Measurement
Range presented particular problems. Early in the 1920s, when navies began to seek mechanical fire-control solutions to the anti-aircraft problem, all but the Germans used coincidence rangefinders. A coincidence rangefinder has two lenses, which observe the target from slightly different angles. The operator matches the half-image from one lens with the other half as seen by the other. When the two half-images form a full image, the angles from the two lenses give target range by triangulation. The observer find the match using a line projecting through both half-images to check that they match up. For a surface ship that was typically a mast or funnel. Unfortunately the most visible line feature of an aircraft, the wing or wings, is horizontal. A horizontal rangefinder splits the image along roughly that horizontal line. Turning the rangefinder vertical split the image at right angles to the wings. The axis of the rangefinder had to be at right angles to the line of sight. The elevation angle of a vertical rangefinder (which the US Navy called an altimeter) gave the aircraft’s angle of elevation. At long range that did not work. It turned out that sight angle was difficult to measure at all until the aircraft was well above the horizon – which meant that attempts to measure elevation rate were frustrated.
The alternative was a stereo rangefinder. As in a coincidence instrument, a stereo rangefinder has two lenses, but instead of forming images they feed into the operator’s two eyes. The operator perceives objects in an exaggerated depth of field. He finds the range by moving a marker in depth to match the apparent distance to the target. The Royal Navy tested stereo rangefinders after the First World War, but became convinced that operators would lose their stereo capability under stress, for example during a protracted engagement. It is not clear to what extent this conclusion reflected Barr & Stroud’s desire to keep selling its coincidence instruments; it may be that stereo rangefinders did not become entirely reliable until the 1930s. The US Navy adopted them at about that time.
A spotting glass is a related instrument. It too has two lenses, one feeding into each of the operator’s eyes. In effect it deepens the operator’s field of view, so that he can see how far one object is from another – how far, for example, a shell burst is from the aircraft target. Spotting glasses and stereo rangefinders are so closely related that the latter was sometimes converted into combination rangefinders and spotting glasses. The US Navy used stereo spotting glasses well before it adopted stereo rangefinders, and their success may have inspired adoption of the stereo rangefinder. A gunnery school held on board Oklahoma in the spring and early summer of 1941 used a Mk V spotting glass atop No 2 turret as an instructional rangefinder, since by that time all the battleships in the Pacific had anti-aircraft directors using stereo rangefinders.
The observable data could, moreover, be misleading. To an observer on the ground (or on board a ship) an aircraft moving at steady speed on a steady course does not seem to be moving steadily at all. The observer sees the aircraft at an angle, and his impression of speed (in three dimensions) is given by the way that angle changes over time. The closer the aircraft, the faster the angle seems to change. To some extent range and speed are entangled, because if the gunner mis-estimates range he will also mis-estimate speed, given what he can see (the rate at which position angle is changing). Similarly, speed and range are entangled in the rate at which the deflection angle changes.
Wind affected anti-aircraft warfare far more than surface fire. Unfortunately wind affected aircraft and shell differently, and wind could also vary considerably between the surface and the altitude of an air target. For this reason, late in the 1930s US anti-aircraft officers wanted each task force provided with an aerological unit; but that was clearly impossible for an isolated unit or a small convoy escort. Even if the solution reached by a fire-control computer was perfect, gunners might still miss altogether if they misestimated the effect of wind on projectiles. In reality wind varied with altitude. To make calculations simpler, gunners and system designers generally used a fictitious ballistic wind, constant at all altitudes. Ballistic wind was a function of trajectory, i.e., of range.
None of the information fed to the gunner was precise. All rangefinders, for example, had inherent inaccuracies. Shortly before the Second World War, an officer commenting on anti-aircraft fire control pointed out that because of inherent rangefinding (but not angle of sight) errors, an aircraft flying straight and level over a ship might seem to be climbing or diving at a slight angle. Gunners seeing up this nonexistent vertical motion might correct fire, only to discover that their corrections threw their fire off the moving target.
However good the solution or prediction a fire-control system produced, it had to contend with three other factors. First, there is a difference between gun (ballistic) and rangefinder range. For a variety of reasons, a gun aimed to fire (say) 6700 yds will generally throw its shell to a slightly different range. Second, the gun aimed at a point in space will distribute its shells around that point in a pattern. The size of the pattern is indicated by the distribution of half the shells (pattern size equates to the modern idea of Circular Error Probable, or CEP, which is frequently cited for long-range missiles). The US Navy referred to the Mean Point of Impact (MPI), in effect the centre of the pattern of bursts. Control was exerted to move the MPI onto the target. Pattern size corresponded to the spread of salvos in surface warfare. Third, to make matters even more interesting, the fuses themselves do not always go off at the intended time: there is a fuse pattern. The gun pattern is a distribution at right angles to the line of fire, so it is reflected in misleading results in elevation and bearing (deflection). The fuse pattern is a distribution in time of bursting, so it is a distribution in range. The US Navy referred to fuse range.
Gunners rely on observation to correct their fire. Patterns and range errors complicated spotting once fire began, because a spotter might correct aim based not on what the fire-control system was trying to do, but on outliers in all the patterns. On that basis corrections might throw aim further and further off – they were said to ‘pyramid’.
Spotting was complicated by the nature of anti-aircraft fire. Instead of making splashes at unambiguous ranges (short or over), an anti-aircraft shell was burst by a time fuse, which might use either a powder train or a clock mechanism. Either mechanism might not explode on time, creating an erroneous apparent range error. Virtually all the time-fused anti-aircraft guns described in this book were hand-loaded, so the time between fuse-setting and insertion of a round into the breech varied from round to round, further complicating system performance. Calculation had to take into account this dead time between calculation and firing. Small-calibre anti-aircraft guns were contact-fused (or did not explode at all).
In surface fire, gunners fired a salvo, spotted the