With the start of the cold war and the increasing threat of nuclear attack in the 1950's, the Royal Observer Corps (ROC) was given the added responsibility of reporting nuclear bursts and monitoring fall-out. Because of this, the construction of underground monitoring posts throughout Great Britain & Northern Ireland was carried out to give fall-out protection, usually at the same location as the aircraft monitoring post which usually took the form of a pre-cast concrete "Orlit" or brick room often raised a few feet off the ground to aid visibility. The ROC posts consisted of a 15ft entrance shaft which gave access to two rooms, one containing a chemical toilet and the larger 15ft X 7ft 6ins monitoring room which was furnished with canvas chairs, folding table, shelf, cupboard and a pair of metal-framed bunk beds. This was the standard furniture for all underground posts although some were customized to suit individual needs with more cupboards and tables, Dexion racking, comfy chairs, bedside curtains. A ventilation shaft with two louvered vents was located alongside the entrance shaft with a second air shaft at the other end of the room. Lighting was provided by a 12 volt battery located behind the monitoring room door. With a few exceptions, where the post was constructed within existing buildings, this layout remained the same at all locations.
The Fixed Survey Meter (FSM) was introduced in 1958. The indicator unit was mounted on the table and connected to the surface by cable running through a pipe on the end of which was fitted the ionization chamber. The Bomb Power Indicator (BPI) was introduced at the same time, consisting of a baffle plate mounted on a steel pipe at the surface. The Ground Zero Indicator which consisted basically of a pinhole camera with four holes facing the cardinal compass points. A piece of photographic paper was placed in-front of each hole and in the event of a nuclear burst, the image of the fireball would be projected through one or more of the pin holes. From these, the bearings and elevation of the burst could be calculated. The GZI was mounted on a convex metal plate, usually located on top of the vent shaft next to the entrance but it necessitated somebody emerging into the open air to retrieve the sheets of photographic paper. All posts were also issued with hand operated ‘Secomak’ or ‘Carter’ sirens and maroons. Communications between posts and Group HQ's was by GPO telephone lines A line of telegraph poles half way across a field are often still visible and give the site away and in 1964 the conventional headset and microphone was replaced by an 8 inch square box containing microphone and amplifier. In case of emergencies, all Group HQ's and one post in each cluster were equipped with VHF radios and radio masts, these posts being known as master posts. For more information, click here