Coast Artillery fire control system In the U . S . Coast Artillery , the term fire control system was used to refer to the personnel , facilities , technology and procedures that were used to observe designated targets , estimate their positions , calculate firing data for guns directed to hit those targets , and assess the effectiveness of such fire , making corrections where necessary . In brief , the fire control system in use from about 1900 through WW2 involved observers , often situated in base end station s , using optical instruments ( like azimuth telescopes or depression position finder s ) to measure bearings and / or distances to targets ( usually moving ships ) . Both horizontal and vertical base position finding systems were used . These observations were communicated to personnel in battery plotting room s , who used a mechanical device called a plotting board to indicate the target ' s observed location on a map of the area . The red " 1 " on the diagram at right indicates this first stage of the fire control process . Once several positions had been plotted for the target ( blue circles on Figure 1 at left ) , plotting board operators estimated the position of the target at the instant a salvo , fired by the battery , was expected to land . This position was called the set forward point ( green square in Figure 1 ) , since it involved " setting forward " the target ' s expected position ( assuming continued forward travel at the same speed and in the same direction ) during two intervals of time : ( 1 ) the dead time between the time the observation had been made and the time the guns were actually fired at that target plus ( 2 ) the time of flight - - the time the projectile spent in the air before hitting something . The set forward point was expressed in terms of firing data : a range ( in yards ) and an azimuth ( a compass heading in degrees ) at which the gun / s should be pointed by the gun crews in order to hit the target . Before these firing data were sent to the guns , however , they were corrected for a variety of " non - standard conditions " , like temperature ( which affected the explosive power of the powder charge ) or wind strength and direction ( which affected the flight of the projectile ) . The red " 2 " in the diagram at right indicates this stage in the fire control process . Special devices , like the deflection board ( for corrections in azimuth ) or the range correction board ( for corrections in range ) were used to produce corrected firing data . The final stage ( the red " 3 " in the diagram at right ) had to do with using feedback from the battery ' s observers , who spotted the fall of the projectiles ( over or under range , left or right in azimuth , or on target ) and telephoned their data to the plotting room so that the aim of the guns could be corrected for future salvos . The importance of timing . Fire control in the Coast Artillery involved a sequence of steps that was carried out over and over again while a target was being tracked and fired upon . First , observers sighted on the target and sent their observations to the plotting room . Next , plotters calculated the target ' s position and probable future movement , as well as adjustments to range and azimuth ( direction ) . Then firing data were sent to the batteries and used by gun crews to point their guns . The guns were fired . Finally , spotters might spot the fall of the projectiles , sending this information back to the plotting room for use in correcting fire . After this , observers would sight on the ( new ) position of the target , beginning another cycle . The time period between sequential sightings by the observers was called the observing interval . It was typically set at 20 sec . for controlling batteries of larger guns . Not only did everyone in the fire control system have to stay in synch ( for example , knowing which one of a series of sets of fire control data they were working with at any given time ) but certain functions ( notably the making of target sightings by the observers and the firing of the guns by the gun crews ) had to be carried out at precise intervals if the accuracy of the system was to be maintained . To enable all battery personnel to stay in synch , a time interval bell ( or buzzer ) would be rung in every observation or spotting station serving the battery , in the plotting room , and at each gun , using bells or buzzers wired together with a centrally located master clock . Five seconds before the start of the next cycle , the bell would ring . After a one second delay , it would ring again . And after another one - second gap the bell would ring a third time , and on this third ring the observations were made again and / or the guns were fired . A well trained battery could observe , plot , adjust , and transmit the firing data to its guns , which could then be loaded and laid , all before the next 20 - second bell , at which point the guns would be fired . If for some reason the firing data were not received in time by the guns , or if a hold - up or a misfire occurred , then firing happened at the end of the following interval . In such a case , the command " Relay ! " ( re - lay ) was given at the guns .