The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

Even so, neither the news emerging from France nor the fact that the
King felt an appeal to God was necessary was hardly encouraging. Most
British people did appear to be keeping calm, but few could have doubted
the seriousness of the situation, even if perhaps not just how grave it was.
Daidie Penna, for one, was not sure what to think. On the 22nd she and her
husband were wondering whether she should take the children back to Port
Isaac in Cornwall, where they had evacuated to the previous autumn. The
next day, things seemed brighter again. She met one man who said,
‘Yesterday we reached rock bottom – now the tide will turn.’ By the 24th,
she felt even more encouraged. ‘News fairly good,’ she noted. ‘Apparently
we are holding them again and also cutting in on the spearhead of their
attack.’
Those close to intelligence sources, however, were struggling to see
much cause for hope. Britain appeared to be gripped by the biggest threat to
her independence she had ever faced, one that was eclipsing the Spanish
threat in 1588 and even that of Napoleon’s France at the turn of the
previous century. Deputy Director of Air Intelligence at the Air Ministry
was Group Captain Tommy Elmhirst. It was Tommy’s job to head the
German Section, responsible for calculating the strength and potential of the
Luftwaffe, a position he had held since the middle of January. This he had
found a singularly depressing job, for by the spring the section had a
reasonably accurate picture, albeit slightly overestimated, of the German air
force, its formations, reserves and, above all, its strength and the location of
front-line units. Compared with the RAF, the Luftwaffe strength seemed so
immense Tommy feared the outlook appeared very depressing.
His sources were mostly from signal intelligence, or ‘sigint’. Since
1935, the RAF had maintained a radio intelligence service known as the ‘Y’
Service for listening in to and collating low-grade wireless traffic, usually
between aircraft, low-grade radio and telephone traffic, and other signals
traffic such as navigational beacons. More recently, the service had acquired
high-grade ciphers encrypted by German Enigma coding machines. The
Government Code and Cipher School (GCSC) at Bletchley Park had begun
to break general Enigma traffic regularly during the Norwegian campaign
and the Luftwaffe key fairly regularly since January. Luftwaffe Enigma
traffic was not yet being deciphered with particular speed, but, combined
with Y Service sigint, it had enabled Tommy and his team to build up a
picture of the Luftwaffe that was disturbing to say the least. What worried