Royal Observer Corps - Observer Post

Physical Appearance of a ROC Post

All these years since the ROC stand down makes it difficult to find photographs of the surface features of a nuclear monitoring post in a state of readiness. Malcolm Sayers has kindly allowed me to use these photographs of his model to illustrate the salient points.
Surface Features of a ROC Nuclear Monitoring Post
ROC Post Model
This is a typical view of the exterior of a ROC monitoring post ready for action. A locked entrance hatch leads to a 20 ft vertical ladder down to an underground chamber 7ft x 16ft x 7ft high. The main room houses the observers, their instruments and welfare facilities. A chemical toilet is located in a small room at the bottom of the ladder. Conditions were very primitive, there was no running water, gas or mains electric.
The only communications with the outside world is the loud speaker intercom device, connected to UKWMO Group HQ and the other posts within the cluster. There was no other means of two way communication as there wasn't a telephone provided. The post had a Warning Receiver to receive instructions when to sound the hand operated siren or maroon. The designated master post in each cluster had a radio for a backup to the landline intercom device.
When not in a state of readiness the radiac tube cover and Bomb Power Indicator (BPI) baffle were removed and replaced by blanking plates. The Ground Zero Indicator (GZI) camera was unbolted and stored inside the post. The pump-up aerial mast was deflated and stored in the ladder shaft.
Originally the only air supply was via two louvered vents, one on each end of the chamber, which meant it could be quite damp. Latterly some posts were fitted with power operated fans. Some posts leaked ground water which had to be pumped out before it could be manned. One has to admire the dedication of people prepared to lock themselves away in these cold and damp conditions for 2 to 3 weeks if there had been a nuclear attack on the U.K.
Looking Down the Hatch
Looking down ladder
Looking down the hatch into the post from the step, we can see the hatch counter weight. At the top left is the ventilation grill and control plate. The grating at the bottom of the ladder covers the water sump. Next to this is a hand operated pump and a vertical pipe to expel the water. The toilet door is on the right and the observation room door at the bottom of the photograph.
Nuclear Monitoring Room
Monitoring Room
This excellent model of the underground parts of the ROC Post as they were at stand down in 1991. It shows the layout of a typical observation room at a master post in an unbelievable level of detail. It is very sparse and functional with just a handful of instruments and communication devices. A 6" striplight or a bulb provide the only lighting.
We can see the radiac survey meter (Geiger Counter) and bomb pressure indicator. These instruments are described in detail further down this page. The communications equipment seen here Teletalk intercom, warning receiver and a radio within its wall box. These described on a separate page accessed from the website menu.


Monitoring Nuclear Weapon Detonations

A Ground Zero Indicator (GZI) was employed to record the position of a nuclear detonation. This consisted of 4 pinhole cameras in one enclosure. One of the pinholes can be seen in this photograph. The camera is bolted onto a bracket on the outside of the post on the entrance hatch.
Ground Zero Indicator External and Internal Views
GZI external
Facing each pinhole is a cassette containing photographic paper. This is marked with a matrix of lines giving the bearing and elevation. Light from the fireball passes through the pinhole and marks its position on the photographic paper. Following a detonation the observer exchanges the cassettes and returns to the monitoring room to examine the markings.
Bomb Power Indicator Pressure Wave Baffle
BPI Baffle
To detect and measure the size of the shock wave from a nuclear detonation, a baffle plate above ground collects the pressure wave and feeds it into a pipe down into the post below. When the post was not manned the baffle is replaced by a sealing plug. At the other end of the pipe is a wall mounted pressure detector known as a Bomb Power Indicator (BPI). It's needle displays the highest peak pressure of the blast wave and remains there until reset by the observer.
Wall Mounted Bomb Power Indicator
Bomb Power Indicator
Immediately a pressure wave is recorded, the observer waits 10 seconds to ensure this is the maximum reading and resets the instrument. A call is made to the Post Display Plotter (PDP) in Group HQ using the Tele-Talk unit or Radio. The spoken message in the format TOCSIN, followed by the Group name, Post Number and time. The PDP responds and the post gives the pressure reading in kilo-pascals.
Plotter replies:THANK YOU - OUT
When one minute has elapsed from the last BPI reading of 2.0 kPa or more, the number three observer goes outside to replace the GZI cassettes holding the sensitised paper and bring the exposed ones down for examination.
For each of the spots recorded by the GZI the readings are passed to the PDP. The format being, time of retrieving the cassette, bearing direction, elevation and spot size in degrees of the fireball and if it is touching or clear of the ground;
Plotter replies:HORSHAM TEN POST - OVER
Plotter replies:THANK YOU - OUT
Group HQ staff use triangulation to determine the exact point of detonation once they have bearing directions from two or more posts. Having determined the position of the detonation, Group HQ can estimate the yield (size) of bomb from the pressure wave readings received from the posts. Knowing the distance from each reporting post to the detonation, the height of the centre of the fireball can be determined from the elevation readings taken by the GZI. Elevation is important as a detonation at ground level (ground burst) will create fallout. Whereas a similar bomb exploding so its fireball did not touch the ground (air burst) would create little fallout. It was assumed the enemy would use a combination of air and ground burst weapons.

