[2009]

Wartime Bletchley was the centre for Britain's development of a ‘gold standard’ of intelligence professionalism, which the UK and US then applied to the challenges of the Cold War. I have argued elsewhere that, despite the limitations in its understanding of the Soviet Union, intelligence became a steadying influence on Western governments. On balance it was an antidote to fears that we might be hit by a Soviet bolt from the blue, or that new Soviet weaponry would suddenly outclass Western military forces. The legacy of Bletchley's professionalism was a force for sanity on these and other questions, and part of this intelligence gold standard was the basic skill of interception – plucking radio signals out of the sky – on which most of the Sigint process depended. This is a note on the introduction to this skill which I was given when I first joined GCHQ.

After its move from Bletchley at the end of the war GCHQ had retained part of the site as a training school for the civilian radio operators still employed by the three service ministries as well as GCHQ. As a newly joined trainee I was sent to Bletchley in the winter of 1952–3 to learn something about interception. I have mixed memories of the time: no Milton Keynes existed, and old Bletchley town was not exactly a centre for swinging youth. But I was introduced there to the craft of signals interception, at a time when the basic technology and skills were still essentially unchanged from wartime.

To understand interception as it was then, I was taught to think of two distinctive features of communications in the immediate post-war period: high frequency (HF) radio, and the morse code used on HF transmissions. Radio in the First World War had used the low frequencies now found on the long and medium wavebands of domestic receivers, but by the Second World War military communications had largely moved to HF, or what was then labelled ‘short-wave’. This technique used radio transmissions at frequencies that bounced off layers of the ionosphere and facilitated long-distance contacts, sometimes through multiple bounces. The heights of these ionospheric layers at any time determined what signals could be picked up, and where and when, but they were constantly changing, particularly between night and day and by the different seasons of the year.