The story of the allies using inflatable tanks and other military equipment to trick German reconnaissance planes into thinking the allies would land near Calais is a well known one. But how did they hide their real army from reconnaissance planes? Surely their actual tanks would have been just as visible as inflatable ones. Wouldn’t the Germans have noticed their were two army’s in south England?
They didn't. It was well known by the Germans that there was an invasion imminent they just didn't know when it would come or where it would come.
While the code name for the entire invasion effort was Operation Overlord, there were numerous other operations occurring within the larger project. One of those was Operation Fortitude. Fortitude was the code name for the deception plan intended to confuse the Germans as to when and where the invasion would occur and to convince them that it would come anywhere BUT at Normandy.
The operation was split into multiple smaller operations each with their own codename such as Fortitude North and Fortitude South. The purpose of North was to suggest to ther Germans and encourage the suggestion that the invasion target would be Norway. Phantom military formations were created in Scotland and fake radio traffic was created to create the impression that large concentrations of troops were stationed in the area.
Fortitude South was more involved as the area into which it fell was within range of German reconnaissance aircraft. The objective of South was to convince the Germans that the Pas de Calais was the invasion's target. To do this, the fake First US Army Group (FUSAG) was created and stationed in the south of England and Patton appointed to its command. Fake buildings, aircraft and vehicles were also set up and German reconnassiance aircraft were intentionally allowed to slip through the Allied fighter net to photograph those areas. This allowed the German High Command to draw their own conclusions as to where the invasion would come.
Of course, the build up of the real invasion force couldn't be entirely hidden either. Troops and vehicles could be concealed but ships could not, especiually in the numbers required for a successful invasion. In the months leading up to the invasion, Luftwaffe reconnaissance identified invasion preparations around the Portsmouth-Southampton area. By the end of April, the Luftwaffe had identified Allied shipping in such quantities that it estimated five and a half divisions could be transported across the channel. By the end of May, that estimate had risen to 16 and half divisions. It was also noted that there was little buildup on eother coast of Scotland, casting doubt that Norway would be the target. Meanwhile, German suspicions were encouraged by continuous fabricated reports by double agents working for MI5 that detailed troop movements, numbers, unit insignia and social events around the South-East of England, further strengthening the belief that the Pas de Calais was the real target.
Sources: GAf (German Air Force, Luftwaffe] and the Invasion of Normandy - 1944 by Oberst Walter Gaul
Fortitude: The D-Day Deception Campaign by Roger Hesketh