(typed in soluble ink)   Copy No. 135__
Most secret
Standing War Order of the B. d. U. No. 172
  Collation of intelligence and observations regarding enemy position-finding appliances.  
1) Distribution of equipment.
  Equipment with position-finding apparatus has been chiefly observed on destroyers and only in isolated cases on watch vessels and the larger trawlers.  According to intelligence received, submarines are not equipped with position-finding apparatus.  Stationary position-finding installations in and in front of particularly important ports and bases must be reckoned with.
2) Acoustic detection.
  Reports as to acoustic detection of enemy position-finding apparatus differ widely.  Some boats have observed impulses similar to those of our own apparatus, only deeper in tone, other boats report a regular chirping sound, both noises were heard over the whole boat and in the hydrophone.  Other boats speak of a ticking, like that of a watch, a noise like that of the Atlas lead, also an easily oriented swelling and decreasing sound, others again of a short metallic tapping against the side.
          A few boats have also reported a curious rustling sound, as though shingle were being thrown against the side.
          In the acoustic phenomena resembling those of our own apparatus, an impulse of 7 seconds was often definitely established.
3) According to reports received, the English position-finder chiefly appears to give exact depths.
4) Observed methods of attack.
  According to observations received up to now, the English appear to work in many cases with a combined hydrophone and depth-sounding apparatus.
          When the boat had been reported, sighted or observed (torpedo-track, torpedo detonation), they probably try and get its bearings as accurately as possible (destroyers stop for the purpose) and then follow it up in the direction ascertained, sounding incessantly, so as to located the boat by simple depth sounding.
  From this there arise the following possibilities of defence:


          As soon as the position-finding vessels stop, stop likewise or move at creeping pace; when position-finding vessels start on again, do the same, in certain cases at a higher speed, with considerable alteration of course.
          This procedure has been successfully carried out by some boats.
          If the foregoing manner of defence against position-finding is not successful, remain at the bottom as far as possible, by way of defence against depth-sounding.  This appears to be particularly efficacious in the case of great depths, since differences of  6-8 metres are then difficult to estimate.
          If the bearings have been lost, the English destroyer appears to steer a searching curve (similar to our Berghoff curve).  As this course is built on the assumption of a constant course and speed on the part of the U-boat, it should be possible to outmaneoeuvre it accordingly.  It is possible that in the above described procedure the bearing is obtained by means of a horizontal sounding apparatus.
          A position-finding apparatus similar to our S-device must in any case be reckoned with.  Experiments with our S-device have shown that the quality and strength of the echo depends on the submerged depth of the boat one is sounding for, and mostly decreases as this depth increases.
          By listening to the enemy's sound impulses the most favourable depth may therefore be determined in certain cases.
          The more faintly the impulses are heard, the smaller is the echo that the enemy detects.
          *"It is always best to show a narrow silhouette.  At a greater distance it is better to take the enemy from ahead than from astern, until the most favorable depth is ascertained.  Lie on the bottom only if there is no other possibility left"
5) Conditions of position-finding.
          The accuracy of position-finding was at first very poor, of late it is often very good, but only in a few cases continuously effacious and successful.  An unfavorable effect on position-finding is produced by a marked water stratification, such as is found in front of estuaries and in certain sea areas.  A stratification of this kind is e.g. well known to us in the Skagerrak, appearing specially in the spring, early summer and autumn, and rendering our own position-finding very
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  difficult.  Shoal water with greatly varying depths (sands) and many wrecks also render position-finding very difficult or almost impossible, since there is unusually not only one echo but a number of echoes occurring at once, making it very difficult to determine, and still more to seize, the quarry.
          *Experience shows that the efficiency of the S-Apparatus diminishes considerably in waters with stratification.  Water stratification and varying density of seawater occur after a long spell of irradiation by the sun and a calm sea, and with great intensity at the meeting of waters of a different nature, e.g. at the meeting of the slightly salt waters of the Baltic with the much saltier waters of the North Sea in the Skagerrak and Kattegat, also in the Gibraltar roadstead (channel) at the edge of the Gulf Stream and in the Gulf Stream itself, and in other places.  (See the atlas of seawater density).  These water stratifications produce a deflection of the sounding ray, so that the echo no longer returns to the receiver.  The position-finding apparatus, under these conditions, loses its efficiency altogether, or retains it only for quite short distances.
          From this it may be inferred that the efficiency of the enemy's position-finding apparatus is mostly less in summer than in winter, also in waters with a high degree of stratification (Skagerrak, Kattegat, west coast of Norway, Pentland Firth, Gulf Stream, Gibraltar Channel.)
6) This collation is for the purpose of including all observations and intelligence received up to the present, and of attempting to indicate the position-finding technique and tactics of the English.  Further observations will be needed to complete or correct the picture, details which appear unimportant in isolation have their significance.  All observations must be entered by the boats in the war log and the intelligence report.
                                                                    signed Dönitz
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