Allied Air Reconnaissance and the D-Day Landings

The key to the successful Allied landings in Normandy on 6 June 1944 was information superiority. Where tactical intelligence was concerned, the Allies possessed a decisive advantage derived overwhelmingly from air reconnaissance. The following account of pre-Overlord air reconnaissance activity is largely based on narratives produced by the Air Historical Branch (RAF) after the war.

The intelligence collection and exploitation task that supported Operation Overlord was the primary focus for Allied intelligence over more than two years, and not source of tactical intelligence was more important than air reconnaissance. At first, the accumulation of air imagery occurred less through the medium of special sorties with set objectives than via the thousands of routine sorties flown over northwest Europe. Almost every photograph taken during 1942 and 1943 revealed key details required by the invasion planners.

It was not until 1944 that the air reconnaissance requirements for Overlord were placed under a SHAEF Reconnaissance Committee (G2 Division), which worked through the inter-service Air Reconnaissance Intelligence Section at Headquarters AEAF to apportion air effort, coordinate and approve requirements. As far as the RAF was concerned, 34 Wing was placed directly under the control of the AEAF and was later transferred to Second Tactical Air Force (TAF). No 39 Wing operated under 83 Group, working with Second British Army, while 35 Wing came under 84 Group supporting First Canadian Army. USAAF reconnaissance resources within IXth Air Force included one Photographic Reconnaissance Group and one Tactical Reconnaissance Group. Strategic requirements beyond the resources of the AEAF squadrons had to be requested through Air Ministry Intelligence, and were then allocated to the 106 Group squadrons at RAF Benson or the VIIIth Air Force’s reconnaissance squadrons (7th Photo Group). However, in May, a new strategic reconnaissance tasking body known as the Joint Photographic Reconnaissance Committee (JPRC) was created.

A Spitfire XI of 541 Squadron, RAF Benson.

Danesfield House, otherwise known as RAF Medmenham,
where the Allied Central Interpretation Unit was based.

The First Phase interpretation section at RAF Benson.

Early reconnaissance tasking for the Army (originally under GHQ Home Forces) was undertaken by 140 (Army Co-operation) Squadron and included requirements to photograph beach defences and gradients stretching from Calais to Cherbourg and round to the Brest Peninsula as far as the mouth of the Loire, and to provide general imagery cover for the revision of military mapping right across northwest France and Belgium. The photo-reconnaissance squadrons based at RAF Benson also contributed to these tasks. The Army Photographic Intelligence Section was responsible for interpreting and collating the steady stream of imagery to keep the Chiefs of Staff and the various planning staffs updated on the development of German defences.

In May 1942, GHQ Home Forces established a special interpretation section to extract all the information obtainable from air imagery of a strip of the continent 30 miles wide, extending from Den Helder in The Netherlands to the Spanish frontier. In the summer of 1943, their work became more directly connected with the invasion plans, and included the preparation of oblique panoramas of the French Channel coast. Employing imagery from distant and close oblique sorties, they constructed mosaics of the coast annotated with any information likely to be useful the planning staffs, and to Allied forces approaching from sea or air. The topography of cliffs and beaches, heights, gradients, beach exits and footpaths were a few of the details included.

In March 1943, a separate detachment was established at RAF Benson to interpret and monitor German rail and road movement in northern France. Their intelligence products were used to inform high-level appreciations of enemy troop movements and their location at Benson would later help to guide the Allied air interdiction programme - the isolation of the Normandy battle area - with the minimum of delay.

The Army Photographic Interpretation Section was transferred to 21st Army Group in July 1943, and in the following December was attached to the Theatre Intelligence Section of COSSAC (later of SHAEF). It was during this period - in August - that Normandy's Calvados coast was selected as the Overlord landing area, and the Allies were subsequently able to concentrate their collection and analysis efforts to a much greater extent than before, and to map the future theatre of operations in ever greater detail. Right up to the end of May 1944, complete sets of maps and traces were produced and continuously updated, drawing on the very latest air reconnaissance photographs.

Detailed mapping was constructed from air imagery, in this
case showing Strongpoints 'Hillman' and 'Morris' south of
Sword beach; anti-glider obstacles appear as tiny white
dots near both strongpoints.

