World War 2, Western Australia, Rottnest Island, Coast Artillery Instrumentation, Depression Position Finder Type 5 Mk1

Depression Position Finder Type 5 Mk2 (Found in the Battery Observation Post, Signal Ridge). The battery observation post was equipped with two Depression Position Finders of different size covering the approaches to Fremantle harbour. Each instrument was so positioned that when their telescopes were correctly laid on a target, the range and bearing from the battery pivot gun (H1) was determined and automatically transmitted electrically to the battery switch in the battery plotting room. Information could be switched either to the fire direction table or to the guns. Fitted to each instrument was a coordinate roamer which travelled on a “block square point” chart recording all movements of the target thus enabling course bearing and speed to be read.