How the amazing aviation innovations behind D-Day laid the foundations for modern air traffic control

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 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 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.

The plan for deception operations, including the Taxable and Glimmer ‘fake fleets’, and Mandrel jamming aircraft. Operation Titanic involved dropping half size mannequins behind enemy lines in northern France to create more confusion. Each ‘Rupert’, as they were called, included loudspeakers playing looped recordings of battle sounds, and firecrackers which would detonate randomly. Picture Credit: MoD, DEFE2/502

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 21^st Century.

An OBOE/9000 Ground Radar Station (Art.IWM ART LD 5780) image: a radar station with two metal frame mounted radars amidst a series of nissen huts surrounded by red brick walls. Copyright: © IWM.

Under the protection of the electronic countermeasures supplied by Mandrel, the first of the 1,200 transport aircraft and 150 gliders took off at 2256 on the 5 June. These were the six Handley-Page Halifax bombers towing Horsa gliders from Tarrant Rushton, undertaking Operation Deadstick. In these gliders were D Company , 2^nd Battalion of the Oxfordshire and Buckinghamshire Light Infantry Regiment, who were to land next to, and seize, the bridges over the Caen canal and Orme river, on the eastern flank of the invasion area.

The routes these aircraft, and those that followed, were meticulously planned to ensure deconfliction from any other aircraft flying that night. Many of the leading aircraft made use of what was called ‘Eureka/Rebecca’. This was another form of secondary radar, where a radar pulse was received and then a reply was sent back. The ground station, named ‘Eureka’ after the Greek for ‘I have found it!’ would receive a pulse from the aircraft, and send a reply on a separate frequency. This was received by ‘Rebecca’, fitted in the aircraft, allowing the relative position of aircraft to the ‘Eureka’ beacon to be displayed (on a more pragmatic level, ‘Rebecca’ was so named through a contraction of the phrase ‘Recognition of Beacons’).

Ships fitted with ‘Eureka’ were moored in the middle of the Channel, providing guidance to allow the aircraft to follow their designated routes over the sea. The first soldiers to drop, known as pathfinders, carried their own ‘Eureka’ beacons which they would, upon landing, set up to guide the main force of transport aircraft to their drop zones. ‘Eureka’ was later developed into the Beacon Approach Beam System to permit aircraft to land in poor weather, a precursor to the Instrument Landing System we still use at airports today.

Other vessels performed other, vital roles. Three especially valuable ships sailed with the invasion fleet that night. Fighter Direction Tenders had been developed to detect enemy aircraft, and direct the Allied fighter response until land-based facilities could be set up. Displacing nearly 4000t and roughly comparable to the size of a contemporary Royal Navy Destroyer, they carried 180 RAF personnel, in addition to the Royal Navy crew of 100. They provided all elements of the system that could be found within the British air defence network, including the customary filter rooms and air control rooms known from many war films and newsreels. The current capability residing with the RAF’s No.1 Air Control Centre can trace its lineage in deployable air surveillance and control to these early pioneers.

At 0016, the first Horsa glider, piloted by Sgt. Jim Wallwork, touched down metres away from the Caen canal bridge. Just five minutes later, both bridges were in Allied hands and were successfully held until relieved by airborne reinforcements later that morning, and Commandos from the invasion beaches in the afternoon. These are now known as Pegasus Bridge and Horsa Bridge, named after the cap badge of the Parachute Regiment and the gliders that landed the troops.

Though the fighting was fierce, and many lost their lives, the efforts of those scientists, inventors, planners and military personnel enabled Operation Overlord to succeed, paving the way for the liberation of Western Europe.

The great pace of technological advancement during the Second World War took us into the electronic age, and as we reflect on the events of 75 years ago, and those lost on both sides, it is also worth remembering  that many of those inventions, borne out of desperation and necessity, subsequently went on  to transform how we travel and help make flying the safest  way to travel.