In order to invade Britain, the Germans had to have control in the air over the English Channel, otherwise the RAF and the Royal Navy would have been able to destroy their invasion force before it reached the shore. It has been postulated by many naval experts that due to the type of flat-bottomed barge built by the Germans, simply running a Destroyer Squadron at full speed through their ranks would have caused many to capsize in the wake from the ships. The troops and their equipment would have suffered heavy casualties, and the invasion effectively stopped with little or no gunfire. The Luftwaffe's command of the air was therefore vital to any plan for an invasion fleet to successfully cross the Channel, to prevent British sea or air forces from interfering with the operation.
Interestingly, the German Navy, Army and Air Force each had their own plans and ideas as to how and where the invasion should be launched. There seems to have been little co-operation between the German armed forces, and despite the impressive build up of barges and other equipment in the Channel ports, the actual detailed planning for the operation, code-named "Sea Lion", was never really thrashed out. All depended on the success of the Luftwaffe it would appear, before the invasion was to be taken seriously.
Starting on 10 July 1940, the Luftwaffe attacked shipping convoys in the Channel and Channel Ports. They also suspected the importance of the British radar masts and attacked the stations on the South coast, damaging some of them very badly.
One of the aircraft types used in these raids was the Junkers Ju87 'Stuka' dive-bomber. These were very accurate, and had been particularly successful earlier in the war when there was no effective fighter opposition. But when dive-bombing, they were very vulnerable to attacks against them and the Hurricanes and Spitfires of Fighter Command found them easy prey. Because of their heavy losses, they were withdrawn from the battle in mid-August.
In this stage of the battle, the Luftwaffe was in effect probing the British defences - looking for weaknesses before a major assault could be launched to exploit them.