air defence

air defence defined in 1939 year

air defence - Air Defence;
air defence - Principles and Methods of Air Defence by Air Ministry. This description of how one of the major problems of total warfare was faced is written with the full authority of the Air Ministry. For further relative information see under Air Raids; Anti-Aircraft Command; Balloon Defences; Britain, Battle of; Fighter Command; Flying Bomb; Radar; Royal Air Force, etc.

The three cardinal principles of air defence are warning of attack, interception of raiders, and counter-bombing of enemy air bases. Enormous advances in method have been made since the First Great War, but these principles have remained the same.

Although the first German bomb fell on British soil in Dec., 1914, enemy airships continued throughout 1915 to attack without serious opposition, and it was not until the spring of 1916 that the defences noticeably improved. Warning of approach was the first to be systematically organized. Observer posts were sited at suitable points round the coast and occasionally out to sea. They relied on the human eye aided by binoculars, night glasses, and telescopes, and on devices to amplify the sound of enemy aircraft and enable their direction to be judged. Communication was by telephone to a central control.

Definite attempts to organize the warning system as part of a comprehensive air defence organization including fighter aircraft, anti-aircraft guns, and searchlights, were not made until the summer of 1917, when the reversion of the German attack to bombing aeroplanes after the failure of the Zeppelin raids made it imperative that the defence of London should be more effective. The formation of a separate air defence command for London produced such an improvement that in the autumn of 1917 the Germans turned to raiding by night instead of by day.

At first the most effective form of defence at night was found to be the "box barrage," in which Anti-Aircraft guns concentrated a cone of fire round an attacking force with the intention of forcing it to fly through the gunfire at some point. Later, methods of interception at night with the aid of searchlights were evolved, and in the biggest German raid, that of May 19, 1918, out of 40 Gothas which attacked London three were shot down in air combat and three by anti-aircraft guns. The armistice of Nov., 1918, found London with 284 guns, 377 searchlights, and 11 fighter squadrons.

In Sept., 1939, guns, fighters, and searchlights still formed part of the defence, but the discovery of radar (q.v.) had vastly altered the picture. In the First Great War the human eye and ear, instrumentally aided, were the only means of discovering hostile aircraft. From the earliest days of the Second Great War, Great Britain had in readiness a system of detecting aircraft before they came within human sight or sound. Air defence could now be regarded as in five stages: radar, giving the earliest warning of approaching aircraft; observers plotting their course overland; fighters attempting to intercept; guns, aided by searchlights, trying to shoot them down; and finally barrage balloons preventing accurate bombing of the target itself. Smoke screens, camouflage, and decoy targets combined to make accurate bombing more difficult. The effectiveness of all these stages of defence depended largely on the first - the early warning of approach.

Radar stations positioned round the coast were able, under favourable conditions, to detect German aircraft even before they crossed the enemy-held coast. They were able to give the course, speed, height, and approximate number. At the start of the war the system covered only the approaches to London, but it was later extended to other vital parts of the coast.

Observation Posts

Once overland, the spotting and plotting of the enemy raider became the responsibility of the Observer Corps, which in April, 1941, became the Royal Observer Corps. The corps consisted of about 32,000 personnel, of whom 23,000 gave spare-time service. Each received special training in the observation, identification, and plotting of aircraft. They manned more than 1,000 R.O.C. posts situated round the coasts and vantage points inland.

Observation posts were equipped with instruments for spotting aircraft and measuring height, speed, and course. This information, together with the number of aircraft and an indication whether they were friendly or hostile, was passed by telephone to an R.O.C. centre. The average centre served between 30 and 40 posts, forming an R.O.C. group. Between them the groups covered the whole of the British Isles.

The information from the R.O.C., together with that from the radar network, was combined and passed to the operations room of the Fighter group. Here the stage of warning ended and the stage of interception began.

Defence of Great Britain, 1940-41

At the time of the Battle of Britain, Fighter Command was organized into four groups, each divided into geographical sectors. A sector contained a headquarters and operations room of its own, together with one or more fighter airfields. At Command, at the groups, and at the sectors, the operations rooms differed in size, but each included a large table-map showing the position and course of aircraft by means of counters moved by members of the Women's Auxiliary Air Force known as plotters. Through head telephones worn throughout their tour of duty, the girls received minute-by-minute information and transferred it to the table-map. Overlooking the table sat the officers responsible for conducting the interception and defence. At Fighter Command headquarters, in addition to the cornmander-in-chief, there was the commander -in-chief Anti-Aircraft Defences, the Royal Observer Corps commandant, liaison officers from the Admiralty and from Bomber and Coastal Commands, and a ministry of Home Security official.

The C.-in-C. exercised general control over the opening of Anti-Aircraft gunfire and use of searchlights through the C.-in-C., Anti-Aircraft Command. He also controlled the balloon barrage and the civil air raid warning system. Group commanders decided which sector should meet any particular raid, and sector commanders detailed the fighter units. Normally, in order to prevent the shooting down of friendly aircraft, Anti-Aircraft Command were allotted specified areas in which gunfire alone provided the defence, known as inner artillery zones. Other areas were detailed for fighters alone, and yet a third type of area would be the responsibility of either guns or fighters, depending on the orders in force.

When warning of the approach of enemy aircraft was received, simultaneous warning was sent out to the fighter airfields, to the gun batteries, to the balloon barrage, and to the authorities of the town or area which was thought to be the target. At the airfield the fighter aircraft took off and were directed towards the raiders by their sector operations room controllers over the radio telephone. Gun batteries opened fire at any enemy aircraft within range in the zones allotted to ground defence. Balloons were raised to the best defensive height. Sirens sounded to warn the target towns. (See Air Raid Warning.)

