The Height & Range Finder
The gun batteries were isolated geographically and strategically. Some sites were close to pillboxes and other installations, others not so much. Apart from a direct telephone line with the operations room, and, later on, a wireless radio set, the sites could not communicate with the rest of the island. That being said, every day they did receive the all essential and updated weather reports multiple times. Radar or observation posts around the island would keep watch for enemy formations or any hostile activity. In every battery a dedicated team of spotters would also keep track of all air activity.
If an approaching enemy aircraft is spotted, either via radar or visually, a report (as detailed as possible in terms of direction, altitude, angle, and speed) would be sent directly to the Anti-Aircraft Operations Room (AAOR) deep below the earth in Valletta at Lascaris. Once the formation has been identified as hostile, if the operations room permit the engagement, the order is sent out to the respective battery/batteries to fire directly or through barrage. Until then (or, as Agius notes, until the Gun Position Officer (GPO) was convinced its an enemy target, p. 30) as the foreign aircraft approached, the gunners would take up their assigned positions on the guns or, just as importantly, on the nearby instruments.
On the 3 (20 cwt), 3.7 and 4.5 inch guns normal anti-aircraft gunnery, using calibrated sites, good practice, and well-trained eyes, would simply be ineffective. Thus several devices were used to aid anti-aircraft guns. Early on in the war sound-locators were used to help detect and orient the guns to their target. Later on, new devices were put in use which could actually lay the gun with impressive accuracy. Two instruments became commonplace to both Light and Heavy anti-aircraft positions, the ‘height and range finder’ and the ‘predictor’. The former would consist of a stereoscopic device with two lenses far apart. Both relied on the engineers within the RAOC to keep them running smoothly. One report stated that heat effected the operation of height-finders and sun-shields were fitted in response. Later on, a device to ‘agitate the air’ resolved the issue, by which time Radar was giving accurate height readings.
These awkward-looking, tube-like devices came in many sizes, from the small 180cm UB-2, UB-3, to the UB-7 and more. The largest of which used on land, the UB-10, was 18 feet wide. Using prisms (one on each end) and measuring their alignment, they could calculate the distance between the ground and enemy aircraft with remarkable accuracy.
Although they seem complicated, at heart they are only using very basic trigonometry. One mirror stays stationary and reflects an image at an angle of 90°. The other can swivel until it is optically aligned with the other towards the target, producing a very precise angle. Using this acquired angle and the known distance between the mirrors, the instrument could give a reading of the object’s range. Thus, Malta’s guns could be set as needed to meet the enemy high up in the skies above.