In the early days at Fort Siloso, guns were sighted by means of Tangent Sights. Within a few years, the Depression Range Finder (D.R.F.), recently invented by Capt. Watkin R.A., came into use. One of these was at the 7 Inch RML Gun Emplacement On Mount Siloso. Capt. Watkin also invented a Depression Position Finder (D.P.F) as well. Both of these instruments had a significant effect on ranging the guns and establishing the position of a target vessel. A D.P.F. could very quickly establish the range of a target, and this was passed to the guns which could be set without the need for Tangent Sights.
In 1896, with plans for the 6-Inch QF Battery for Mount Siloso, came plans for a Battery Command Post (B.C.P). The B.C.P. would provide fire control for the gun. It would contain a D.P.F., Chart Room and a Telephone Room — all seen as essential for good fire control.. This was quite a small building on one level.
The telephone reached Singapore in 1879, with a very small exchange being created at Collyer Quay. The military could see the advantage of a telephone system and soon had their own lines.
Over the years, the B.C.P. would be expanded into the Battery Observation Post (B.O.P.) seen today. The B.C.P. was built on the site of a store room, located at the top of the stairs leading up to Mount Siloso.
By 1912, there was the need for improved Battery Command facilities for Fort Siloso and a major rebuild of the Command Post was undertaken. This added a new top floor and a Post for the Direction of the Fort’s Electric Lights (Searchlights). The result is a building which is practically the same to that which is seen today.
When the Fort was modernised during the 1930s, further modifications were made to the B.C.P. These included a new room on the top floor. The B.C.P. was renamed as the Battery Observation Post (B.O.P.). Also renamed was the Electric Light Director Post. This became the Searchlight Director Station (S.L.D.S.). The Lights themselves becoming Coast Artillery Searchlights (C.A.S.L.) or Defence Electric Lights (D.E.L.)
A Position Finder (P.F.) was constructed close to the B.O.P. It was built on top of the old No.2 Q.F. Gun Emplacement. It is possible that this was built to replace the P.F. on Mount Imbiah. On Mount Imbiah was Fire Control West which directed the gun on Mount Imbiah and those at Fort Siloso. The Mount Imbiah gun was decommissioned during the 1930s, and the responsibility for fire control went to the new Faber Fire Command on Mount Faber. This Command directed all the guns on the south and west of Singapore. Mount Imbiah was not abandoned, and the Magazine there became the Reserve Magazine for Blakang Mati.
Glass windows would never have been fitted to the B.O.P. Glass shards scattered by an explosion would have flown around the inside of the causing injury to anyone inside. In action, only shutters being used for range finding would be left open.