User:Pgrig/base end station

A cutaway drawing of a battery's fire control systm.

A cut-away drawing of a Coast Artillery battery, showing its two base and stations (upper left), the plotting room (with plotting board), and the directing point (between the two guns), with their displacement from it.

Base end stations were used by the U.S Coast Artillery as part of systems for locating the positions of attacking ships and controlling the firing of seacoast guns, mortars, or mines to defend against them.^[1]. Some base end stations resembled pillboxes or small bunkers dug into the ground overlooking coastal waters. Others were located in tall fire control towers which could contain as many as 10 stories and rise 100 ft. or more above ground. Still others were camouflaged to resemble seaside homes or cottages. Some base end stations had anti-aircraft observation positions on their top levels, or harbor observation radar antennas on platforms above their roofs.

These stations were used between the 1890s and 1946. In the later stages of WW2, radar and electro-mechanical gun data computers were used increasingly to direct coastal guns, and the optical observation systems using base end stations became a back-up. At the end of the war these stations were declared surplus and were sold off to public and private owners. More than 100 of these stations likely still survive today, as part of state preserves or under private ownership, Many have been razed to clear sites for new development.

Horizontal Base Systems

In a horizontal base system (like that in the diagram above), two base end stations were located at precisely surveyed points ^[2], one at each end of a base line, or a line between them of known length and azimuth ^[3] . At each of these stations was an observation instrument (such as an azimuth telescope or a depression position finder (DPF) capable of making a precise measurement of the bearing of a distant target (usually a ship) from the station. When the stations at each end of the baseline had made their measurements, they communicated these to a plotting room (or fire control center) which used a plotting board (a sort of a graphical analog computer) to locate the position of the target.

Making this system work properly required the two base end stations to take bearings on the target at precisely the same time. To enable this, a bell or buzzer (called the time-interval bell) was rung automatically at fixed intervals (usually every 15 or 20 seconds) in all observing stations across a harbor defense system^[4] (note the dashed communications lines in the diagram above). One soldier, using the telescope or DPF, would track the target. At the sound of the bell, a second soldier would read the azimuth (at which the instrument was pointed) off a scale on the instrument and telephone this reading to the plotting room.

At the plotting room, staff would use a plotting board to pinpoint the position of the target from the data sent in by the base end stations. Once this position was noted, it had to be converted to a set forward point, or the position the target was estimated to have reached at the time the shells fired would actually hit it. A number of other corrections also had to be made before the guns were pointed (in angle and azimuth). These included corrections for height of tide, wind direction and strength, atmospheric pressure, temperature, type of projectile fired, type and amount of powder used, etc.^[5]

Some examples

Base end stations took a variety of forms. Some were multi-story towers, either square or round in plan (or both), and rising from 20 to 100 feet above the ground. Some looked like forest fire watch towers. Others appeared to be small buildings, or were disguised as seaside cottages to camouflage their purpose. Still other base end stations resembled small pillboxes, dug into the ground, and usually looked down over the channels they defended.

Ft. Miles Round FCT — Ft. Miles Round Fire Control Tower

Base end stations were often assigned to particular batteries of guns or mine fields in a harbor defense system. In some U.S. harbors during WW2 there were 20 or more of these base end stations, Often from 10,000 to 15,000 yards apart, and tied together by telephone lines running through switchboards. These stations could be used flexibly in different combinations or by different gun batteries as ships moved through the area, or in case a given station was damaged by enemy action.

Base end station cottage — Base End Station Cottage

Spotting Stations

In WW2, each base end station was often combined with a spotting station. This meant that a separate observation instrument (often an azimuth telescope) was included in the structure, on a separate pedestal mount (or tripod), alongside the depression position finder (DPF).

Often the role of the spotter was to observe the fall of fire from the guns of a supported battery, telephoning back to the plotting room whether the shells were falling short or over, left or right.

In harbor defense plans and other documents, such as survey data sheets ^[6] prepared by the U.S. Coast and Geodetic Survey (USCGS), these combined stations were often labeled according to the tactical numbers of the gun batteries they were designed primarily to serve and their number in the series of stations serving that battery. Thus a station marked B 4/2 S 4/2 referred to base end station #4 for battery #2 and spotting station #4 for battery #2.

Multi-level fire control towers often contained several base end and spotting stations, each on a different level and each serving a different battery of guns in the harbor's defenses. These multiple stations were usually stacked up precisely above one another, so that the coordinates of their instruments needed to be surveyed only once for all levels.

Vertical Base Systems

References

^ For a detailed description of base end stations and their role in fire control for the U.S. Coast Artillery, see "FM 4-15, Coast Artillery Field Manual: Seacoast Artillery Fire Control and Position Finding, U.S. War Department, U.S. Government Printing Office, Washington, D.C., November 5, 1943.
^ The position of the station was usually measured (surveyed) as the "pintle center" or mounting point of its primary observation instrument.
^ In this case, azimuth is synonymous with "direction" or "compass bearing." In the U.S. Coast Artillery, azimuths were measured from South (instead of North) in a clockwise direction.
^ This bell also rang in the plotting room and at each gun in the battery. The cycle of bells governed the speed with which the plotting room crew had to work. If a gun did not receive "fresh" firing data by the end of a timing cycle, it had to wait until new data were received during a later timing cycle.
^ Coast Artillery manuals describe a myriad of adjustments and corrections that were to be computed and applied to the firing data sent to the guns. Since no Coast Artillery gun in the continental U.S. was ever fired under combat conditions, it is hard to tell how this complex system would have functioned in practice. Drills were held fairly often, though, and often achieved shots that fell within 50 yds. of their targets.
^ [1] This is a sample data sheet for a base end station in Nahant, MA that is also a spotting station. The National Geodetic Survey (NGS) which publishes this sheet replaced the precursor agency, the U.S. Coast and Geodetic Survey (USGS).

[1] For a detailed description of base end stations and their role in fire control for the U.S. Coast Artillery, see "FM 4-15, Coast Artillery Field Manual: Seacoast Artillery Fire Control and Position Finding, U.S. War Department, U.S. Government Printing Office, Washington, D.C., November 5, 1943.

[2] The position of the station was usually measured (surveyed) as the "pintle center" or mounting point of its primary observation instrument.

[3] In this case, azimuth is synonymous with "direction" or "compass bearing." In the U.S. Coast Artillery, azimuths were measured from South (instead of North) in a clockwise direction.

[4] This bell also rang in the plotting room and at each gun in the battery. The cycle of bells governed the speed with which the plotting room crew had to work. If a gun did not receive "fresh" firing data by the end of a timing cycle, it had to wait until new data were received during a later timing cycle.

[5] Coast Artillery manuals describe a myriad of adjustments and corrections that were to be computed and applied to the firing data sent to the guns. Since no Coast Artillery gun in the continental U.S. was ever fired under combat conditions, it is hard to tell how this complex system would have functioned in practice. Drills were held fairly often, though, and often achieved shots that fell within 50 yds. of their targets.

[6] [1] This is a sample data sheet for a base end station in Nahant, MA that is also a spotting station. The National Geodetic Survey (NGS) which publishes this sheet replaced the precursor agency, the U.S. Coast and Geodetic Survey (USGS).

[1]

[2]

[3]

[4]

[5]

[6]