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

pointing in slightly different directions but also just overlapping. One of the
beams would be represented by a ‘dot’ signal, the other by a ‘dash’, which
would be heard through the headset of the pilot or navigator on board an
aircraft. The two beams would be set so that when the aircraft was flying on
the correct course, the two sounds would merge into one continuous note. If
the pilot deviated he would hear more dashes or dots depending on whether
he veered left or right.
If RV was right, then Knickebein would be a further note that would be
heard the moment the two beams actually intersected. This would thus be
set at a specific target point. The Y Service was immediately put on the
alert for any more information; interrogations were also targeted at finding
out more. At Boscombe Down, near Salisbury, the Blind Approach
Development Unit, which had been experimenting with Lorenz-type beams,
was also called upon to help by sending up aircraft using captured Lorenz
receivers and trying to pick up beams. It found nothing. RV was sure that
one of the beams was coming from Cleves, but he needed to know the
location of the second one. This was the key. By good fortune, a
breakthrough came almost immediately from another piece of paper
salvaged from a shot-down bomber. This fixed the second beam in
Schleswig-Holstein. More pieces of the jigsaw were quickly found; Felkin
was doing his work well. First, the team at the BAD unit discovered that the
Cleves beam was set at 31.5 megahertz, then that Schleswig-Holstein was
set at 30.0 megahertz. They now had hard information on the size of the
two beams, so the next step was to confirm their existence by discovering
and flying along them for themselves.
It was only eight days since the decoded Cleves message, but events
had moved very quickly. RV now found himself urgently called to a
meeting with the Prime Minister at Downing Street. It was already in
progress when he arrived. Sinclair, Newall and Dowding were there, as
were Lindemann and Beaverbrook. So too was Portal, C-in-C Bomber
Command, as well as Watson-Watt and Tizard. RV had never met the Prime
Minister or many of the other war leaders gathered there but straight away
he correctly sensed there was tension in the air – before his arrival,
Lindemann and Tizard had clashed, the latter insisting that ‘bending’ beams
was not possible. As RV now listened for a few minutes to the continuing
discussion, it became clear to him that none of them had really grasped the
essence of what had been discovered. Eventually, when Churchill turned to