Entering service in 1927, the predictor was a vital component of any HAA battery. Using mechanical calculation, it could produce gun-laying instructions. In other words, it could predict the future position of aircraft based on their projected trajectory. In Malta, although only 5% of LAA engagements made use of predictors (Rollo, p. 264) (possibly to conserve fuel from their thirsty generators). HAA found the No 2 (1938) predictor much more necessary. Heavy anti-aircraft guns would be engaging targets at much higher altitudes, up to 13,000 metres. The shell delay (the time it takes for the shell to travel from the gun to its target) would have been far longer and hence much more difficult and time consuming to calculate.
HAA positions would rely on nearby Vickers predictor crews to calculate or rather predict the future position of aircraft so that they could be aimed accordingly as quickly and accurately as possible. The range finder would provide the height to be fed into the height computer just on top of the predictor. After the crew would input wind-speed, distance and other factors, one operator would track aircraft horizontally the other vertically from two opposing scopes attached to the device: one would maintain the angle of elevation, the other one keep the instrument pointed at the target. In practice, what this translates to is keeping the target in the middle of their lense as accurately as possible. Acquiring what is known as the ‘quadrant angle’, the future position would then be exported to the gun dials by the third man. The last task was just as important as the other. No. 4 would keep his eyes fixed on the ‘drums’ and the dials displaying the ‘vertical rate’ to assign the correct time on the fuzes.
Back inside the command position adjacent to the predictor and height finder, a plotter (later using a G.L Mark 1 plotter) would do his best to record enemy movements from observer data and telephone communications. There were many other roles integral and essential to the operation of the guns.
The new defense became not only the anticipation of the adversary’s actions, but their prediction. The speed of new weapons was such that soon a calculator would have to prepare the attack and ceaselessly correct the control elements in order for the projectile-shells and the projectile-plane to become one: this apparatus was called the “Predictor.” This automation of pursuit brought on, after the war, the extraordinary development of data processing and those famous “strategic calculators” that upset the conduct and politics of war. -Bunker Archeology, Paul Virilio.