Data could be received and transmitted by telephone, or directly via dedicated electrical systems. Locations of plotting rooms in coastal defence installations varied greatly; they could be in low-rise structures such as base end stations (usually colocated with observation equipment in a two-story structure), taller fire control towers, in gun battery structures, or in bunkers separate from gun batteries. The British Watkin position finder system for coastal artillery, which entered service in the 1890s, did not require a plotting room due to mounting a depression position finder (DPF) on a "range dial", similar to a plotting board. An electrical system moved bearing and range dials near the guns as the DPF was manipulated by an operator.
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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]). A Depression Position Finder (on the left) and an Azimuth Scope in the same base end station. 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.
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Surprisingly, his shots were harmless, in part because the fort's commander ordered an immediate blackout. The commander also refused to permit his men to return fire, which would have revealed their position. Spotting the enemy gun flashes with a depression position finder indicated the submarine was out of range.Webber p.
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Watkin's system included automatic electrical updating of range and bearing dials near the guns as the position finders were manipulated, and a system of remotely firing the guns electrically from the position finding cell. The improved system was trialled in 1885 and widely deployed in the 1890s.Coast defence range finding at victorianforts.co.ukMajor Watkin's Position Finder at victorianforts.co.
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On 20 April 1942 the tower received a No. 3 Mark 4 telescope and a No. 10 Director. On 1 May 1943 a Mark II Position Finder was added to the fire control equipment on the tower. Soldiers of the 3rd New Brunswick Coast Brigade manned the tower from 1941 until it ceased operations in 1944.
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The magazines of the 9 inch guns, however, opened to a courtyard to the rear. A short distance behind the battery was a barracks and a small parade ground. Behind these, on Skinner's Hill, was the Fire Commanders Post, which gave orders for gun-laying, the Position Finder Cell and Marine Signals. In 1953, the last Imperial Defence Plan was issued, under which the local units were tasked.
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A Mk. III Accurate Position Finder (APF) ready for action, with its antennas raised and the carriage levelled. The cable behind the antennas runs to a winch on the front of the cabin that raises them into operating position. This GL Mk. IIIc APF is limbered for transit, with the antennas locked down. The cabin has been opened to display the electronics in the back of the consoles.
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It was easy to use, highly accurate and was combined with automatic electrical updating of range and bearing dials near the guns as the position finders were manipulated. Soon after the DRF was developed, Watkin developed the Position Finder (PF), a family of several devices which, when two were used in a horizontal base system, gave a more accurate range than the DRF. Some of these were called electric position finders. In some configurations, both horizontal base and vertical base ranging became possible.
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Crewmen raise the antenna boom of the Zone Position Indicator (ZPI). The Canadian design became known as the Accurate Position Finder, or APF for short. At the time, the solution to rapidly switching a microwave signal between two antenna leads had not been solved. As a result, there was no way to use a single antenna for both transmission and reception, and the team initially considered systems with one transmitter and one receiver, or a single transmitter and four receivers.
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Auxiliary Territorial Service (ATS) girls at the Royal Artillery Experimental Unit, at Shoeburyness, using the Window Position Finder to sight shell bursts in the air or water, 1943. The eastern terminus of the London, Tilbury and Southend Railway (c2c line) is at Shoeburyness railway station, services run to London Fenchurch Street in the city of London. The eastern end of the A13 is at Shoeburyness. The MoD Shoeburyness site at Pig's Bay is situated nearby and the facility is run by the company QinetiQ.
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These systems used instruments such as the depression position finder (DPF) or the coincidence rangefinder (CRF) to measure the range and/or azimuth to the target. 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.
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A base end station might be a two-story structure with a plotting room or other instruments or facilities on the lower level. By the 1920s coincidence range finders, self-contained horizontal base instruments, were in use along with the other methods. Though these could be used quickly, these had baselines of only a few feet, reducing their accuracy and maximum effective range. 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.
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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.
