Coast Artillery fire control system
bearing or deflection) instruments, preferably widely separated, and a communications system to transmit data to a plotting room and then to the guns.
A base end station might be a two-story structure with a plotting room or other instruments or facilities on the lower level.
[2] Vertical base rangefinding used a single depression position finder (DPF) mounted as high as possible above the water level; these were derived from similar British devices and adopted beginning in 1896.
The need to be progressively higher above water as gun ranges increased was a severely limiting factor for the DPF, and they were usually supplemented by horizontal base systems.
Some of these computers received data directly from communicators that were connected to observation instruments in fire control stations or from Coast Artillery radar equipment.
[6] In brief, the fire control system in use from about 1900 through WW2 involved observers, often situated in base end stations or other fire control towers, using optical instruments (like azimuth telescopes or depression position finders) to measure bearings and/or ranges to targets (usually moving ships).
[note 4][8] 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.
Next, plotters calculated the target's position and probable future movement, as well as adjustments to range and azimuth (direction).
Finally, spotters might spot the fall of the projectiles, sending this information back to the plotting room for use in correcting fire.
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.
In Coast Artillery parlance, the term "correction" usually referred to changes in estimated range or deflection (direction) that were made prior to firing.
Adjustments were usually made by observing and plotting the fall (splashes) of the shells fired and reporting by how much they were left or right in azimuth or over or under in range.
The higher altitude readings were needed for firings of the 12-inch (305 mm) coast defense mortars, which sent their shells on very high trajectories.
This device yielded index numbers that were either fed to the plotting room and used to correct readings on a plotting board, were used as input to a "deflection board" (see below) or were telephoned to the batteries and used by gun crews to make offsets directly on the range wheels or sights of the guns themselves.
Values for the individual factors (#1 through #7 above) had to be obtained by plotting room personnel from battery officers or from the hourly meteorological message.
Since precise measurements of muzzle velocity (factor #1) often could not be made, estimates were used, based upon the size of the powder charge being fired and the characteristics of the individual gun being used.
Like many other pieces of Coast Artillery fire control equipment, the deflection board was a mechanical analog computer that used methods of similar triangles to solve the problems of correcting fire for wind speed and direction, drift of the projectile, and angular travel of the target during the observing interval.
