From the naval point of view the success of D-Day depended to a great extent on the efficiency of R.C.M., or Radio Countermeasures, in "blinding" a watchful enemy established on the northern coast of France. The object of R.C.M. was to stop the Germans using effectively their own radar equipment. It was in March 1941 that the Royal Navy began to take active radio countermeasures against German batteries on the French coast that were firing on British coastal convoys as these passed through the Straits of Dover.
One way to give protection was to stop the Germans on the far shore seeing the ships in the dark with radar. Radio experts stationed at posts along the south coast of England listened fro enemy radar signals as our ships passed, and jammed them as soon as they were heard. So well did the operators and scientists follow every German move that when the large convoys were passing through the Straits for the invasion of Normandy only about six ships of some 2,000 were hit by enemy gunfire.
To mystify the enemy, German radar in the area where the invasion was to occur was jammed; and in certain other places, where no landings were intended, the enemy was made to detect on his radar screens indications that suggested the approach of an assault force. The transmitting-room of one of the Dover jamming stations had a Wellsian appearance, and the aerials might have been tubular steel scaffolding. The aerials of the monitoring receivers, which listened to the enemy's signals. Further countermeasures had to be devised to combat the German radio-controlled bombs; scientists invented equipment that enabled a ship's wireless operator to jam the radio control and no ship so fitted was hit by one of those missiles. This successful warfare waged against the enemy's radar was always a race against time, and the scientist behind the fighting man should have a large share of the credit. More often than not he was told the enemy had perfected some device and the countermeasures was required within a few hours. He managed it, somehow.