As early as 1929, Dr.Kramer and Dieckmann had invented the first aircraft ILS system. The system was demonstrated
in 1932. In 1935, this ILS system was described in Prof. Dr. H.E. Hollmann's book "Physics and Technique of Ultrashort Waves" from which the picture below
is taken. By 1935, both the Lorenz Co. and the Telefunken Co. were in full swing producing ILS and installing them at airports around the world. In Germany,
under the sponsorship of Lufthansa, ILS were installed at Breslau,
Königsberg, Munich, Nurenberg and Stettin. By 1938, the Germany companies had
also delivered and installed ILS at airports in Kastrup-Dänemark, Malmö-Sweden, Wilna and Warsaw-Polen, Prag-Tschesolavakia, Budapest-Hungary and Sidney
and Melbourne-Australia. This system was also installed in England with the name "Standard Beam Approach."
The X-Beam and Y-Beam Systems were also used for navigation. In 1939-1940 during the "Battle of
Britain", German bombers used radio beams operating at 33.3 MHz and
later at 71 MHz. The system had the code name "Knickebein" during
WWII and navigation was possible up to 350 km.
Below is a brief description of how ILS works.