Editing Channel Dash (section)

===Ultra===

{{See also|Enigma machine|Ultra (cryptography)}}
[[File:Bristol Beauforts 217 Squadron in flight.jpg|thumb|left|[[Bristol Beaufort]] torpedo-bombers of 217 Squadron, RAF [[Coastal Command]]]]
Ultra was the code name used by [[United Kingdom|British]] [[military intelligence]] for [[signals intelligence]] obtained by breaking German [[radio]] and [[teleprinter]] communications, including signals encrypted by Enigma, a German [[Electromechanics|electro-mechanical]] [[rotor machine|rotor cipher machine]]. The decryption was carried out at the [[Government Code and Cypher School]] at [[Bletchley Park]] and the information was passed on to operational commands.{{sfn|Hinsley|1979|p=346}} From May 1941, Bletchley could read the Enigma ''Home Waters'' setting used by surface ships with few failures or interruptions, which combined with the PRU and reports from agents kept watch on the ships at Brest. By April 1941, the British knew that the three ships had been hit but not the extent of the damage. {{sfn|Hinsley|1994|p=124}}

From 16 to 23 December, Enigma decrypts showed that the gunners of the ships were on the Baltic, conducting gunnery training. Next day, the Admiralty warned that an attempt to break out was likely.{{sfn|Hinsley|1994|p=134}} On 25 January 1942, the ships were photographed in the harbour and two short periods in dry dock by two ships were seen. From the end of January to early February, torpedo boats, minesweepers and destroyers joined the big ships; together with news that the battleship ''Tirpitz'' in Norway had moved to the south, this led the Admiralty to issue an appreciation on 2 February that the three ships were going to attempt to sail up the channel and sent the signal ''Executive Fuller'', the order to begin the operation to prevent the German Fleet from breaking into the North Atlantic.{{sfn|Richards|1974|p=363}} Next day Enigma and RAF photographic reconnaissance (PR) found that the number of German ship reinforcements from Brest to the [[Hook of Holland]] had risen to seven destroyers, ten torpedo-boats, more than {{nowrap|30 minesweepers,}} {{nowrap|25 [[E-boat]]s}} and many smaller craft.{{Sfn|Hinsley|1994|p=132}}