It was applied in Korea by way of two bunkers one, the command centre and the other a forward observation post, the AP.
The AP being, manned on a continuous basis by a small party of surveyors and signallers and in the command bunker other surveyors and signallers. The duties were rostered through each 24 hours, alternating with seven days at an AP and seven in the command centre. An officer and sergeant were in overall command in the command bunker and a signaller continually manned the wireless set.
Four microphones were placed and surveyed in by Surveyors on forward hills and that covered our area of capability. From these Signallers laid combat wire lines from each microphone back to the command bunker and where was a recording machine. The signallers also ran a line from the AP direct to the command bunker.
The AP was located further forward in the infantry lines. This was so that any gun firing from the enemy would be heard there before being registered at each microphone.
When the AP Surveyor heard enemy gun fire, he pressed a button that started the command centre recording machine. He also called out a Shell Report to the signaller who would be sitting down below in the bunker to relay by wireless. The report would normally consist of – “Baker Sound/1 gun/magnetic compass reading/grid square of where it landed (if witnessed/)” plus any other info. eg.“failed to explode/propaganda leaflets” (usually delivered by mortar) etc.