Supermarine Spitfire PR.XI
The Mark XI PR Spitfire relied on speed and agility for protection. Travelling at 400 miles per hour, the aircraft, which flashed low over the Merville Battery at 1,000 feet, had been ‘cottonized’. By stripping out their guns and radios, the weight of their airframes had been reduced to allow them to carry a configuration of photographic reconnaissance equipment. The sortie that flew over Rommel and his party were fitted with F.24 cameras with fourteen-inch focal lenses, which enabled them to take low-level oblique-angle photos. The aircraft could also carry F.52 cameras with larger lenses to take vertical pictures from altitudes of up to 30,000 feet, but the mission they flew over the Merville Battery required greater detail and that meant going in low.
Each Spitfire carried one sideways-looking F.24 mounted in a porthole behind the cockpit on the port side of the fuselage. Producing a five-by-five-inch negative, each exposure provided a coverage of 1,667 by 1,667 yards of ground. Once enlarged, the negatives produced a 1:12,000 scale image, with sufficient detail to pick out a single man, or a group of men running for cover. But it wasn’t just the scale that was important. Two additional F.24s, with smaller five-inch lenses, were fitted under the wings and angled towards each other so they could take overlapping photos of the same target. When consecutive photos were viewed with a stereoscope, they gave a three-dimensional effect, akin to looking at a still from a modern film with 3-D glasses.
Achieving the 3-D effect depended on the interval of exposures between frames being matched against the speed of the aircraft. Consequently, flying a low-level ‘dicing’ mission to get the necessary detail was a tricky and intricate business that required skill and entailed risk, as, lacking height, the pilots were more vulnerable to enemy aircraft above them and flak below them.
To capture the right image, the pilots’ navigation had to be spot-on; even a small deviation from the pre-planned flight path could lead to missing the target by several hundred yards. Flying close to the ground at high speed, the lead pilot had little time to line up his aircraft and aim the camera by aligning a tiny black cross etched on the blister of his bubble canopy with a small black strip painted on his aileron. At the same time he had to mentally calculate his speed and adjust the camera control box on his joystick to ensure it was set at the right exposure to start taking the pictures.
Trusting that his wingman was with him, the aviator glanced at his airspeed dial, checked his bearing and then focused on the crosshair and target alignment. He thumbed the camera control switch as the terrain of green fields and hedgerows flashed past beneath him; at the moment when all three points of reference lined up, he flicked the switch on his control column and the cameras started taking photographs at one-second intervals.
Surprise was also an important part of the aircraft’s protection and the pilot put more faith in it than in the single sheet of armoured plating in the back of his seat. He knew he had to get the alignment right first time. There would be no second chance. He had to bounce the target and get in and out fast, before the anti-aircraft gunners had time to react and fill the air around him with flak. If he missed the target, or his photos were not of the right quality, he would not be allowed to revisit the target for several days.
The pilot in the lead Spitfire was concentrating too hard to realize that the gunners of the single 20mm 38 Flak gun at the battery had failed to get their cannon into action against him or his wingman. He hoped he had got what he wanted as he flicked off the camera control switch and pulled back aggressively on his joystick to begin climbing hard for altitude.
The danger was far from over. The pair of reconnaissance aircraft may have been too fast for the crew of the 20mm gun mounted on the cookhouse bunker of the battery, but they still had to run the gauntlet of the nearby anti-aircraft positions stationed along the coast if they were get home safely with the information they had captured. By now the Germans were alerted to the presence of enemy fighters in the area and thick black puffs of exploding flak and tracer marked the air as the Spitfires pulled up steeply to reach a height of 5,000 feet to give them a chance of evading enemy fire as they crossed the coast. This is where the two-stage supercharged Merlin 60 engine, with its excellent climb rate, did its business.
The engines screamed for power, calling on the maximum performance of their 1,560-brake horsepower to get their aircraft out of trouble. Both pilots fought against the crushing effects of the G-force, as the horizon dropped away below them and their airframes surged upwards. They were not out of it yet. As the blood began to drain to their lower extremities, they struggled to remain focused. The need to keep scanning the skies above them and the rear-view mirror for enemy aircraft was paramount, as was checking that the aircraft’s fuel gauges, oil pressure and heading were all still good.
Within an hour of clearing the coast the two aircraft were back at their base at RAF Benson in Oxfordshire. As their props feathered and wound down at their stands on the apron of the airfield, the ground crews of the squadron’s photographic section were already waiting to meet them. While the crews switched off their engines, unstrapped and climbed out of their cockpits to head for debriefing by the intelligence officer in the operations room, the magazines of the cameras were being unloaded. The film was taken to a requisitioned manor house in the neighbouring village of Ewelme, where it could be developed. Within forty-eight hours the negatives had arrived at RAF Medmenham in Buckinghamshire for detailed analysis.
Medmenham was a sister organization of Bletchley Park and specialized in photographic intelligence and interpretation. Much of the work they conducted was in 3-D using a stereoscope. The stereoscope was a bi-optical viewing device, akin to a pair of magnifying spectacles mounted on a small four-legged metal frame. When positioned over an overlapping air photograph, it gave the 3-D image. Being able to view captured images in three dimensions was crucial, because it brought the enemy landscape and installations they studied to life, and allowed the interpreters to examine features of an object, such as the angles of shadows, to make assessments about the height of a particular structure or weapon types. It was a monotonous and painstaking task, but the photographic intelligence work at Medmenham was instrumental to the planning of D-Day in assessing enemy dispositions, strengths and capabilities.
In the run-up to the invasion, the teams of interpreters at Medmenham would study and file reports on 16 million photographs of enemy-occupied territory. It was an immense undertaking and the interpreters worked round the clock to provide those charged with planning D-Day with vital information. The majority of effort was concentrated along the coastline between Calais and Cherbourg, but no one area received particular attention so as not to give away the intended location of the invasion.
