Networks Era 1 and Era 2. Or use the quick links underneath this topic.