Growing allied success in direction finding was becoming a serious concern for the Kriegsmarine by the mid-point of the war. Their first response was to emphasize the use of short signals (Kurzsignal) by U-Boats to communicate tactical information such as course, enemy reports, position grids or weather reports. Based upon extensive codebooks which had  tables that converted complete reports into a four figure group, each Kurzsignal had a strict format to communicate the message. These four figure groups that were further encrypted using the Enigma machine prior to transmission, limited broadcasts to less than a minute.

            In the spring of 1943, Dr. Berndt of the Telefunken firm proposed a method to further compress these signals to what would now be referred to as burst encoding, as a further counter measure against D/Fing. The first trials of the system, known as “Kurier” was prototyped in the summer of 1943. After another year of development, the first sea trials were conducted in the summer of 1944, but failed due to the levels of humidity and temperature in the U-Boat’s radio room. This problem was overcome by fitting a synchronous electric motor to the device to precisely time the revolutions of the “Geber”, the KGZ 44/2 pulse generator. The Geber was an aluminum disk 25 centimeters wide with a series of 85 small adjustable iron bars around the perimeter that could be set to create a pulse when the disk rotated over a magnetic head. Each pulse was 1 millisecond long and there was a 3 milliseconds gap between each pulse. One bar pushed in represented a dot, two bars pushed in adjacent to each other represented a dash, and an unset bar represented a pause between pulses. The Geber could be programmed to send a three figure group (the identity) followed by a four figure group, (the Kurzsignal) within 460 milliseconds. This was a serious threat to Allied Naval intelligence, as no Kurier signals were ever successfully D/F’d.    

            The receiving station was equipped with three Phillips CR 101 receivers, each with its own antenna and all tuned to the same offset U-Boat frequency according to a complex broadcast schedule. The incoming signals, converted into pulses on a CRT then photographed onto film, were decoded upon development. The system, given a top priority, was still in development when the war caught up to it. The receiving station at Bornau, in eastern Germany, was forced to relocate when the Russians approached in the spring of 1945. It evacuated to Bokel, near Neumünster in Schleswig-Holstein, where it went operational on April 27^th, 1945. Only a few days later, on 2 May, OKM ordered it to destroy its gear, smashing up the equipment and burying it on the grounds.

            Bokel and its Kurier station was a known target for TICOM, and was occupied on 8 May by a 30 AU team accompanied by members of TICOM Team 6. A German naval officer, Lieutenant Poeschke and two NCOs, specialists in Kurier operations, were still there. TICOM team members remained in place until the third week of May, collecting documents and equipment for shipment back to Bletchley Park, until a representative of the Admiralty’s Signal Division was located to take over responsibility.

            Like many examples of German advanced technology, the Kurier system reappeared for use in the Cold War, when the Soviet Navy in the early 1960s developed burst encoding systems for their own submarine communications.

References:

Kurzsignalen on German U-boats  

Ralph Erskine. " Kriegsmarine short signal systems--and how Bletchley Park exploited them". Cryptologia;  Jan 1999; 23, 1.