Often the ground lines were disrupted by shells or mortar bombs and required repair, and then the order to start the recorder would be transmitted by wireless.
Once the AP surveyor stopped sending, those at the command bunker would remove the film from the machine to enable them to register the difference in sound metres between the respective microphones and plot these readings on a map of the forward enemy held ground. The four readings will give three intermediate values and hopefully supply a reasonable trisection. After adjusting for wind atmospheric pressure and humidity, the adjusted trisection will appear over a position where an artillery piece could fire have fired from. Its co-ordinates then capable of establishment and where then counter bombardment fire could be directed.
Our AP consisted of a small but stoutly constructed bunker leading off a forward infantry defence trench and situated with a view across the valley below and the paddy fields of the Samichon Valley. Rising were the North Korean and Chinese held hills to our front. Behind these enemy mortars and small howitzers hidden in reverse slopes of the closer hills, could be pushed out on railrails to fire a few rounds before being hurriedly pushed back into its safe tunnel.