According to [this article] (http://edition.cnn.com/2014/06/05/opinion/opinion-d-day-myth-reality/), the allies had around 6,000 ships approaching Normandy. How did the Germans not see this and continue to believe the invasion would come through Calais? Surely the Germans had functioning radar in the area, although I've heard their radar technology was behind that of the allies. I am also aware that the Germans weren't very skilled in terms of intelligence gathering. But still... 6,000 giant hunks of metal in the British Channel approaching you and nobody notices?
There were two main reasons for this. Firstly, the Germans had no real way of making visual contact with the fleet until it was close to the Normandy shore. They had no real maritime patrols, either from aircraft or from surface craft. In the air, the Luftwaffe had always been reluctant to commit aircraft in support of the Kriegsmarine. This was exacerbated by Allied air superiority, which attritted those patrols that were sent out. This meant that there were no aircraft available to fly patrols over the Channel on the 5th-6th July 1944. Similarly, Allied naval superiority meant that German warships were only committed when they were certain to engage targets, instead of carrying out patrols. Additionally, the Allies had learned from the experience of Dieppe. At Dieppe, the landing force had run into a German surface patrol, delaying part of the landing, which partly led to the disaster that would ensue. As a result, heavy forces were deployed to sweep the Channel clear of any German ships that did show themselves (though none did). With the force travelling through the night, there was nothing to spot them until they reached the coast at dawn on the 6th.
Secondly, the Allies made a major electronic warfare effort to jam and confuse the German radar network. The Germans had a network of radar equipment along the French coastline, using the naval Seetakt system. The British carried out two operations against this radar net simultaneously with Overlord, called Taxable and Glimmer. Both operations were very similar, with bombers dropping radar-reflective chaff and small boats towing barrage balloons. By acting in concerto, they simulated the approach of an invasion fleet. Taxable targeted the area around Le Havre, while Glimmer was aimed at the Pas de Calais. By confusing the network like this, it led the Germans to dismiss reports of the real invasion fleet. It also distracted their responses and available patrol craft; as they considered the Pas de Calais a much more likely invasion target, ships and aircraft were directed to this area, and away from Normandy.