This was the system used for the defence of Britain in the summer of 1940. By Aug. 12, 182 of the German aircraft employed to attack Channel convoys and ports had been destroyed by the Hurricanes and Spitfires of Fighter Command. On Aug. 15, 180 enemy aircraft were shot down, the record being reached on Sept. 15, when 185 were brought down. From Sept. 8 to Oct. 5, 3,291 day patrols were made by fighters.

A similar system, suitably adapted, was used to intercept the night raiders during 1940-41. Sector operations rooms sent up the fighters and directed them towards the attacking force - a process known as vectoring. The big difference between day and night interception was that from early in 1941 the night fighter carried its own radar set capable of detecting hostile machines to enable the hunting machine to "home" on to its quarry.

Once within range, the night fighter was often able to pick up the enemy on his own instrument, and finally to get within visual sight of the aircraft and shoot it down. The most successful night was May 10, 1941, when night fighters shot down 29 enemy raiders. The total for the mouth was 111, double that for April and five times that for March. The German attack on Russia in June, 1941, signalled the end of large-scale air attack on Britain, so that the problem of whether or not the British air defence could make the night raids as prohibitive in cost as day raids was never finally answered.

Fighters and Anti-Aircraft Guns

Throughout the war R.A.F. fighters were of three main types - the single-seater single-engine type, such as the Spitfire and the Hurricane, the two-seater single engine, such as the Defiant, and the twin engine, such as the Beau-fighter and Mosquito. Comparable American types of aircraft were used, notably the Mustang and the twin-fuselage Lightning. The Hurricanes and Spitfires were at first armed with machine-guns, but were later converted to fire cannon or guns, or both. Heavier types of fighter were used to project rocket missiles.

Generally speaking, just as radar was the first line of the warning system and the observers the second, in the interception system the fighters were the first to meet the enemy and the guns the second. Once an enemy raider had penetrated the fighter screen, he was the responsibility of Anti-Aircraft Command. This command was also divided into geographical areas, but its guns were positioned chiefly around or in large towns or important target areas. Anti-aircraft forts, largely manned by personnel of the Royal Navy, were also sited round the coast.

Instruments for the laying and firing of guns were much improved between 1939 and 1944. The biggest single step was the adaptation of radar to enable guns to be ranged and directed on to an unseen target. This device mainly replaced the expensive and often ineffective box barrage or massed shell-fire - a relic of the last war - as a defence against night raiders. It also eventually superseded the searchlight as a means of searching for high-flying raiders.

Another development was the mobile anti-aircraft gun, which made swift concentration of defensive fire possible, and the antiaircraft rocket, the cheapest and most effective form of defence against low-flying raiders.

The average heavy Anti-Aircraft battery contained up to eight guns, being connected by telephone to the gun operations room and through this to the Fighter Command operations room, itself the link with balloons and searchlights. Rocket batteries and machine-gun units were part of a much more elastic organization, centred largely on industrial targets.

Until the opening of the flying bomb attacks in June, 1944. Anti-Aircraft Command was comparatively inactive in Great Britain. Many batteries were transferred overseas, and others taken over by the Home Guard. By the autumn of 1944 Home Guard gunners were manning rocket batteries and 27 heavy gun batteries. The attacks with flying bombs necessitated a sudden revival of Anti-Aircraft activity. Batteries were massed on the coast in the known path of the bombs, and with practice and improvement in predictor control eventually achieved upwards of 80 p.c. success in hitting their targets. One day in August, 68 out of 96 flying bombs were shot down, and only four reached their target, London.

Fighter Command also experienced a great revival of defensive activity. The new Tempest fighter appeared, and achieved many successes in chasing and shooting down flying bombs. The most notable development from some points of view, however, was the success achieved by counter-bombing. For a whole year the launching sites of the flying bombs in France and Holland were attacked by bombers and fighters, more than 100,000 tons of bombs being dropped. As a result it was estimated that two-thirds of the potential air effort against London was stopped at source. Counter-bombing was also the method of defence adopted against the V2 rockets, launched chiefly from bases in Holland. This counter-bombing also brought into prominence the work of the R.A.F. photographic reconnaissance aircraft and interpreters, who first detected the well-hidden enemy launching sites.

The part played by balloons in the defence of Great Britain was unspectacular but important. They were never intended to stop bombing, but they could and did force the bombers up to heights at which - for the Germans - bombing was inaccurate and heavy Anti-Aircraft fire most effective. By the time the Allied air offensive on Germany was launched, bomb sights had become so improved that height was little handicap. (See Balloon Defences.)

From 1942 onwards air defence became a matter of more urgent concern to the Germans than to the British. A radar system similar to the British - and actually copied from it - was the backbone of the warning system. Day and night fighters were employed, as well as searchlights, balloons, and anti-aircraft guns. Anti-aircraft fire, or flak, was relied upon by the Germans to an enormous extent, particularly from heavy-calibre rail-borne guns.

The Germans developed skilful camouflage, extensive smoke screens, and cleverly built decoys and dummy buildings to a much greater extent than did Great Britain, where lack of space prevented full employment of devices of this kind. The increasing tempo and accuracy of Allied bombing forced them in the later stages of the war to practise widespread dispersal of industry and even the construction of entire factories underground. They also produced the last word in interception - the rocket fighter. None of these forms of defence, however, availed against the weight of the British and American attack.

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