Position finding equipment was designed to collect accurate and rapid data to:
Correctly identify the target.
Determine the speed and direction of the target.
Determine the true range of the target from the directing point of the battery at any instant.
Predict the position of the target at any instant in advance of its present position.
Make ballistic corrections due to atmospheric or other conditions existing at the moment of firing.
Make corrections due to range difference between directing point and location of the gun.
Make adjustments to place center of impact of fire from guns on target.
Determine deflection to be set on gun so that when fired, it will allow for the travel of the target during the time of flight of the projectile, and effect of wind and drift on the flight of the projectile.
For a battery of 6-inch caliber or larger the operation of determining firing data for a moving target was divided into the following steps:
- Tracking-which included observing and plotting successive positions of the target.
- Determination of the set-forward point-which predicted the future position of the target, that is, its predicted position at the end of the projectileâ€™s time of flight.
- Relocation-which determined the range and direction of the future position of the target from the directing point.
- Calculation of firing data-which consisted of converting the relocation data into firing data for use in pointing the guns.
The three standard systems of position finding used by seacoast artillery were:
- horizontal base
- vertical base
- self-contained base systems
In all of these systems the procedures were similar. They differed only in the method of locating the target in tracking. The first step in all position finding systems was the identification and location of the position of the assigned target with respect to the observation stations (base end stations) of the battery.Â This operation was called tracking and consisted of locating, at regular intervals of time by observation from one or more stations, successive positions of the target, and plotting those positions on a plotting board. The time interval between successive observations was called the observing interval and was 15 to 30 seconds in length (this varied from battery to battery). The observing intervals were indicated by TI (time interval) bells or buzzers which sounded simultaneously in all stations of the battery.
HORIZONTAL BASE SYSTEM
Description. In the horizontal base system, the target was located by the method of intersection used in surveying in which the direction of the target from two known points was determined. In the triangle involved, one side and the two adjacent angles were known. The solution was made graphically on the plotting board. The system required a base line on the ground, the azimuth and length of which had been accurately determined by surveying; two observation stations, one at each end of the base line, in each of which was mounted an instrument for measuring azimuths; a plotting room with plotting board; and the necessary communication lines. By 1944 most major caliber batteries had six to ten different possible baselines. The plotting board represented to scale the field of fire of the battery. On it was located to scale, in their proper relation to each other, the observation stations, the base line, and the directing point (the point for which the firing data are to be determined). The figure above illustrates the relation between the installations in the field and the set-up on the plotting board.
Operation. The observers at the base end stations sighted and followed the target with the vertical cross wires of their instruments on a designated point on the target assigned by the battery commander. At the sounding of the TI bell, the observers stopped following the target with their instruments while the readers read the azimuths and then resumed tracking. Each reader was equipped with a telephone head set connecting him to an operator, called an arm setter,in the plotting room. There the successive observations were plotted on the plotting board. The plotting board had an arm corresponding to each of two observation stations with a means of setting each arm in azimuth. Each arm setterset his arm to the azimuth read by the corresponding reader. The point of intersection of the arms represented the position of the target at the instant the observations were taken. This point was marked by the plotter. The operation was repeated at the sounding of each successive TI bell. The points were called plotted points. A line joining the plotted points represented the track or path of the target.
VERTICAL BASE SYSTEM
Description. In the vertical base system, the target was located by the offset method used in surveying, in which the direction and distance of the target from a known point were determined.Â The direction was determined by reading the azimuth as in the horizontal base system. The distance was determined by the depression angle method which involved the solution of a vertical right triangle of which the known leg was the effective height of the observation instrument above sea level, the hypotenuse was the line of sight from the observer to the target, and the unknown leg was the desired range to the target. The known angle was the complement of the angle between the hypotenuse and the known side, corrected for refraction. It was the angle through which the line of sight was depressed from the horizontal to intersect the target and was called the depression angle. The triangle was solved mechanically by the observation instrument called a depression position finder (DPF). This system required but one observation station, the azimuth and range to the target being read from the same instrument.
Wikipedia: the Depression Position Finder
Operation. The observer tracked the target in azimuth with the vertical cross wire as in the horizontal base system. At the same time he tracked the target in range with the horizontal cross wire. In the plotting room, only one arm of the plotting board was used. The azimuth and range was received from the reader at each sounding of the TI bell. The arm setter set the arm in azimuth and repeated the range to the plotter who marked the point at that range by means of range graduations along the edge of the arm.
SELF-CONTAINED BASE SYSTEM
Description. In the self-contained base system, the target was located by the offset method as in the vertical base system. The azimuth (direction) and range were determined by means of a self-contained range finder of either the coincidence (coincidence range finder or C.R.F.) or the stereoscopic type.
Operation. The operation of tracking with this system was similar to that with the vertical base system except that azimuths may have been read from a separate instrument.
Fire control stations dot the coastline near many major American harbors. People coming across these nondescript military structures frequently, if erroneously, call them pill boxes, bunkers, lookout posts, or machine gun nests. In actuality these stations are remnants of the once-extensive network of army seacoast defenses that protected the United States during the first half of the 20th Century. These stations served as the eyes for seacoast guns that surrounded the harbors, taking measurements to enable gunners to precisely locate enemy ships, and controlling the submarine mines that guarded the harbors.
Technically, battery fire control stations for use with vertical-base systems were not base end stations, but the term was generally used for all position-finding stations. These stations came in several styles, usually reflecting the eras when they were constructed and the location where they were built. The first fire control stations, crows nests, were small stations set into the battery parapets. The first separate stations at most harbor defenses began to appear just after the turn of the century. Army engineers frequently grouped five or six stations within a few dozen yards of each other, or several stations were located in large, multi-story frame buildings. Many clusters of fire control stations, especially primary stations, were within existing army forts. Secondary (and higher) stations were usually located some distance away. When possible, they too were located on government land, such as another fort, but often, especially during World War II, they were on small, military reservations purchased or leased for the purpose.
Where high land was close to the water, stations were often originally built on the reverse slope of hills; only their tops high enough to see or be seen. Later, simple rectangular structures were buried with only the top two or three feet exposed. A vision slit, usually with steel shutters, faced the sea. Interiors allowed just enough space for a cramped crew of soldiers to operate their telescopes and communications equipment. At other coastal locations, such as along much of the Gulf and South Atlantic Coasts, the land was relatively low and base end stations had to be elevated. Most of these stations consisted of a one, two, or three-floor house atop or otherwise integrated into a wood, metal, or concrete tower.
Restored Plotting Room, Battery Osgood-Farley, Fort MacArthur Museum, San Pedro, CA (Joe Janesic)
Restored Battery Commander’s Station, Battery Osgood-Farley, Fort MacArthur Museum (Joe Janesic)
Defense Stations at the Harbor Defenses of the Columbia River (For symbol key download this PDF: Symbols 1940)