With two entire US Airborne divisions and numerous Commonwealth detachments parachuting into Normandy in the small hours of June 6th, did the Germans catch on to the fact that a much larger invasion was to succeed it or did they just treat the airborne phase as an isolated situation until the main invasion force arrived a few hours later?
I know the plan with Overlord in general was to try and catch the Germans relatively unawares, but I wonder how much of the element of surprise was lost when the paratroopers flew in, given that it was a good four hours before the amphibious assaults began.
The Western Allies employed massive deception actions in order to distract the Germans on D Day. One of the key components was operation Titanic. Titanic involved dropping hundreds of dummy paratroops away from Normandy to hopefully draw German forces away from beaches.
Some of the dummies had fireworks attached to simulate gunfire. Others had explosives that would sound like mortars, and would destroy the dummy, leaving just the chute behind for the Germans to find. The British even sent some actual SAS troops who played recordings of gunfire and of men talking to make it sound like thousands of men were landing. Most of the SAS men were killed.
The result was everything the Allies could have hoped for. The Germans were put on alert when they started receiving reports of paratroops. But general Hans Speidel actually ordered the alert canceled for his men when they found dummies. General Rundstedt ordered the 12th SS panzer division to Lisieux to deal with a theoretical landing there. And the 352 reserve division was ordered away from the Omaha and Gold beaches and the actual 101st Airborne drop zones in order to search the woods and bocage area to the south, for what proved to be dummy paratroops. The fact that the actual paratroop drops were screwed up actually added to the confusion. The German commanders were receiving reports all along the coasts, and for some distance inland, of paratroops everywhere. And what they kept finding was dummies.
In addition the Allies launched a massive air campaign. Over 14,000 sorties were flown by the Allies from dusk June 5th to dusk June 6th. And a lot of these sorties were well away from Normandy. In addition to just raw deception, they were also targeting transportation and communication links. Bombers and fighters were attacking bridges, rail stock, radio towers and more.
The French also started a major sabotage campaign on the night of the fifth. Partisans were out targeting the same kinds of things as the air forces were. In the dark it is hard to tell the difference between a partisan and a paratroop. And it is even harder to communicate it back to your commander when the radio tower has been knocked down and the phone line has been cut.
Mary Barbier's D-day Deception covers the deception operations including Titanic quite well.