In 1953 the Operational Evaluation Group of the US Navy produced a report on the effectiveness of the decrypted intelligence during the Battle of the Atlantic. They tried to calculate how the decryption helped both sides. Here is a little bit about their report , it is 113 pages long.
The duration concerned is from 1 July 1942 to 31 March 1944 and this has been divided into four periods corresponding roughly to the signals intelligence available to both sides.
- 1 July 1942 to 31 December 1942. During this time the Germans were reading Allied signals with some success , the Allies were not reading the German traffic.
- 1 January 1943 to 31 May 1943. The Allies began reading the German traffic in late December 1942 and read it sporadically throughout this period. The Germans also read the Allied traffic , sporadically but more successfully than in the pervious period.
- 1 June 1943 to 15 September 1943. during this period the U-Boats did not operate against the North Atlantic convoys. The Allies killed a large number of U-Boats and changed their naval convoy cipher in June 1943 , the Germans then had no decrypted intelligence. The Allies read the German traffic less successfully than in the previous period but effectively exploited whatever intelligence became available.
- 16 September 1943 to 31 March 1944. The Germans only succeeded in reading an unimportant part of the Allied convoy communications , the results were of little use to them and in early December even this source dried up. The Allies read the German traffic completely and currently.
The four rotor version of the Enigma machine was introduced in February 1942 and was not cracked until December 1942. The Allies started to monitor U-Boat communications in June 1941 until the four rotor machine appeared. A lot also depends on how long it took to decrypt the messages. It looks like it wasn’t until June 1943 that Bletchley Park was on top of it.
The Germans were desperate to stop the convoys , they were being used to build up to an invasion and they knew they would not be able fight on two fronts at the same time. They had enough submarines to keep about 40-60 at sea at all times after Pearl Harbour and over 100 for the nine months from October 1942 to June 1943. The great majority of these were small 500 tons type VII C. They had a long transit time from the French ports to the most promising operating areas , as a result the average time spent on station was 16 to 20 days unless they could be refuelled when they could remain there for 32 to 36 days.
Good intelligence on convoy movements would help them make better use of their time instead of reconnaissance work. The Command of the U-Boats was convinced that more sinking would result if a promising contact was attacked by a large number of U-Boats rather than distributing the boats among several simultaneous targets. They needed good intelligence to be able to attack this way.
Since aircraft reconnaissance was only available to the Germans to a limited extent , and hardly at all with regard to US-UK convoys , the decryption of Allied radio communications containing information on sailing routes , rendezvous with escorts , and sometimes current positions was their best source of intelligence. With the exception of several periods of comparatively short duration , the Germans read the convoy codes for nearly one and a half years after the entry of the US into the war. From June 1943 on they were able to read practically nothing except some message which gave stragglers’ routes and early rendezvous points. The submarine command suspected that many of these were deceptive and designed to lead U-Boats into traps. Even this dried up in December 1943 when the Allies began using reference points instead of definite locations. This remained the situation up to the end of the war.
The Allies handled their intelligence problem in the U-Boat war by evaluating all the information of U-Boat movements that came from every possible source in order to plot the probable locations of as many U-Boats as possible. This gave a guide for evasive routing or diverting convoys and vectoring the task groups or other forces for the specific mission of hunting down the submarines. A very important source of intelligence was provided by the heavy radio traffic carried on by the Germans , necessitated by the firm control that the submarine command kept on the boats in order to carry out group operations effectively.
The German radio communications were exploited in three ways
- By Direction Finding
- By decrypting the radio messages
- By identifying the transmitter by means of the transmission characteristics in two ways
- TINA , a method of identifying a radio operator by his sending characteristics.
- Radio fingerprinting , by analysing the electrical characteristics of the transmitting station.
Radio Finger Printing (RFP) was the process used to catalogue a specific transmitter through its distinct characteristics with the aim of locating it at a future date. The idea was to identify individual transmitters by their emitted waveform. The rectified signals were applied to a cathode ray tube, (without a time base), and photographed onto moving film.
TINA was the method used to recognise specific radio operators by their Morse code “fist” and habits. One definition has the code name of TINA as being derived from the Latin word “tinea” which meant “worm”. TINA was the process that involved studying the distinctive characteristics of particular Morse code operators to identify and tracing the locations of those operators, for which might show that a particular operator has changed ships which may indicate damage or destruction to the previous ship or even a re-assignment. Each operator had a distinctive touch, or ‘fist’. Some were slower while others were jerky; some held down the key or paused between dots and dashes for different lengths of time and so on.
The early TINA recordings of Morse were made with a siphon-pen recording on paper tape using a device known as an undulator. At a later date, Morse transmissions were recorded on 35 mm film with a slow running RFP machine. This allowed measurements to be made through considerable noise conditions (QRN). This permitted the desired waveform to be distinguishable from the ambient noise.
Radio Finger Printing and TINA could be used to identify an individual U-Boat even if the message could not be decrypted.
In the next part , details of how this intelligence was used by both sides in their attempts to defeat each other.
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