Another tracking system, the high-frequency radiogonio (“Huff-duff” after the initials H .F.D.F. of the device). It was also passive, operating solely on the transmission of their position by the U-boats. These used to surface every day to convey their report and position to the General Staff in Germany. The Huff-Duff originated as a by-product of electronic research done for radar. Fixed stations on land picked up the faint emissions of the U-boats and thus obtained their position; receivers on ships at sea could also be used in conjunction with stations on land, in order to obtain even more precise positions. A search area was thus delimited where we could send corvettes and frigates equipped with ASDIC.
Although the Germans probably did not know of the existence of the "Huff-Duff", they certainly thought such a spotting was possible. They had therefore produced and put into service the “Kurier”, which transmitted “in jet”, at high speed, the essential information. The radio operator of the submarine composed in advance a table of data such as latitude, longitude, identification and ready-made phrases. Then he pressed a button, and the whole message was transmitted at very high speed in the form of a rapid "jet" of impulses; the operation lasted a few seconds, which reduced the time during which the transmitter could be detected.
It did not matter so much when the radar became operational. The story of radar has been told so often that we won't go into it here; we will only say that the various models of aircraft radars became the main detection instrument, without, however, entirely replacing the other devices.