On a hill north of London there is a large country house called Bentley Priory. Underneath it, in nineteen forty, there was a room.
The room had a huge table in the middle covered by a map of Britain and the sea around it. Around the table stood young women with headsets and long wooden rakes, like the ones used to move chips across a casino table. Above them, on a balcony, stood officers who did nothing but watch.
Everything that happened in the sky over southern England that summer passed through that room first.
Almost nobody who worked in it was over twenty-five. Most were between eighteen and twenty-two. They were paid roughly two thirds of what a man in the same job would have been paid, and they were forbidden, by law, to tell their families what the job was.
It is tempting to imagine radar producing a clear picture, like something on a screen in a film. It produced nothing of the kind.
What arrived at Bentley Priory, down dedicated telephone lines, was a stream of spoken numbers from twenty different coastal stations, and the numbers disagreed with each other constantly.
One station would report a raid of about twenty aircraft at fifteen thousand feet. The station forty miles along the coast, looking at the same formation from a different angle, would report about fifty aircraft at twelve thousand feet. A third would report two separate raids, because the formation had briefly split.
Height was the worst of it. Estimating the height of an aircraft from the shape of a blip was closer to an art than a measurement, and being wrong by five thousand feet meant sending fighters to the wrong altitude, which meant sending them to be shot at from above.
And all of it was arriving at once, continuously, in a room where a delay of four minutes meant fighters climbing too late.
The job was called filtering, and the word is exact. Out of that contradictory noise, one agreed track had to be produced for each real raid, with a number, a height, a bearing and a speed.
There was no formula. A filterer learned the personality of each station. She knew that one particular set tended to overestimate numbers, and that another was reliable on height but slow to report, and that a certain stretch of coast produced phantom echoes in some weather. She held perhaps thirty of these adjustments in her head at once and applied them in seconds.
She also had to decide, continuously, whether two reports were one raid or two, and whether a track that had gone quiet had turned back or simply dropped below the radar's reach.
Every track that came out of that room was, in the end, a judgement made in a few seconds by a young woman in her late teens or early twenties, and the fighters were sent on that judgement.
The tracks then went out to the fighter group headquarters and down to the sector airfields, each of which had its own table and its own plotters, so that the same picture existed at every level at the same moment.
The table itself is worth describing, because it was a machine made of wood and people.
Each raid was represented by a small block carrying its number, its estimated strength and its height. Plotters standing round the edge pushed these into position with their rakes as new information came through their headsets.
On the wall was a clock, and it was not divided into twelve hours. It was divided into five-minute segments coloured red, yellow and blue. Each block was marked with the colour that had been showing when it was last updated. A controller glancing at the table could tell instantly, from the colour, whether he was looking at fresh information or something four minutes old and therefore possibly a lie.
That single idea, colouring information by age, is the sort of thing that seems obvious once somebody has done it and never occurs to anybody beforehand.
Along another wall was a board showing every squadron in the group and its state: at readiness, released, refuelling, in the air, in combat, landed. The controller looked at the table, looked at the board, and made his decision. Then a young woman relayed it, and forty miles away a telephone rang beside a deckchair.
They belonged to the Women's Auxiliary Air Force, which everybody shortened to the WAAF, and which had been formed less than a year before the war began.
They had joined expecting cooking, driving and clerical work, and a great many did exactly that. But the service grew to something like a hundred and eighty thousand women, and it turned out that the most demanding technical jobs in air defence were being handed to the newest and youngest organisation in the air force, because there was nobody else left to hand them to.
The secrecy was absolute and it was permanent. A plotter could not tell her parents what she did. She could not tell a boyfriend. If asked, she said she was a clerk. Some of them kept saying they were clerks for the next fifty years, out of habit, long after there was anything left to protect.
The work itself was exhausting in a way that is easy to underestimate. A watch was eight hours of unbroken concentration, standing, in a room with no daylight and poor air, and the consequence of a lapse was not a reprimand but a squadron in the wrong place. Several accounts mention that plotters came off watch unable to speak for a while.
There is a particular unfairness in this that is worth noticing. The pilots were photographed and interviewed and had a phrase written about them by the Prime Minister. The women who told the pilots where to go were, officially, not there.
The sector stations were airfields, and airfields were targets, which meant the operations rooms were bombed too.
On the last day of August nineteen forty, the airfield at Biggin Hill in Kent was hit hard and accurately. The operations block took a direct hit. The building was on fire and the ceiling was coming down.
Two WAAF women, Elspeth Henderson and Helen Turner, stayed at their posts. Turner was on the switchboard and kept it working. Henderson stayed on the direct line to group headquarters, passing information, and only left when the building was actually burning around her and she was ordered out.
Both were awarded the Military Medal, which at that time was a very unusual thing indeed for a woman to receive.
The fitters filled the craters, and the squadron flew from Biggin Hill the following morning.
A third woman at Biggin Hill, Joan Mortimer, had spent the raid running out onto the airfield with red flags to mark unexploded bombs so that the returning aircraft would not land on them. She was awarded the Military Medal too. All three medals were announced together, and the citation for one of them noted, in the flat language of official documents, that she had displayed exceptional courage and coolness.
The whole arrangement is now called the Dowding System, after the man who built it, and it is generally reckoned to be the first integrated air defence network anywhere in the world. Every air defence system since works on the same principle: gather, reconcile, distribute, decide.
The colour clock was real and several of them survive. If you ever visit a preserved operations room you can see one on the wall, and it is worth standing in front of for a minute, because it is a very simple answer to a very hard question.
It took until the nineteen nineties for most of this to be publicly told, and by then a good many of the women who did it had died without their families ever knowing.
Here is something to think about tonight. Everyone remembers the pilots, and they should. But the pilots could only be in one place at a time, and something had to decide which place. Why do you think we find it so much easier to remember the person at the sharp end than the person who worked out where to send them?