The success of Overlord was in part dependent on detailed topographical map information about the beaches and coastal towns along the French coast. British experience of Galipolli in the First World War, with the loss of 100,000 dead or wounded troops, meant that detail was necessary to ensure the invading army did not get stuck on the beach.
Aerial photographs helped identify likely locations but, to obtain more detailed views, the Government asked the BBC to appeal for holiday photographs and picture postcards of unspecified coastal areas of France.
However, as it was known that the beaches were in parts underpinned by ancient forests which had turned into peat bogs before becoming submerged, much more detailed information on the target beaches and their approaches was required. Local conditions such as the composition of the beaches, hidden underwater banks, German defensive obstacles, depth of water, tidal conditions etc. would all be taken into account in the planning of the project.
On 16 January 1944, X20 planned to spend four days off the French coast. Commanded by Lt K.R. Hudspeth DSC* RANVR and Sub. Lt. B. Enzer RNVR, with the COPP (Combined Ops Pilotage Party) comprising Lt. Cdr. Nigel Willmott DSO DSC RN, and the same divers, Major Scott-Bowden and Sergeant Ogden-Smith.
X25 X-class submarine, the type used
During the daytime, periscope-level reconnaissance of the shoreline and echo-soundings were performed. Each night, X20 would approach the beach, and the divers would swim ashore to take soil samples, collected this time in condoms.
The divers went ashore on two nights to survey the beaches at Vierville-sur-Mer, Moulins St Laurent and Colleville-sur-Mer in what became the American Omaha. On the third night, they were due to go ashore off the Orne Estuary (Sword), but by this stage fatigue (the crew and divers had been living on little more than benzedrine tablets) and the worsening weather caused Hudspeth to shorten the operation, returning to HMS Dolphin on 21 January 1944.
At Cairnryan, just north of Stranraer, south west Scotland, a "life size" reproduction of the beaches was constructed. This allowed the planners to assess the effectiveness of the current landing techniques and the movement of men and machinery over the terrain.