Monitoring Fallout Radiation

Fixed Survey Meter Detector Head, Ceiling Flange and Protective Dome
Radiation measurement
From the safety of the underground room the observers can monitor fallout radiation levels using a Radiac Fixed Survey Meter (A Geiger counter) Its probe head is mounted on a rod which is pushed up a tube extending from a flange in the ceiling up to the surface and into the protective grey tube. These parts may be seen in the photo above.
If the radiation level approached the maximum range of the instrument, the head would be withdrawn down the tube and refixed at a point where it indicates a tenth of the fully extended level. This would be repeated if the radiation approached the maximum range again, so now the indicated reading would be one hundredth of the surface reading.
The fixed survey meter PDRM82, fits into a slot in the monitoring room desk so it can be easily read with the cable to the probe head exiting from under the table.
After the Attack Warning the observers make regular readings of the radiation levels. When the radiation level rises indicating fallout, the post initiates a call to the Post Display Plotter (PDP). If the reading rises to 0.3 and a Fallout Warning Black has not been issued via the carrier receiver, the post will set off the maroons to warn the local population of the fallout.
Plotter replies:HORSHAM TEN POST - OVER
Plotter replies:THANK YOU - OUT
Once the time cycle has started the PDP will call every post in the cluster on a five minutes cycle enabling Group HQ to plot the progress and path of the fallout plume.

Local Weather Reports

Selected posts known as ROCMET posts, had rudimentary equipment to allow them to make simple meteorological reports. These are passed to Group HQ for relaying on to Sector Headquarters. If the United Kingdom Meteorological Office (Met Office) was knocked out and couldn't produce weather forecasts, UKWMO Sectors could generate their own forecasts to enable the path of fallout to be calculated.

Survey Meter Gallery

Radiation Meter Top View
Side Views
Slotted into monitoring room desk

ROC Post Communications Overview

The primary means of communications was the Loud Speaking device known as a 'TeleTalk' connected to a landline private circuit to the Group Headquarters. The same private circuit also brought the carrier signal from the exchange to the Warning Broadcast Receiver, a listen only device. The post designated as 'Master Post' in each cluster also had a VHF radio as an alternative way to contact Group Headquarters if the landline failed.
In 1981 a Home Defence Review had recognised the need to improve and update the communications and equipment in use in the ROC. During the life of the posts there were two distinct equipment eras, these alternative devices are shown below, and full descriptions given in the Post Communications page or the WB400 / WB1400 pages, found in the website menu.
Contrary to some belief there wasn't a telephone in the post. Neither did it have way to speak to the local police station. This later myth may be as a result of the police being the source of the warning broadcasts.
TeleTalk Device the Primary Means of Communication
When switched on, the TeleTalk would be in listen mode on the private circuit. If the 'Post Display Plotter' (PDP) in Group HQ spoke the message would be heard at all the posts in the cluster. To speak back, a switch was depressed, and the observer spoke towards the loud speaker grill, the other posts in the cluster would hear the reply to Group. To initiate a message to Group, the observer would press the 'CALL' button to attract the PDP's attention. As the arrangement is very similar to an intercom, the posts within one cluster could talk amongst themselves without involving Group.
Radio Backup at the Master Post
Radio Equipment
If the 'TeleTalk' landline link back to Group was severed but the local landlines to the other posts in the cluster were still working, the master post would collect the readings and messages from the cluster's posts and radio them through to the PDP in Group HQ. Not all Groups had first generation radios, Belfast No.31 didn't have their's until the second generation came along.
Carrier Receivers
Carrier Receiver
In common with all Warning Points, the ROC Nuclear Monitoring Post had a Carrier Receiver allowing them to receive the National Attack Warning, Fallout Warning and All Clear allowing them to warn the local population.

Public Warning Functions

ROC Posts have both a observing and monitoring role and to act as warning points for the local population. If there is not a ROC post near a village, this role as warning point may be carried out by the local police or a trusted individual such as postmaster or pub landlord.
In common with other warning points, the Post has a Carrier Receiver to alert the observers of an Attack Warning that they would pass on to the public by sounding their hand operated siren. This siren is normally kept packed in a crate, but when the post is manned for a war situation, it will be unpacked and placed outside the post in readiness.
Hand Operated Siren
Hand Operated Siren
If the post gets a fallout warning message from Group HQ either directly via the TeleTalk or relayed via the Carrier Control Point to the Carrier Receiver or should the local external radiation reach a predetermined level of 0.3, the Post will sound the fallout warning by means of a pyrotechnic maroon.
The maroon is a large firework that shoots three shells high into the air which explode in a bang-bang-bang sequence. Due to the danger of handling live maroons containing explosives a training kit was provided for use during exercises. Live maroons would be only issued during the transition to war.
The training kit consists of a training maroon using bulbs to indicate the firing of each tube and a dummy maroon made to look as closely as possible like the real thing with the same packaging, but marked 'Dummy'. The kit also includes the same instruction card as the genuine maroon. The instruction card is reproduced in the gallery.
The same controller box is used for both the genuine and training maroon. The maroon controller connects via six-way plug and long black wire, so the user would be at a safe distance when this powerful firework ignites. The controller contains a dry battery, but an adapter (with the red wiring) allows it to be operated from a car battery or the post lighting / radio battery.

ROC Post Communications Next : Please Use Menu

or Click Here => ROC Post Communications - An in depth description of the equipment.


Maroon Training Unit
Instruction Card