After the Nazi occupation of the Czech lands in March 1939, the Germans began to search for illegal transmitters that helped the domestic resistance communicate with foreign countries. They were equipped with very modern radio reconnaissance stations, which allowed relatively accurate tracking of agency radios. They radio targeting service used mobile radio direction finder vehicles masked with civilian ads. After determining the approximate location of the transmission, the soldiers came in with perfectly masked belt radio direction finders developed in 1942 and produced by Viennese company Kapsch. These machines brought the Gestapo members all the way to the door behind which the illegal radio transmitter was operating.