The stereoscope was Medmenham’s secret weapon; while the British and Americans worked in three rather than two dimensions, the Germans did not. Scouring the prints of the Merville Battery for every detail, the interpreters could give an appraisal of the progress of the casemates’ erection, noting that two had been completed and that two remained under construction. They could also provide an estimate of the thickness of the concrete protection, pick out the detail of perimeter defences and compare the progress of the work against later photographs. This was all important information, but the study of the photographs could not confirm the calibre of the guns at Merville. It was a vital piece of missing detail, as the position of the battery and the frantic work being conducted to improve its defences were of profound concern to the man responsible for planning the Allied invasion of France.
By January 1944, Lieutenant General Sir Frederick Morgan had been working on the planning for D-Day for several months. At a meeting in Washington in May 1943 the Combined British and American Chiefs of Staff had taken the final decision to invade Nazi-occupied north-west Europe and had set a provisional date of May 1944. Morgan had been selected as the Chief of Staff Supreme Allied Command. As COSSAC, he and a small team of Anglo-American officers were responsible for the joint planning of the largest, most complex combined arms operation ever to be mounted. They had decided to codename it Operation Overlord and had also decided that it would take place in Normandy.
COSSAC’s detailed planning had identified the advantages of landing in Calais, but their work also confirmed that it was heavily defended. The disaster of Dieppe highlighted the need to avoid landing in areas of main enemy troop concentrations and had been reinforced by the experience of Allied landings in Sicily and Italy. German troop dispositions in Normandy were more thinly spread and could be more readily isolated by bombing the bridges over the Seine, which the bulk of German reinforcing units would have to pass across. Intensive reconnaissance and intelligence analysis had also confirmed that the gently sloping beaches along the Cotentin Peninsula were suitable for a landing and were within the range of Allied fighter cover. Additionally, the terrain and inland road network were suitable for the logistic build-up of a beachhead and subsequent breakout towards Paris.
While Normandy offered clear advantages to Morgan and his planners, it also had its problems. One was a matter of logistics. Cherbourg was the nearest major port most suitable for the subsequent logistical build-up behind the main beach landing, but its heavy defences precluded a direct assault from the sea. This was overcome by the simple, but ingenious decision to take a port with them in the form of the Mulberry portable harbours and by laying fuel pipelines under the Channel. But the issue of neutralizing the gun battery at Merville was an altogether thornier problem, which could not be overcome by the application of physics and science alone.
On the other side of the River Orne from the Merville Battery lay the small seaside resort of Ouistreham, where the mouth of the river flows into the Bay of the Seine and the beaches of Normandy start their long curve west along the flat shelving shoreline of the Cotentin Peninsula towards Cherbourg. Codenamed Sword by the Allied planners, the beach at Ouistreham was at the eastern end of the Allies’ chosen landing area. It was also where the left-hand British assault division would touch down on the morning of D-Day.
Given their position, the guns at Merville were capable of sweeping the entire length of the beach with artillery fire. Drawing on the lessons from Dieppe, where British and Canadian troops had been caught out in the open shingle as they disembarked from their landing craft, Morgan and his team were in no doubt of the devastation that a well-defended battery could wreak on troops struggling to get ashore on Sword. Additionally, the position of the guns meant that they would be capable of engaging ships out to sea as well as the slow-moving landing craft as they ran into the beach.
Although it had not been confirmed by photo reconnaissance, the COSSAC planners suspected that the guns were 150mm-calibre field howitzers. While none of the artillery pieces had been captured on camera, when viewed under a stereoscope the shape and shadow at the rear of the casemates indicated that large entrances were being constructed. If they were naval ordnance, the guns would have been bolted permanently inside the bunkers and would have no need of large rear entrances. Consequently, photographic interpretation suggested that the casemates were designed specifically to take field guns, which could be manhandled in and out of the concrete shelters.
Given the extraordinary lengths the Germans were going to in order to protect the guns, it was logical for the planners to deduce that they would be one of the heavier and more valuable Wehrmacht field types. The largest standard field gun the Wehrmacht possessed was schwere or ‘heavy’ Feldhaubitze 18. It had a calibre of 150mm and could hurl a shell weighing sixty pounds over ten miles, a distance that brought every inch of Sword Beach within range and meant that vessels could be engaged several miles out at sea.
While the Allies placed a heavy emphasis on PR aircraft to gain intelligence on the Atlantic Wall defences, they were not their only source of information on the preparations being made on the other side of the Channel. The build-up of German troops at the beginning of 1944 and the frenetic building activity had not gone unnoticed by the French Resistance. Eugène Meslin was the Vichy government’s chief engineer in Caen and was responsible for handling relations with the Todt Organisation. Meslin was also the head of the Resistance’s intelligence section at their western headquarters based in the city and his job meant that he was ideally placed to conduct the principal Resistance task of spying on the German coastal defences and reporting on their progress to the Allies. Through his network of fellow engineers and artisans working for the Germans on the defences, the details of every pill-box, wire entanglement and gun emplacement were being reported back to London by Meslin’s outfit.
Louis Bourdet was a member of Meslin’s network and had been subcontracted to work on the gun position at Merville. When completing electrical work at the battery he had managed to slip his hand into the mouth of one of the guns. Once his shift had finished, Bourdet raced home and measured the span of his hand with the aid of a piece of paper and a ruler. The ruler showed 120mm and the information was duly fed back to London. The Germans did not possess 120mm-calibre field guns, but the measurement of Bourdet’s hand suggested that the guns in the casemates were definitely larger than the Wehrmacht’s other standard-issue field howitzer, the smaller leichte or ‘light’ Feldhaubitze 18, which had a calibre of 105mm.