High-level vertical and low-level oblique imagery was used 
to map the German defences in microscopic detail for the 
Allied landing forces.

One of the earliest participants in the Overlord task was the Model Section of the Allied Central Interpretation Unit (ACIU, based at RAF Medmenham). As early as June 1942, when the pre-Overlord plans known as ‘Round-Up’ were being prepared, the planners requested large-scale models of the continent, extending a considerable distance north and south of the possible areas of attack. The models of the coast were to show contour relief on the seaward side down to low-water level. All models required for the early planning stages were completed by November 1942 and illustrated different areas of the coast from Dunkirk to the Gironde. No 140 Squadron and the Benson PR squadrons shared the task of collecting the necessary imagery and, as the commitments grew, the accommodation demands at Medmenham for making and storing the models became enormous. In spite of the intervention of requirements for such operations as Dieppe, North Africa and Sicily, the work of the Model Section continued apace. A number of American model makers joined the section at the end of 1943 and helped to produce models of the American landing beaches.

Three basic types of model were produced:

1. Those of large areas of Northern France showing communications, built-up areas and rivers. These were produced at a vertical scale of 1/12,500 or 1/6,250 and 32 originals of this kind were made, each measuring 5 x 3 ft. Plaster copies were then produced and sent to the various formations concerned.

2. Those of the coastline areas up to a depth of 12 miles inland. Those of selected operational areas were made on the scale of 1/50,000 and showed land and surface variations, topographical features in detail, defences, tide lines and communications; they were in fact complete replicas in miniature of the actual locations. In all, 63 of these model sections were made, each approximately 16 ft square; from them, 186 copies were constructed and dispatched to the planning staffs.

3. Those used for to plan the airborne landings. Two of these models were made to a scale of 1/900 and included the most minute detail. The air approach to the airborne objectives, notably Pegasus Bridge, was simulated by moving film cameras over the models.

Many models were also made for use after D-Day, but those employed for planning and briefing the landings on 6 June made a very real contribution to the Allied victory.

Photos 1 and 2 show models rather than the actual landscape
around the Merville Battery, shown in photo 3, illustrating the 
extraordinary detail and sophistication of the pre-Overlord 
modelling effort.

In June 1942, the Allies began the task of compiling a comprehensive library of photographs of France, Belgium and Holland. Sorties over the continent became more and more frequent, increasing from an average of 310 per month in December 1943 to 1,085 per month by May 1944. The reconnaissance aircraft under the AEAF flew 3,215 sorties between 1 April and 5 June 1944, while 106 Group and the American squadrons flew 1,519 Overlord sorties in the same period. One RAF Mobile Field Photographic Section alone made more than 120,000 prints for the Army in a period of just two weeks before D-Day.

It is worth bearing in mind that this immense Allied collection effort was conducted overwhelmingly by lone, unarmed aircraft, which were required to fulfil their tasking in hostile airspace protected by an integrated air defence system. While speed and high altitude were usually enough to keep them beyond the reach of enemy fighters, missions had often to be flown at medium and low altitudes where the air-to-air or ground-to-air interception risk was much greater. Low-level missions were referred to as 'dicing' for the simple reason that they involved dicing with death.

Radar plots of reconnaissance missions were easily distinguishable from other Allied air activity, and the Germans were known to monitor reconnaissance plots very closely in the search for intelligence on future Allied intentions. For this reason, the Allied planners divided France and the Low Countries into six regions and maintained a weekly record to monitor the number of sorties flown over the actual Overlord area in comparison to the rest of the northwest European coast. This record was used to balance air reconnaissance cover evenly across the six regions and so prevent German suspicions from being aroused by a disproportionate amount of flying over any particular area. Such was the importance attached to operational security that requests for reconnaissance cover of key Overlord objectives were occasionally rejected because the resources needed to fly 'cover' missions over other areas were unavailable.

One of the early tasks undertaken by the Army interpreters at the ACIU involved studying and plotting all enemy batteries in the coastal areas, and detailed studies of selected batteries were completed in October 1942. From these studies, it was possible to report enemy activity and identify correctly all the defence building proceeding on the Channel coast in connection with light and medium coastal guns. As D-Day approached, immediate interpretation reports were issued to record all changes to batteries and give estimates of the amount of damage inflicted by Allied bombing. A check on enemy coastal artillery conducted later in 1944 showed that photographic interpretation had succeeded in identifying 90 per cent of the batteries constructed, the remaining 10 per cent being mainly mobile flak batteries.

