Battle of the Beams
The Battle of the Beams is a name given to German attacks on Britain after the end of the Battle of Britain, defined as when Germany cancelled its planned invasion, Operation Sea Lion. German bomber crews were not trained in navigation at night, and required external radionavigation aids, generically called "beams", to find their targets. Britain countered with a wide range of defenses, but none more important than electronic warfare, most specifically electronic attack on the German navigational aids, such as Knickebein, X-Geraet and Y-Geraet.
British scientific intelligence organization, under R.V. Jones, determined that the German night bombing attacks were guided by electronic navigation signals sent from occupied Europe. Once the systems were understood, it was possible to mislead the bombers, so that they bombed open country, or, on a few wondrous occasions, became so disoriented that they landed at airfields in the U.K. 
Interfering with German night bomber navigation was the first challenge for the Royal Air Force. When bombers did navigate successfully, they had to be destroyed by fighters or anti-aircraft artillery.
"During the human struggle between the British and the German Air Forces, between pilot and pilot, between AAA batteries and aircraft, between ruthless bombing and fortitude of the British people, another conflict was going on, step by step, month by month. This was a secret war, whose battles were lost or won unknown to the public, and only with difficulty comprehended, even now, to those outside the small scientific circles concerned. Unless British science had proven superior to German, and unless its strange, sinister resources had been brought to bear in the struggle for survival, we might well have been defeated, and defeated, destroyed." — Winston Churchill
Changes in German goals
After the cancellation of the invasion and the end of the Battle of Britain proper, German objectives changed. No longer trying to achieve air superiority, the focus was now on damaging British production and morale. To reduce their bomber losses, almost all raids were at night.
Previously, during day attacks, ground-based radar needed to get interceptors into visual range of the bomber formations. With the Germans attacking at night, they forced the Royal Air Force into much more difficult night interception, guided either by a limited number of short-range precision ground control radars, or much more primitive airborne radar.
Going to night bombing, however, was not an unmitigated advantage for the Germans. While the fighters might not be able to find the bombers, the bombers now faced a much greater problem in finding their targets. German aircrew were not trained in celestial navigation as were British crews, so they needed assistance in even trying to find a city-sized target. The German answer to this problem was to provide their bombers with external, radio-based navigational aids.
The starting point for the various systems was the Lorenz blind landing system, termed the Ultrakurzwellen-Landefunkfeuer or LFF. This was a High Frequency (HF) system operating at 38 MHz. As implemented for instrument landings, its purpose was assisting the aircraft to line up with the center of the runway.
In the Lorenz system, a set of three antennas, near the start of the runway, transmitted a pair of beams. One was modulated with short "dots" and one with longer "dashes". As long as the pilot stayed on center, the dots and dashes would merge into a steady audio tone. Drifting to the left or right would change the sound to dots or dashes, which would tell the pilot if he were drifting to the left or the right. He would then steer away from the drift until the steady tones returned.
The British were not going to be obliging and install Lorenz transmitters on their soil, so Germany had to develop similar systems that could be located on German-controlled territory, but still provide the bombers, over British soil, with useful directional indications.
British night defenses
- Jones, R. V. (1978), The Wizard War: British Scientific Intelligence 1939-1945, Coward, McCann & Geohegan
- Churchill, Winston (2005). The Second World War, Volume 2: Their Finest Hour. Penguin Books Ltd. ISBN 0141441739.
- Kopp, Carlo (January/February 2007), "Milestones: Battle of the Beams", Defence Today