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An American Warner-Swasey depression position finder, illustration from a 1910 manual Military instrument contracts were an important line of work for the company.. The U.S. government referred many problems concerning such instruments to the company during the Spanish–American War (1898). Instruments produced included "range finders of several types, gun-sight telescopes, battery commanders' telescopes, telescopic musket sights, and prism binoculars".. Presumably, the range finders included the company's depression position finder.FM 4-15, Seacoast Artillery Fire Control and Position Finding, pp. 49–51 During World War I, three important kinds of instrument were produced: "musket sights, naval gun sights, and panoramic sights".
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The two highest land elevations surveyed for Devils Battery observation posts were Flandrum Hill, Cow Bay and Caldwell Road between "A-23" barracks and "Radio 16" aircraft direction radar (Scott Drive). Each elevated point had a four- storey reinforced concrete Victoria Fortress Observation Post, with plotting rooms and telephones connected to the Halifax fire command post. Two other three-storey observation posts were erected, one at the new radar installation at Osborne Head, and the other above the hill behind the three turrets. Each look-out was manned with a crew of two on the top storey using a Depression position finder looking out towards the harbour for any suspicious targets.
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From the extreme southeast end of East Point, these guns, with their 12-mile maximum range, could command the entire ocean front of Boston Harbor and could reach all the way south to Hingham, MA. The guns, which could be fired rapidly, were mounted roughly 250 feet apart, and a small, wooden Battery Commander's station, containing a depression position finder and a coincidence range finder was built between them. (See the image gallery below for plans and dimensions of this battery.) This 155mm gun battery is believed to have been the only one of its type in the Boston Harbor defenses, and the surviving platform of Gun 1 is thought to be relatively rare in New England.
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The DRF was invented by Captain H.S.S. Watkin of the Royal Artillery in the 1870s and was adopted in 1881.1885 DRF Handbook1892 DRF Handbook It could provide both range and bearing information on a target. The device's inventor also developed a family of similar devices, among them the position finder, which used two telescopes as a horizontal base rangefinding system, around the same time; some of these were called electric position finders. Some position finders retained a depression range finding capability; some of these were called depression position finders. Watkin's family of devices were deployed in position finding cells, a type of fire control tower, often in configurations that allowed both horizontal base and vertical base rangefinding.
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After these adjustments, the plotting board represented a true analog of the harbor being defended (see figure at left above) and was ready for use in fire control. This customization of the board to its site, however, was also a weakness. It meant that the battery's fire control system was limited to using only the one baseline and only the two base end stations associated with that baseline. If one of the two base end stations was put out of action (due to enemy fire or a communications casualty) the battery would have to switch to a less precise method of fire control, such as vertical base observation (using a depression position finder), the use of a self-contained rangefinder instrument, or aiming its guns directly, using their own telescopic sights.
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A depression position finder (DPF) was an observation instrument that was used in the fire control system of the U.S. Army Coast Artillery Corps and predecessors from circa 1901 through 1945 to locate targets in range and/or azimuth as part of the process of directing the fire of a battery of coast defence guns or mortars. It was one of many technologies introduced to US coast defences as part of the wide-ranging Endicott program. These instruments, which contained telescopes on massive, finely geared mountings, were located in various types of fire control towers (or smaller facilities) such as base end stations, DPF bunkers, or built into concrete gun emplacements. The American DPFs were functionally similar to the British device of the same name, but their data were usually relayed by telephone to a plotting room instead of directly to the guns.
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A Watkin depression position finder on a range dial in a position finding cell, Dover Castle, England O represents the observer, at a distance OM above sea level (and the target on the sea). The range to the target is determined by taking the cotangent of the depression angle d times the height of the instrument OM, to yield the range MP. But due to the curvature of the earth, the true range is MT. To achieve this, the DPF instrument adjusts the observer's height from OM to ON, and then correctly measures the range as NT. Alternatively, the depression angle could be adjusted to d, but this method was not used. An American DPF (left) and azimuth scope (right) in use Warner- Swasey DPF, illustration from a 1910 manual The remains of three American base end stations with their concrete DPF mounting columns. These columns date to about 1910.
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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.
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