A low-level oblique showing the progress of construction 
at Merville.

The impact of Allied bombing at the Pointe du Hoc battery.

Another sphere of imagery collection focused on German underwater obstacles, which were deployed in ever-increasing numbers off potential invasion beaches in 1943 and the first half of 1944. Identifying and plotting these obstacles proved to be another exacting task. During the Dieppe raid in 1942, several landing craft had run into steel stakes driven into the sand below the water line. In the spring of 1943, ground intelligence revealed that similar obstacles were being deployed at a number of coastal resorts, and oblique photographs taken by a 541 Squadron Spitfire on 12 May 1943 showed curved rails off the shore at Quineville. A meticulous air survey subsequently revealed no further underwater obstacles off the rest of the coast of occupied France, and it was later established that Quineville was an experimental area.

In early 1944, Second TAF’s 35 Wing photographed the sea bed off the Normandy coast, and imagery obtained on 20 February showed that the work of laying underwater obstacles had commenced off the landing beaches. From then on, photographic cover of the beaches was continuously maintained to watch German progress, and this helped the Allies to observe a further development: the Germans were attaching mines to the obstacles. During an Allied air attack on a coastal battery, one aircraft dropped a stray bomb in the sea 700 yards off shore, and the strike photographs taken at the same time showed a series of 14 small explosions. When the photographs were taken, the obstacles were covered by the tide, but when they were plotted and compared with other imagery of the area, they showed that the explosions had occurred within the depth of the belt of obstacles.

During April and May, American PR Lightnings with nose-mounted cameras operated repeatedly along the Normandy beaches, flying only feet from the ground along the lines of obstacles to reveal the various types, how they were mined and the methods of erection. New moving film cameras were also used in the final weeks before the Allied landings to obtain large-scale photographs of the obstacles from 6,000 feet. Daily sorties were flown over the beaches before D-Day, and the latest information was obtained and issued to the various army headquarters right up to the evening of 5 June.

Low-level air reconnaissance of German beach obstacles 
allowed the Allies to understand their design and monitor 
the progress of construction.

Forward-facing camera installation on a Spitfire for taking
oblique photographs at low level; the risk involved in these
missions is underlined by the visible flak damage.

Air reconnaissance missions also captured imagery of German anti-air landing obstacles. Until the beginning of 1944, photographs of Normandy showed very few areas of open ground obstructed by poles, but Allied interpreters noticed in March that an abnormal amount of tree felling was taking place in the Overlord area, and it was assumed that a programme of obstruction against glider landings was about to start. Careful notes were prepared of all the areas so obstructed, but the obstacles did not impede the Allied airborne landings on 6 June.

The multiple tasks of photographing the German defences, minefields, anti-tank obstacles, anti-tank guns, observation posts, and enemy troop movements were undertaken for Allied ground forces. However, the combined PR organisations of 106 Group, the US PR Wing and Second TAF’s PR units executed a number of additional missions of more specific interest to the air forces. These included photography of the Luftwaffe on French airfields to maintain an accurate picture of potential German air strength, and of airfields immediately after Allied air attacks for damage assessment purposes.  They also used survey cameras to photograph 91 small areas in Normandy so that sites for future Allied airfields could be selected after the ground had been captured. This involved flying over the areas of interest at 9,000 feet, despite the increased risks involved. The task had been completed without loss by April 1944, mainly by 34 Wing's Mosquitos.

Thirdly, immediately before D-Day, the USAAF Wing photographed bridges over the Loire, which involved a considerable amount of low-level flying but provided invaluable information to the Allied bomber forces about the progress of their campaign to sever German road and rail communications across northern France.

Air reconnaissance supported the interdiction campaign,
providing target intelligence and imagery for post-strike
analysis.

