The next big question was how to mark the swept channel. Not only were buoys only possible for a short inshore part of the course but there were also dangers in placing more buoys outside Brest. In the first place they might be spotted by British reconnaissance. Also the tendency in those mid-war days was to remove buoys — and to anchor new ones might puzzle French port officials and cause suspicion.
To avoid this, Ruge arranged for some of his small mine-sweeping craft to anchor as "mark-boats" along the most important points of the route, while the operation was in progress. When the captains of the mine-sweeping craft came to open their secret orders they were mystified. For each contained instructions to sail to a certain position in the Channel at a time laid down, and anchor there to act as a "living buoy." They had to endure the hazards of air attack with no idea of the vital importance attaching to their role. But no explanation was possible.
Meanwhile, based on Ruge's information, Admiral Saal-wächter's staff officers in Paris prepared the navigation charts. They were delivered to the ships in Brest by "safe hand" officer messengers with the rank of naval captain. Yet in spite of the most careful planning, several of the mark-boats were off position on the day. But no operation, however well planned, can be perfect.
Next to mines, the greatest danger to the ships was the unseen eyes of radar stations strung along the English Channel coast. The Germans already knew from the reports of Captain Brinkmann of the Prinz Eugen that the British were ahead in radar. But they were still not certain how far ahead.
Even before the outbreak of the war they were curious about British radar. Then in the spring of 1939 350-foot aerial masts were erected from the Isle of Wight to the Orkneys, German Intelligence marked them down as radio transmitters. But General Wolfgang Martini, the Head of the Luftwaffe Signals Service, was not satisfied that they were.
He was suspicious of the fact that these latticed aerials were on different wavelengths from the crude German Freya and Wurzburg radars, which German firms were developing. Was this possibly more advanced radar than his own country had? At a meeting with German Air Force chiefs, Göring and Milch, Martini came forward with a startling suggestion. He suggested a Zeppelin reconnaissance. When Göring asked, "Why not use an aeroplane?" Martini explained that only an airship could remain stationary in the air to record a series of signals. Göring ordered one of Germany's two remaining Zeppelins to be converted into an airborne radar spy.
One night in May 1939 the 776-feet long Zeppelin LZ 127 left Frankfurt and headed over the North Sea with aerials rigged underneath her gondola. She flew towards the Bawdsey Research Station at Orfordness, Suffolk, where tall masts could be seen. Aboard her was General Martini. While the Zeppelin cruised along the Suffolk coast technicians in her gondola manned special radio detectors. All they picked up was a loud crackling.
At Bawdsey, the radar operations staff gazed in astonishment at the largest "blip" they had ever seen travelling slowly across the radar screen. They guessed correctly that it was a German airship carrying out radar investigation. As she flew along the east coast the Zeppelin picked up only more crackling in her receivers. General Martini landed in Frankfurt still as ignorant of the development of British radar.
A month before the outbreak of war — or 2 August 1939—the Zeppelin made a second trip. Her instructions were to keep fifteen miles off the east coast of Britain and note the wavelengths, strength and position of all high-frequency signals. Martini was not aboard this time but sent his senior officer, Lt.-Col. Gosewisch. Once again no transmissions were detected — and British radar did not pick up the airship.
But she was seen. Coastguards in Aberdeenshire reported her and two RAF fighters took off from Dyee and identified the airship. But she was well outside territorial limits as Martini had ordered.
She cruised near the British naval base at Scapa Flow, catching glimpses of British warships through the clouds before setting course back to Germany, again without detecting any high-frequency signals.
A month later, when war broke out, such Zeppelin cruises off the coast of Britain were impossible. In spite of the two abortive Zeppelin flights, General Martini was convinced that Britain was ahead in radar. He persevered in trying to find out more about their sets.
The fall of France in 1940 gave him his opportunity. Special teams of Luftwaffe and Navy signal experts were sent to the Channel coast to find out whether instruments similar to their radar had been established on the English south coast. Until then the Germans had no definite proof that the British were ahead. When these radar intelligence receivers recorded a number of English radar sets on meter and decimeter wavelengths, reconnaissance planes helped to pinpoint their locations for an exact map of all existing radar. From these precise observations, the Germans were able to deduce a good deal about the development of the British radar devices. Martini decided to jam them.
During the next year jamming stations were established in Ostend, Boulogne, Dieppe and Cherbourg. They were equipped with very efficient directional beam antennae, synchronized with the search impulses of the British equipment. They could achieve effective jamming from Cherbourg to the Isle of Wight. Several aeroplanes also had jamming equipment.
When told of Operation Cerberus, General Martini personally directed his "interference" operations. At dawn each day during January English radar stations had a few minutes of jamming, deliberately made to appear like atmospherics. Every day the length of the jamming increased slightly. By February British radar operators were wearily accustomed to this interference. They reported it as "caused by atmospheric conditions."
This brilliant, painstaking radar plan was to play a decisive part in delaying the British defences.
The day after the New Year conference a staff car appeared on the quai Lannion. Ships' officers recognized the visitor it contained as the famous Battle of Britain fighter ace Col. Adolf Galland.
Galland was a popular figure with the Navy, which was unusual because the Luftwaffe — and especially its chief Hermann Göring — was not. Not only did the Navy feel that the Luftwaffe was the younger, more favoured child but it resented the fact that Göring had used his influence with Hitler to resist any attempt to give the Navy its own Fleet Air Arm, as the Royal Navy had.
Although Scharnhorst carried three reconnaissance aircraft in her hangar, they would be useless in this operation, which needed a vast Luftwaffe force. Col. Galland came aboard Scharnhorst to discuss this air cover with Admiral Ciliax and his Chief of Staff Reinicke. With them he worked out an elaborate air defence plan of fighter forces under his command.
Galland's headquarters were to be at Le Touquet, geographically in the centre of the operation. He set up one command post at Caen for the early part of the break-out, and another at Schiphol in Holland for the last leg.
The problem was to have the greatest possible number of aeroplanes at all times because, as Hitler had predicted, the success of the break-out depended on how soon the British could mobilize the full force of the RAF against the surprise appearance of the German ships.
Owing to the great demands jn Russia the Luftwaffe was short of aircraft. But three fighter groups were available with 250 fighters and 30 night-fighters. To cover the battle squadron, all the 280 fighters stationed along the Channel coast would be brought into action. Night-fighters would give protection before dawn and as soon as dawn broke 16 day fighters would be constantly overhead. Each flight would last thirty-five minutes. Ten minutes before they left another wave would arrive. This meant that for twenty minutes of every hour there would be 32 fighters overhead.