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 Entrance and GZI

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.

Looking Down the Hatch

Looking down the ROC Post ladder

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 to sound the hand operated siren. 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 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.

Monitoring Room

ROC Post Monitoring Room

This excellent model of the underground parts of the ROC Post, shows the layout of a typical observation room at a master post in an unbelievable level of detail.

We can see the teletalk intercom, warning carrier receiver and radio within its wall box. 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 is described on a separate page Communications / ROC Post ERA 2 accessed from the top navigation bar.

ROC Nuclear Monitoring Post Clusters

Post Numbers in the York Group at Closure

York ROC Posts drawn on map

Posts are grouped together to form a cluster of typically in three but maybe from two to five posts. Each post is uniquely numbered within the Group area, the first post is always designated as 10. One post in the cluster is selected as the master post, identified by its post number ending with a ZERO or FIVE.

The master posts numbered 10, 15, 20, 25 etc with the remaining posts within the cluster numbered sequentially. This results in a numbering scheme like this in the York Group. In the Group control, one 'Post Display Plotter' handles two clusters during landline operation.

The use of clusters helped minimise the cost of land lines connecting each post to its group headquarters by combining the long distance part of the individual circuits into one.

Cluster Maps for all Groups

View Cluster Maps

Map of UKWMO Groups


I have prepared a map showing all the ROC Posts on a U.K. base map which shows the towns and roads, just like the York one in the previous topic. Individual group maps may be viewed on screen or they can be downloaded for printing. A variety of larger area maps may be downloaded too.

View Group Cluster Maps on Screen ⇒

Click on the map to the right to view the ROC Nuclear Monitoring Posts within each UKWMO Group Headquarters Area.

Download or Print Maps ⇓

Click the button to switch to the map copyright acknowledgement and download page.

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.

Cluster TeleTalk Connection

Private Circuits connecting Cluster TeleTalks

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.

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.

If the 'TeleTalk' landline link back to Group was severed but the local landlines to the other posts in the cluster were working, the master post would collect the readings and messages from the cluster's posts and radio them through to the PDP in Group.

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 are discussed in separate topics, linked from the further reading section below. During the change period, some posts were re-clustered to improve the security of their 'TeleTalk' links.


Ground Zero Indicator

Ground Zero Indicator GZI external view

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.

GZI internal View

Ground Zero Indicator GZI internal View

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.

BPI Baffle

Bomb Power Indicator BPI pressure wave 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 seal. 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 hand.

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 PDP 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

Bomb Power Indicator

Bomb Power Indicator Display

When one minute has elapsed from the last BPI reading of 02 kPa or more, the number three observer goes outside to change the GZI cassettes holding the sensitised paper and bring them down for examination.

For each of the spots recorded by the GZI the readings are passed to the PDP at ROC Group HQ using the Tele-Talk unit or Radio. 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 directionals from two or more posts. Having determined the position of the detonation, Group HQ would 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.

Radiac Meter Head

Fixed Survey Meter Detector Head

From the safety of the underground room the observer would monitor radiation levels using a radiac Fixed Survey Meter (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.

Regular readings of the radiation level from any subsequent fallout are transmitted to PDP in Group HQ enabling them to plot the progress and path of the fallout plume. 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 table so it can be easily read, as shown in image 4 in the gallery. The cable to the probe head exits from under the table.

Selected 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 Meteorological Office (Met Office) was knocked out and couldn't produce weather forecasts, Sectors could generate their own forecasts to enable the path of fallout to be calculated.

Warning Function

ROC Posts had both a warning and monitoring role. The Carrier Receiver would alert the observers of an Attack Warning, they would pass on this warning the public by sounding their 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, 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.