Lastly – perhaps the most important task of all – the PR organisation played a crucial role supporting the neutralisation of the German radar system in the invasion area. As a result of two and a half years of research by the Radio and Radar Section at the ACIU, a full picture of the German radar network had been prepared, and it presented a truly formidable problem. The Allies decided that an essential feature of the preparations for the invasion should be a concentrated onslaught on all the radar facilities within a prescribed area around the proposed landing beaches, and the length of the coast selected stretched for 450 miles from the Franco-Belgian frontier to Cap Frechel in Brittany. On this strip there were nearly 100 radar stations, most of which comprised several separate installations. It was calculated that there was one radar instrument for every four and a half miles of coastline over the whole range, and every one of these had to be silenced if the security of the Allied landings was to be ensured.

On 1 March, the ACIU began the task of producing target material on these installations suitable for briefing pilots to execute the actual attacks. Much depended on the thoroughness of their work, for if any active installation, however small, was overlooked, the security of the whole Overlord operation might have been jeopardised. By 1 April, after particularly intensive efforts, a full report was ready. No fewer than 253 copies were prepared, each of 300 pages (76,000 sheets). The result was known as ‘Rhubarb’ Operations, Appendix XII. To guide the pilots who were to attack the German radar network, this document provided full descriptions of each target, three types of selected and annotated photographs and large-scale plans of each single piece of apparatus, small-scale maps showing their position and a brief description of their functions.

The RAF then conducted an experiment to find out whether rockets would be effective in damaging radar installations, and an enemy radar station in The Netherlands was selected as the target. After the attack, conventional oblique photographs failed to show how much damage had been inflicted, and the ACIU therefore briefed one of Second TAF’s Mustang squadrons (168 Squadron) to collect further oblique imagery at very close range. A sortie flown for this purpose on 16 May took photographs confirming that the attack had been completely successful.

On 22 May, the RAF began a systematic campaign against the German radar network along the northern French coast. After each attack, PR aircraft took oblique photographs for damage assessment, and further high-level imagery was captured after 48 hours and again at intervals of three to five days. Repairs and replacements were closely monitored to ensure that no installation had been returned to operational status immediately before D-Day. On 6 June, only five out of 98 installations appeared to function at all. The Allied counter-radar effort had been completely successful, and it allowed the vast invasion fleets to rendezvous 24 hours before the landings and launch the assault without detection or interference in any form. At least some of the credit for this achievement belonged to the Allied photographic reconnaissance squadrons and the supporting interpretation organisation.

Again, targeting and post-strike imagery; many of the radar,
signals and intelligence targets attacked by the Allies
consisted of multiple installations.

Destroyed German radar at Arromanches.

Coastal Radar Imagery

Apart from maintaining the air reconnaissance effort over the French coast in support of Overlord, the Allied PR squadrons also assisted deception activity against German reconnaissance aircraft. At a number of ports along the southeast coast, the Allies deployed dummy landing craft to support broader deception operations, which aimed to convince the Germans that the landings would be staged in the Pas de Calais rather than Normandy. Allied PR aircraft then photographed the landing craft from the air, and the interpreters listed details that might disclose that they were dummies, when compared with photos of real craft. All necessary steps were then taken to make the dummies indistinguishable from the real landing craft. Air photography of concentrations and dispersals of aircraft, armoured vehicles, army transport and similar objects was also used to assist effective camouflage and concealment.

However, of greater importance in concealing Allied intentions from German eyes was combat air activity and patrolling. The effort expended on preventing German air reconnaissance along the south coast of England ranks as one of the forgotten but crucially important factors in Overlord’s success. The Allied air forces reverted to the tactics of the First World War, mounting constant standing patrols over the channel, which required thousands of monotonous fighter sorties in the months before the landings. Offensive targeting of Luftwaffe airfields in France also took a heavy toll on their reconnaissance aircraft. On the ground and in the air, Luftflotte 3 – the Luftwaffe command responsible for France and the Low Countries, lost 38 reconnaissance aircraft destroyed during the first five months of 1944; several more aircraft were damaged and there were the inevitable non-operational losses too.

And then there was the deterrent effect of the Allied air defences. This cannot be quantified, but the ever-present threat posed by Allied fighters over the channel would have severely curtailed German ambitions, and the prospect of interception doubtless also caused many reconnaissance missions to be aborted in the air. For all these reasons, collection over the southern English ports where the invasion fleets assembled was extremely limited, and the quality of the imagery captured was invariably low. It produced no evidence that came close to correcting German expectations regarding the location or timing of Overlord.