The filtered reports were plotted on the Filter Room map table. The counters red for enemy and black for friendly had numerals to show estimated height and strength, with an arrow to indicate direction, and a reference number for that particular formation. >From the Filter Room balcony the whole map table was watched, and a teller passed details of these filtered plots back to Operations Rooms at Sector, Group and Fighter Command HQ. At each place, the map table was identical. On the wall in every Operations Room there was a special clock, marked to give each five-minute segment a different colour. Each raid's coloured direction arrow was changed (to match the clock), and moved as each new report was received. Providing that the reports kept coming, all the plots would be the same colour. But a lost, or neglected, raid would be noticed because its colour remained unchanged.

Also available to the controller and his staff on their balcony was 'the Tote'. This was a board fitted with coloured lights. It showed at a glance which squadrons were available in 30 minutes, which were at readiness (5 minutes) or at cockpit readiness (2 minutes), and which were in the air.

Virtually all of Britain's radar stations were near the coast and facing seawards. But as soon as the raiders passed over the coast, the Operations Rooms were forced to rely upon an army of volunteers equipped with only enthusiasm, binoculars, an aircraft recognition booklet, and a simple sighting device (see Plate 12).

As Churchill said, it was like going from the middle of the twentieth century to the early stone age. To cater to this transition there were 'Lost Property Offices' which recorded aircraft reported by the Observer Corps but not by radar. (The Observer Corps were only allowed to track aircraft that had been detected by the radar stations.)

The Observer Corps volunteers devoted much of their spare time to aircraft recognition. The system was wholly dependent upon these men for reports of enemy aircraft that had crossed the coast of Britain. On cloudy days this meant that enemy formations over land were reported only on the basis of the sound of their engines.

For the radar system to work, it was essential that the Sector Controllers have an accurate plot of the position of their own fighters, in order to guide them to the raiders.

The direction-finding stations — there were three of them in each sector — took bearings on transmissions from the fighter pilots' radio-telephones. To save the pilots effort, these were automatically switched to transmit for fourteen seconds in every minute of flight. The cross-bearings were translated into a map position in a D/F room, which was usually next door to the Sector Operations Room. There the Sector Controller could watch the movements of the enemy formation and of his own fighters.

Calculating quickly the compass course on which to send the fighter squadrons for accurate interception proved a vexing problem. Not only were pages of trigonometry consulted but a number of small computers were built to assist the calculation. Until one day, watching an exercise, an exasperated Wing Commander said he could judge the interception course by eye alone. He was immediately challenged to do so by the irritated boffins. He picked up the microphone that connected the Operations Room with the fighter pilots and gave them courses, until the two RAF formations taking part in the exercise met in a perfect interception.

The Wing Commander's judgment was greeted by amazed disbelief. Asked to explain how he did it, he said it was a process of imagining an isosceles triangle with the fighters and bombers at each base corner interception would take place at the summit. He gave the course accordingly. It was a rough calculation but quite good enough to become standard procedure. The most common discrepancy, due to the superior speed of the fighters, was no real problem. The fighters were simply ordered to orbit until the bombers arrived.

First reports of incoming attackers were seen on the cathode-ray tubes inside the 'receiver huts' at the radar stations. Estimates of the strength, altitude, and position of the enemy were phoned from here to the Filter Room at Fighter Command HQ (although 10 Group had its own Filter Room).

On the plotting table of the Filter Room women plotters moved markers showing the reports of aircraft. Here very skilled officers had to decide which were friendly, which hostile, and which 'doubtful'. This 'filtered' information was then passed next door, to Fighter Command Operations Room, and simultaneously to Group Operations Room and to the Sector Operations Room of any sector with raiders. All of the tables in these Operations Rooms

[The Royal Navy found a different use for HF/DF when they adapted this method to fix the positions of enemy seaborne radio traffic "Huff-duff stations, first on land and then shipborne, played such an important role in the war against the U-boat that the mast-head site of the radar aerials was used for 'huff-duff aerials instead, and the radar attached lower.]

At the Command Operations Room, decisions were made about sounding the public air-raid alarms and taking BBC transmitters off the air (in case they were used for direction-finding by the German airmen).

At the Group Operations Room the Duty Group Controller watched the plotting table, in order to decide which sector should deal with the raid and how many fighters should be sent to intercept it. It was here that decisions were made about anti-aircraft gunfire, so that RAF fighters could be kept clear of it.

The Sector Operations Room was usually in the Operations Block of the most important airfield in the sector. In the same block there was a D/F room where they received the pip-squeak transmissions from RAF fighters so that these could be shown on the plot too.

The Sector Controller ordered the squadrons under his command to various states of readiness, or 'scrambled' them into the air. As with all the Operations Rooms, girl plotters received the plots over their headsets and used croupier's rakes to move coloured counters on the plotting table. The Controller or his deputies, often NCOs, spoke with the leader of each of the fighter formations while they were flying and directed them towards the enemy. They employed simple code words intended for brevity and clarity rather than security. 'Angels' meant height, so that 'angels ten' meant 10,000 feet. 'Vector' meant steer, so 'vector 180' meant head due south. 'Pancake' meant come home and land. A 'bogey' was an unidentified aircraft and a 'bandit' an enemy one. From the fighter pilots, "Tally-Ho!" meant enemy sighted, 'liner' was cruising speed, and 'buster' meant full throttle.

There were, of course, no radar sets of any kind in the Hurricanes and Spitfires. The fighter pilots were guided entirely by the voice of the Controller, who was watching the plotting-table map. In practice it was found that one Controller could not manage more than two squadrons at a time, so subordinate Controllers assisted.

The importance of the country-wide network of telephone and teleprinter cables is obvious. When the bombing began, it was often only the exemplary courage of Post Office engineers that enabled Fighter Command to continue the interceptions.

Once the enemy had crossed the English coastline the only reports coming into the Operations Rooms were from the Observer Corps posts. They had no radar or detection aids of any kind. Their reports were also filtered (through the Observer Corps Centre at Horsham). These reports then went to Operations Rooms at all levels — Command, Group, and Sector.