It must be remembered that the secret of radar was still shared by only a small number of men and women. Generalmajor Wolfgang Martini of Luftwaffe Signals Service was certainly informed of all the technical advances, and it was he who insisted that attacks must be made upon these Funkstationen mit Sonderanlagen (radio stations with special installations), as they were marked on Luftwaffe target maps. So it was decided to devote the day before Adlertag to attacks upon these stations and upon RAF fighter airfields near the coast. This combination of raids would thus confirm, or deny, that the RAF fighters were depending upon the stations.

The four-engined FW 200s, under Marine Gruppe West at Lorient, had ranged far into the Atlantic, watching the region of high pressure over the Azores. As it began moving, the Air Fleet's orders were dated and put onto the teleprinters. Adlertag was fixed for 13 August, with preparatory attacks the day before.

The RAF also had access to the reports from men eavesdropping on German frequencies. The German weather-reconnaissance units were not asked simply for general information but for weather conditions at the intended targets. As an added risk to security such aircraft sent radio reports while still over Britain. This, and any other Luftwaffe radio traffic, was correlated by a secret unit known as the Y service. And as each German aeroplane was prepared for an operational flight, its radio was tested. Monitoring of these test signals provided intelligence with a fairly accurate guess at the number of aircraft to be used in the following 24-hour period. As Eagle Day approached, the listening service was able to tell Dowding that he was about to be attacked on a scale far exceeding all previous attacks.

Erprobungsgruppe 210 (flight evaluation group) was a recently formed unit. Its primary task of evaluating the new Messerschmitt Me 210A-O under battle conditions had been temporarily put aside. This was fortunate for them, since the Me 210A-O was one of the most notorious aircraft design failures of the war. As far as is recorded, no example of it participated in the Battle.

ErprGr 210's present task was that of evaluating the Jabos, which were Bf 109 and Bf 110 aircraft in the role of a light, low-level bomber that could revert to the fighter role after the bombs had been dropped.

This highly skilled unit had shown enough success to be briefed for the most vital of Adlerangriff's preparatory attacks. At 8:40 a.m. sixteen aircraft took off from the airfield at Calais. Their task was a pinpoint bombing attack upon four radar stations.

Already Bf 109s of II/JG 52 had passed over Dover and chased across Kent. Spitfires of No. 610 (County of Chester), an Auxiliary Air Force squadron, were sent after them. In the dog-fight that followed, the Germans deliberately moved the battle eastwards. This would leave the air clear for ErprGr 210.

Hauptmann Walter Rubensdorffer — a 30-year-old Swiss — brought his formation of Bf 110s along the Channel at 18,000 feet. They approached Dover at right angles, to make the radar operator's task more difficult. It was the Rye radar that picked them up. It was noticed that the track was heading straight at them. The 19-year-old girl operator was slightly irritated by the way in which the Filter Room, at Fighter Command, gave the plot an X code. That meant a report of doubtful origin: possibly friendly aircraft, or a mistake.

The first element of four Messerschmitts peeled off from the formation and dropped towards the 350-foot-tall masts at Dover. Their well-placed bombs rocked the pylons and destroyed some of the huts.

Rubensdorffer took the next element north across Kent to the inland RDF station at Dunkirk, Kent. One of these Jabos dropped a bomb so close to the transmitter block that the whole concrete building moved two or three inches. There were other hits on the huts there.

At Rye the operators were still watching in fascination as the plot came nearer and nearer. Suddenly bombs began to fall upon them. Almost every building was hit, except the transmitting and receiving block. The Filter Room called repeatedly into the phone to find out what was happening. "Your X raid is bombing us," explained the girl primly.

The last element of aircraft hit Pevensey with eight 500-kg bombs. One of them cut the electricity cable and the whole station went off the air. ErprGr 210 turned southwards and raced home, their mission completed. There could have been no better demonstration of the capabilities of the Bf 110 in the fighter-bomber role. Of the four radar stations attacked, only Dunkirk, Kent, remained on the air.

With a 100-mile-wide gap torn out of the radar chain, no fighters could be sent to intercept the formations that now attacked the fighter airfields at Lympne and Hawkinge. The former was only used as an emergency satellite, so the serious damage done to it was not vital to Fighter Command. Hawkinge, however, was important. The damage there included the destruction of two hangars, and station workshops, and four fighters damaged on the ground.

The Stukagruppen also benefited from the radar gap, and ships were attacked as they moved along the Kent coast. In the absence of radar, the fighters arrived late. The Ju 87s all returned safely.

It was nearly noon when the radar station at Poling — which had not suffered an attack — read a very large formation approaching Brighton from due south. This was the whole of KG 51—nearly 100 Ju 88s — plus ZG 2 and ZG 76: 120 Bf 110s. As high-flying escort came twenty-five Bf 109s from JG 53, The Pik As Geschwader, named after the ace of spades emblem that was painted on their fighters.

Before it reached Brighton the whole formation turned west, following the coast towards the Isle of Wight. As they came to Spithead, all of the aircraft except fifteen Ju 88s turned north to Portsmouth. Going through a gap in the balloon barrage caused by the harbour they made a fierce attack upon the docks and the town.

The Hurricanes of 213 Squadron steered clear of Portsmouth, where they could see intense anti-aircraft fire colouring the sky along the line of the Germans' bombing run. The big battleship HMS Queen Elizabeth was in the harbour, and the Ju 88s followed their Kommodore in a steep dive-bombing attack. However, all the ships escaped and a brewery suffered. At least one Staffel had virtually no experience of dive-bombing, and the dive-brakes of one machine jammed in the down position. Two of the Ju 88s fell victim to the anti-aircraft guns. Then the Kommodore of the Edelweiss Geschwader turned to lead his formation out of the target area. The Hurricanes dived upon them. The Kommodore's Ju 88 flew back the way it had come until it was shot down into the sea south of the Poling radar station. None of its crew survived.

Meanwhile, the fifteen Ju 88s that had detached from the main formation turned south across the middle of the Isle of Wight to attack Ventnor radar station from the landward side. The aerials, as tall as cathedral spires and sited on the hills above the town, provided the specially designed radar with very-long-range detection but they were a conspicuous target. The bombers put fifteen 500-kg bombs into the compound, and destroyed every building there. The aerials were also damaged: the radar flickered and died.

Coastal posts of the Observer Corps had been reporting the German formation ever since it approached Brighton.

By the time that the bombs were dropping on the Ventnor radar, Spitfires of 152 and 609 Squadrons were closing on them.

Artfully, Park had put his fighters into the battle piecemeal. Ten thousand feet above the bombers, the highflying escort of Bf 109s circled, waiting for the main body of RAF fighters to arrive. Only when the bombers had lost ten of their number did the Pik As formation leader dive into the battle.