The Germans have developed two types of supersonic mine units; first a twin receiver unit known as the A 106; secondly an echo sounder unit known as the AE 1 when used in buoyant mines, and the AE 101 when used in ground mines.

By May of 1945 these units were in the advanced development stage. The Naval Offi-cer in Charge for the development of naval influence mine units at SVK, Kiel, considered that the AE 1 was ideally suited for use against the U.S. because of the adapability of the unit in deep water.

The Navy unit (AE 1) was designed for use in moored mine of the EMF and SMA types. The Luftwaffe unit (AE 101) was designed for use in ground mines, BM 1000 types and was also considered for use in the moored mine BM 1000 T. The electrical components of the units are almost identical, differing only in those respects peculiar to the mines for which intended. One of the main problems was the proper orientation of the echo sound-er. In the case of AE 1 this problem was not too difficult, since the EMF and SMA mines, being of the moored type, are self orientating; the only change necessary in such mines was the redesigning of the top cover plate. To obtain proper orientation of the unit when used the BM 1000 type mine, a special float was designed. The echo sounder was moun-ted in the float, the float being connected to the mine by a four-wire, rubber covered cable three feet long. The float was carried inside a protective housing and released therefrom by means of an electro-mechanical device, after the mine had settled on the bottom. This release device presented serious design problems and was one of the prin-cipal factors in delaying the development of the AE 101.

The A 106 consists of two magnetostrictive receivers tuned to 22.5 kilocycles per second. One receiver is focused vertically upwards, and the other in a ring having a maximum at about 55° from the vertical. A noise source which subtends an angle of more than 30° from the vertical axis of the receiver produces a greater volatge output from the horizontally focused receiver, and blocks the mine. A noise source which subtends an angle of less than 30° from the vertical axis produces a greater voltage output from the vertically focussed receiver and fires the mine. In this way the mine is made to have a definite range against ships, independet of their loudness, provided this is greater than a very small threshold value. The mine is also proof against acoustic sweeps or underwater explosions which are not directly over it. The receivers are maintained in the correct orientation by mounting them in a small float which is released when the mine is laid and is moored to it with about six feet of cable.

The AE 1 operates in a similar manner to a normal echo depth sounder inverted. It uti-lizes magnetostirctive transducers resonant at 30 kilcycles per second. A ship passing over the mine causes a sudden change in the apparent distance to the water surface, and the mine unit is so constituted that, when the rate of hange of this distance ex-ceeds a certain value, the mines fires. Elaborate precautions have been taken to ensure that the mine will become passive if it receives a signal, such as the noise produced by an acoustic sweep or underwater explosion, at an instant when it is unlikely to receive an echo from water surface or from a ship.

The twin-receiver mine was chosen by the Luftwaffe as an aircraft-laid ground mine in prefernce to the echo-sounder mine, first because of its greater simplicity, and second-ly because it tends to fire under a ship's engine room. The major source of high-frequen-cy noise from a ship is the propeller, and the mine will fire at a fixed distance of about 50 feet ahead of this. It will fire up to a similar distance abeam. The echo-sounder unit on the other hand is triggered when a vessel's bow comes into the transmitter beam, so the mine is fired slightly ahead of the ship unless a time delay is used in the firing circuit. The German Navy preferred the latter unit for the following application. They proposed to mount it in a buoyant mine carrying a charge of 780 pounds, which would normally be moored about 27 fathoms beneath the surface, too deep for normal wire sweeping. When the echo-sounder unit was actuated by a ship's bow, the mine would be released from its mooring cable, and, after a seven-second delay it would fire, having risen about 12 fathmos in the meantime. For this application the echo sounder units were mounted di-rectly in the cover plate of the buoyant mine, and relied on its taking up an orientation which would be approximately vertical. A modified type of the AE 1 echo sounder unit, known as the AE 101 was mounted in a float for use in ground mines. It was proposed to lay these in depths up to 18 fathmos, with a 1,500 pound charge.

In order to prolong the life of the batteries and the tubes of these high-frequency mines, the Germans arranged that they should be switched on only when a ship was in the vicinity. This was accomplished by means of a sensitive carbon button microphone tuned to a 250 cps.

In the AE 1 and the AE 101 mine units the Germans used an ingenious anti-sweep ar-rangement in the trigger circuit. Each time the carbon microphone is actuated, it swit-ches in the echo sounder for a period of 100 seconds. If the mine is actuated for more than about five such periods in rapid succession, it will go passive for a period of half an hour to an hour. This principle against mine-sweepers would seem to have a good appli-cation. However, it must be realized that the system has the limiation that the mine can be made passive by sweepers for a sufficiently long time to allow ships to pass over it in safety.

Prototype models of the two receiver units type A 106 had passed sea trials and shock trials, and stability tests of the float had been concluded. An order for two thou-sand units had been placed with Atlaswerke, and it was said that 200 units had already been made at their works in Gnadenfrei in the Russian zone.

Experimental models of the echo-sounder unit had passed sea trials and shock trials satisfactorily. Twelve prototype units were almost ready for trials.