As aforementioned in the introduction of chapter 8, communication is the workhorse of armies and armed forces. Without a doubt the precise monitoring and evaluation of signals were possible only after the advancements in electronics, which took place back in the firsts years of the XX century and, certainly it was responsible for inspiring the procedures of the modern clandestine warfare.
In this way due to the readiness arisen by the electronic technology, in the several communications modes: continuous wave, phony, facsimile, pulse and so forth, those types of signals could be monitored, analyzed and then duly encoded to become unknown to the enemy.

During WWII, in the countries in conflict the radio waves were daily monitored by using radiogoniometry procedures in order to locate several modes of clandestine transmissions. Due to the lack of trained operators generally radio amateurs conducted this work.

Photograph by Hallicrafter.

Mobile radio monitoring of clandestine transmissions, circa 1944.