When I read the note, I knew immediately that what Pribyl had learned was genuine. The change in Watcher operations had indeed taken place, largely at my instigation as part of the attempted modernization program. The RCMP had been experimenting with this idea with some success. It was called Operation COVERPOINT. No wonder Hoover insisted that his letter be delivered by hand via his deputy Al Belmont, who refused to meet Hollis inside Leconfield House. The letter was handed over at a secret meeting in an MI5 safe house, and Belmont flew straight back to Washington incognito.
"You can see our problem, Peter," said Hollis. "If we make a move against Linney we may blow Tisler, and the FBI are anxious to retain him in place for as long as possible. And if we try to investigate the case by other means, we'll be blown by the Russian source inside the office. Whatever happens, we must get to the bottom of this penetration."
Hollis told me that for the past three months extensive investigations had been made in the Watcher and Watcher support services by Malcolm Cumming and Courtney Young, the head of Russian Counterespionage. It was felt that the leak must emanate from there, but nothing had been found. Finally, Hugh Winterborn had prevailed on Cumming to persuade Hollis to indoctrinate me.
"Have you got any ideas, Peter?"
"Only to string up those buggers down at Cheltenham, sir!"
"I'm sorry. I don't think I follow..."
I explained to Hollis that I had long held the theory that the Russians might be obtaining intelligence through intercepting and analyzing our Watcher communications.
"My father and I did something similar in 1940 on the Sussex Downs. We tracked signals, and managed to plot the course of the British Fleet as it went down the Channel. I'm sure that's how Rogov got the information. It would be relatively easy for them to do it, sir. Just cross-referencing direction-finding of our signals with the records of where their own people go would tell them a lot. Basically, they must always know when we're following them."
I told him that I had repeatedly pushed GCHQ to conduct thorough tests to check if receivers were operating inside the Embassy which correlated with our own communications.
"I'm afraid, sir, it was never high on their list of priorities."
Hollis groaned.
"But can you do it, Peter?"
"Yes, I think so. What we've got to do is try to trace emissions from the receiver."
The principle was simple. Every radio set contains a local oscillator to "beat down" the incoming signal into a fixed frequency which can be much more easily filtered. The local oscillator always radiates sound waves as it operates, and it is these emissions which reveal the presence of a receiver.
"You realize, of course, that this is SIGINT, sir. Strictly speaking we're not allowed to do that work. GCHQ will take my guts for garters when they find out..."
Hollis hunched forward thoughtfully, cupping his hands across his face. There was a painful silence.
"They would need to be told about the Tisler allegation, of course, if we brought them in," he said finally. It was the kind of Whitehall demarcation dispute Hollis understood only too well.
"I could always have a go," I ventured. "If you can square my back upstairs with Cheltenham when they find out, at least we'll know one way or the other about Tisler's source within a few months. If we go to GCHQ, it'll take a year or more to arrange."
Hollis began to collect the files together into a pile.
"I think that is the best course of action," said Hollis, "Keep me informed, won't you."
He looked at me squarely.
"Of course, Peter, you realize what a terrible thing this would be for the Service, don't you? If it's true. I mean. Quite apart from the effect in Washington. A lot of good work will have been wasted."
"Including my own," I thought bitterly, angry at myself for not pushing GCHQ harder over the Watcher radios.
As soon as I got back to my office I contacted Courtney Young and asked him to send over any intelligence reports he had detailing the types of electronic equipment the Russians had either bought in London or imported into the UK since the war. Working through the files of reports I was able to pull together a reasonably accurate picture of the range and types of receiver the Russians were using inside the Embassy. I calculated that the probable range of emissions from their local oscillators was around two hundred yards. That ruled out operating from our static observation posts. But A Branch had been busy for some time developing a radio transparent mobile van with plastic walls. I pressed Winterborn to finish the project as soon as possible. Within a fortnight the van was rigged with an internal power supply and two receivers, one to detect the emissions from the Russian local oscillator and the other to confirm the relationship to the A4 frequency.
One spring day in March 1958, my assistant Tony Sale and I took the van out for the first time. We obtained permission to drive it up Kensington Park Gardens in front of the Embassy as if we were making a delivery to a house nearby. Sale and I sat inside with fingers crossed, earphones clamped over our heads, watching for the faintest flicker out of the amplifier. We made two passes. Nothing happened. The static hummed. We drove across to the Consulate on Bayswater Road and made a pass along the front of the building. As we neared No. 5, the Russian premises, we began to pick up the faint flutterings of a signal. As I tuned the receiver we heard a whistle as it encountered the frequency of the local oscillator. We slowed in front of the front door and the signal gained rapidly in strength, tailing off as we made our way up toward Marble Arch. There was certainly a receiver operating inside the Embassy. But was it tuned to the Watcher frequency?
For the next few days we made a series of passes at various different times of the day and night to try to gain some idea of what times the receiver inside the Embassy was in use, and whether there was a correlation with Watcher radios. It looked as if it were going to be a long, laborious, imperfect task. Then, by coincidence, as we were making a pass in front of the Consulate, a Watcher car drove past the other way, transmitting on the Watcher frequency back to Watcher headquarters. Inside the van our receiver, which was tuned to the local oscillator inside the Consulate, squawked loudly.
"What the hell do you suppose that is?" I asked Tony Sale.
He looked up with a quizzical look on his face. Then the truth suddenly dawned on us both. The Watcher car had just handed us the proof we needed. By transmitting on the Watcher frequency so close to the Consulate the Watcher car had overloaded the input circuit going into the Embassy local oscillator. We had picked up the squawk of pain as its frequency distorted under the overload. In other words, it was proof that the receiver inside the Embassy was tuned to the Watcher frequency.
The implications of this new discovery, code-named RAFTER, were enormous. Not only could we prove beyond any doubt that the Russians were listening to our Watcher frequencies; we could also use the same technique to check the frequencies being listened to on any receiver we could detect inside the Embassy. All we needed was to radiate at the Embassy and listen for the local oscillator overload. The ideas I had nursed since reading the KEYSTONE files were finally in a position to be put into practice. Using RAFTER we could detect which broadcasts from Moscow to illegal agents in the field were being monitored inside the Embassy. RAFTER, potentially, offered us a shattering breakthrough into the hitherto secret world of Soviet illegal communications.