The Function of 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.

The Function of ROC Group HQ

Each Group HQ had thirty to fourty ROC monitoring posts within its control. Each collecting data from their Ground Zero Indicator (GZI) camera, Bomb Power Indicator (BPI) and radiac Fixed Survey Meter (FSM) 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's BPI 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 following the last BPI reading, the GZI cassette papers are 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 from posts on a regular cycle.
The Group operating procedures detailed how these various messages are processed onto forms, the forms are used locally to create plotting information and shared via the telegraph network, with other UKWMO Group and Sector controls. The local data combined with that received from other groups, creates a national picture, this is 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 in degrees clockwise from North. Group would use a minimum of two of these bearings to triangulate the ground-zero of the bomb.
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. As the ground material and debris get sucked up into the mushroom cloud it creates 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.
Air Burst Threshold Height
Bomb PowerThreshold feet
20 Kilotons600
½ Megaton2,200
1 Megaton2,900
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 of 9,500,000 Megaton of TNT in energy.


AWDREY Nuclear Burst Detector Display Unit
AWDREY the acronym for Atomic Weapon Detection Recognition and Estimation of Yield was designed and built by the U.K. 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 height. 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
A sensor head consisting of two banks of 5 solar cells is mounted on the roof of the bunker and connected to 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 and direction. The intensity of the flash and electromagnetic pulse allows 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.
Alerts from the AWDREY are entered on an 'Awdrey Data' form 'AA' and distributed nationally on the telegraph network. Also verbally announced in the Group control, using the format 'TOCSIN BANG' + Group Name + 6-figure time as indicated by AWDREY. For example 'TOCSIN BANG MAIDSTONE EIGHTEEN TWENTY FOUR OH EIGHT'

Nuclear Bomb Size

The peak overpressure reading measured by the Bomb Power Indicator (BPI) at monitoring posts in the vicinity of the nuclear burst are sent to Group. Once the location of Ground Zero has been triangulated, the posts' BPI readings can 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 double sided plastic calculator is 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 be 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 of yield for the different types of detonation.

Bomb Designation

Each bomb is allocated a designation letter 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 and transferred to a form 'BB' then distributed nationally on the telegraph network.

Plotting Fallout

In Group and Sector controls, an illuminated plastic panel overlaid with a map is 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. Air burst weapons are recorded on the panel but no fallout predictions are made.
ROC Group HQ Display Screens
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 Fallout on a Map of UK
Plotting on Screen

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 Map
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' and marked with a Red mushroom in this exercise. The fallout progress is shown for various times in the fallout zone.

The Warning Switchboard Function

A special switchboard, the 'Warning Keyboard', was at the centre of another network of private circuits radiating out to the black telephone on Carrier Control Points (CCP) within the group's area. If fallout is expected in a warning district within the hour, the warning officer calls the CCP responsible for that warning district, requesting them to broadcast a Fallout 'BLACK' message over their carrier system. Later when the fallout has decayed, the All Clear 'WHITE' message is handled similarly.
As a contingency against the national attack warning system failing, group operating procedures included passing the 'RED' message to all their CCP if any bomb detonation had been detected prior to a red warning being received via the Group Control's own carrier receiver.
Warning Keyboard 1st Generation
Warning Switchboard at Group HQ
The first generation switchboard uses a key and lamp arrangement. The Warning Officer throws a key to select the desired CCP within its area, then operates a call key. Incoming calls from the CCP light the lamp associated with the line and the Warning Officer operates the speak key on that line.
Warning Switchboard 2nd Generation
The second generation uses a TX14 Featurephone instead of a special switchboard. Its buttons are programmed with the four digit internal telephone number assigned to each CCP. This number can be dialled from any phone within the building, but only the Warning Officer's Featurephone has them preprogrammed on its buttons.
If a CCP calls the Group, the incoming call rings on this phone, but should it already be engaged on another call, the call transfers to another phone.

Monitored Radiation Levels

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. One PDP handles two clusters of posts. 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.
Both Sides of the Tote Boards
Both Sies of Tote
The lower image 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 shown 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 is shared amongst all 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.

Bomb Calculator Gallery

Bomb Size Calculator - Front
Bomb Size Calculator - Rear

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 socket for the old link to Fiskerton which was the location of the ROC Midland Sector Headquarters at Lincoln.

Group and Sector Communications Overview

The Group and Sector HQs had an extensive communications network of its own. Other topics on this web site explore the communication network in great detail, use the menu to navigate. To summarise:-
General Coverage Receiver
Photo of wireless
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 EB35 Mk.3 receiver on the table 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."
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