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The aviation innovations behind D Day and how they have influenced modern air traffic control

06 June 2019   (0 Comments)
Posted by: Thomas Wakelin
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On Friday 24 May 2019 more than 8,700 flights were handled in UK airspace in what is likely to be our busiest day of the year.

Highly specialised and experienced controllers, assistants, engineers, analysts, software developers and many other people, possessing more than 10,000 years of combined ATC experience, all worked together with sophisticated electronic aids in air traffic control operations rooms, control towers, remote radar sites, communications links and the wider aviation system to ensure this was a ‘normal’, if busy, day.

In contrast, on the 6 June 1944, there were 14,674 flights handled with nothing more than flare guns and signal lamps, paper and pencils, blackboard and chalk, rudimentary radio navigation and an enormous amount of pre-planning, hard work, and determination driven by necessity.

Operation Overlord, the invasion of Normandy, arguably remains one of the most complex tasks ever devised. The statistics of those 24 hours are astounding: 160,000 troops (of which 24,000 landed by glider or parachute), 5,000 vessels and 11,000 individual aircraft took part.

While obviously bearing no relation to each other, today’s air traffic operation can trace some roots back to that day and to similar operations in preceding years.

By 1944, radar, used to great effect by the RAF in the Battle of Britain, had advanced in sophistication, as had an understanding of its application and countermeasures. In the early hours of the 6 June, as the first pathfinder paratroops were jumping out over Normandy, heavy bombers of Nos. 617 and 218 Squadrons RAF were flying race-track patterns over the Channel near Calais and north of the Seine estuary, gradually creeping closer to the coast of France on every circuit. At precise intervals, the crew of each aircraft would throw bundles of aluminium foil strips now known as ‘chaff’ out of the aircraft. These strips, then called ‘Window’, were cut to the wavelength of German coastal radar stations, and would, given the aggregate movement towards France, resemble invasion fleets on their radar screens. This was an effort to sow confusion as to the actual target of the invasion, and perhaps to persuade the German High Command that any troops and fighting reported in Normandy would initially be dismissed as a diversion.

Radar’s susceptibility to interference was also exploited. At the same time, sixteen Short Stirling and four B-17 Flying Fortress bombers were airborne, forming a line from Portland in the west, to Brighton to the east. These aircraft were equipped with radar-jamming equipment called ‘Mandrel’, and drew an electronic veil over the real invasion fleet. None of the long-range German radar stations in Belgium and France could see through to pick up the tracks of the 1,200 transport aircraft ferrying parachute troops, or the 5,000 ships and landing craft, to Normandy. There were, however, carefully planned, intentional holes in this veil, in order to allow those same German radars to detect the ‘fake’ invasion fleet being created off Calais.

Many aircraft that night were using radio navigation systems. ‘Oboe’ was a forerunner of today’s SSR (secondary surveillance radar) where a ground station broadcast was then received by an onboard transponder which then transmitted a reply. The time difference between the broadcast and the reply being received allowed the aircraft position to be determined. The ‘Gee-H’ navigation system was also in use. This was the world’s first hyperbolic navigation system, where the aircraft would send a radio pulse and then measure the time difference between replying signals from various ground stations. The principles of ‘Gee’ were developed by the USA into LORAN for the war against Japan, becoming a navigation aid more suited to the vast distances over the Pacific Ocean. LORAN was then continually modified and served many users, including civil aviation well into the 21st Century.

Read more about this on the NATS website