Base end stations were used by the United States Army Coast Artillery Corps as part of fire control systems for locating the positions of attacking ships and controlling the firing of seacoast guns, mortars, or mines to defend against them.
A "true" base end station was one of a pair of stations at either end of a precisely measured (surveyed) baseline. Once simultaneous bearings from each base end station to a target were taken, since the distance between the stations (the baseline) was known, the range to the target from either station could be calculated through triangulation. If the target's bearing from each station was sent to a plotting room and input to a plotting board, the position of the target could be estimated and firing coordinates for a gun battery could be calculated.
Some base end stations were located in tall fire control towers (FCTs) Sometimes, the terms "base end station" and "fire control tower" were used interchangeably. In general, however, a fire control tower (FCT) was a structure built to raise one or more base end, spotting, or observation stations high above ground level. Some fire control towers contained several base end stations, one on top of another on different stories of the tower, with each station being at one end of a different baseline and being assigned primarily to a different gun battery in a harbor's defensive scheme. Other base end stations resembled pillboxes or small bunkers dug into the ground or on the surface overlooking coastal waters. 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.
Almost all base end stations were equipped at least with an azimuth telescope (similar to a surveyor's theodolite) to enable them to sight accurate bearings to a target. In addition, some base end stations held a depression position finder (DPF) or a coincidence range finder (CRF) that could be used by itself (without reference to another station) to produce fire control data for the gun(s). Sometimes a space holding one of these instruments (if not located on a baseline and paired with a second station) was referred to simply as an observation station.
These stations were used between the 1890s and 1946. At the end of the war almost all of them were declared surplus and were sold off to public and private owners. More than 100 of these stations likely still survive today across the United States, as part of state preserves or under private ownership, although many have been razed to clear sites for new development.
In a horizontal base system (like that in the diagram above), two base end stations were located at precisely surveyed points, one at each end of a base line, or a line between them of known length and azimuth. 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 an analog tracking device called a plotting board to locate the position of the target. Later in WW2, the target bearings could be input to an electro-mechanical gun data computer, which calculated the position of the target and the required adjustments for things like ballistic factors and the locations of the guns that were to be fired.
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 (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.
In WW2, each base end station was often combined with a spotting station. This meant that a second spotting instrument (usually an azimuth telescope) on a separate pedestal (or tripod) was included in the observing room, alongside the primary one (which was usually a depression position finder [DPF]).
While the primary observer (and perhaps the azimuth reader who assisted him) tracked the target, the role of the spotter was to observe the fall of fire from the guns of the 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 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.
Either of these two types of range finding stations could function effectively by itself, measuring both range and bearing (azimuth) to the target, without the need for a second base end station (as with the horizontal base system). Using one of these position-finding systems, while not as accurate as using the horizontal base method, had certain advantages.
Base end stations (also called observation stations) took a variety of forms. Some were multi-story fire control (FC) 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.
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.