Top Secret.
Copy No. 171
    U-boat  "U 249"
    Rec.     26.3.44.
    Reg No:  Top Sec. 28
    Encl.      ---____________
Current Order No. 42 of Feb. 1944.
  Use of the FUMO for warning of aircraft.  
1) Our own radio-location apparatus is an important and reliable means of alarm for the approach of aircraft, especially in case of defect in the Radar interception gear or if there is suspicion of a new type of locating gear used by the enemy and which cannot be intercepted.  By experimenting with "GEMA" apparatus against the He 111 and Ju 88, interception was reliable during numerous flights.
  a) At an altitude of 330 m. to a distance of 12 km.
  b) At an altitude of 100 m. to a distance of 7km.
  Interception of low-level aircraft at lower altitude is not reliable.  In higher altitudes and for larger aircraft, an improvement in above values is to be expected.
  Experiments with the "Hohentwiel" set and reports from boats prove that aircraft at medium or great heights can be measured very accurately from 20-30 kms. distance.  Low-flying planes can thus be intercepted, using FUMO soon enough to be fired on, as has already been done successfully by individual boats.  The keeping of a continuous searching sweep is important, even after picking up target, in order to avoid surprise from another direction, whether by planes or surface craft.
2) FUMB as a means of warning of aircraft has the disadvantage that it is dependent upon the locating activity of the enemy.  With FUMO on the other hand, an aircraft is intercepted in good time, regardless of whether the aircraft itself is using radio locating gear or the type of gear being used.  Doubts frequently expressed whether location of boats can be established by enemy aircraft with Radar interception gear, are unjustified.
  a) Interception device on enemy aircraft is indeed possible, at present however, improbable and in no case proved.
  b) Interception device is of no advantage to enemy aircraft, since it can intercept the boat at the same range with the newest locating devices, and in addition can measure the actual distance.  Thus the enemy has no need of observation.  Only naval forces if equipped with interception gear can perhaps establish a boats position sooner than by using an actual radio-locating set.
  c) It is less important, when under way that the boat is located by the enemy than that it is warned of aircraft and not attacked by surprise.
3) Conclusions:
  a) When under way, and if the boat need not necessarily remain unobserved, switch on own FUMO for warning of aircraft as far as condition of gear and weather permit.  The most favorable turning speed is searching operation 25° per second.  Relieve watch-keeper frequently, since manipulation (of wheel) is very tiring.
  b) If the boat must continually remain unobserved (e.g. in patrolled stretches) own FUMO must not be used.
  c) The instruction of wireless operators in manipulation of FUMO at every possible opportunity is very important.



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