Active sonar research (1947 – ’60s)
Around 1950, as a result of the aforementioned marine committee, the laboratory model of an Anti-submarine Installation (ADI) was completed. Piezo-electric crystals made from Seignette salt (potassium sodium tartrate) were fixed to a thick steel plate which served as counter mass. These crystals resonated at a frequency of 25 kHz. The frequency of this active sonar was adjustable between 17 and 35 kHz. The transmission power was 250 W. In that time this “searchlight sonar” was ultramodern. The sonar system contained everything one could wish for. The arrangement of the crystals in diagonally placed squares allowed a directional sound determination in the horizontal plane between the left and the right quadrants, while the upper and the lower quadrants provided the directional sound determination in the vertical plane. Therefore, this arrangement the detection of the incoming sound direction.
In addition to the electronic part, which was of a completely new design, the transducer was also an own development. The latter contained as active elements Seignette salt crystals manufactured by a specialised Dutch industry. For underwater use, the crystals were enclosed in a watertight housing filled with castor oil and with a rubber ‘sound window’ at the front. The electronic equipment included a part that could eliminate the movement of the own ship as part of the speed determination of the target, a so-called own Doppler nullifier (ODN).
At 03:30 AM, November 6 – 7, 1952, the Panamanian steam-powered freighter Faustus ran ashore north of the Noorderpier near Hook of Holland. At 09:00 PM, the ship broke through the Noorderpier and sank in the Nieuwe Waterweg shipping channel. The wreck blocked the access to the port of Rotterdam. The experimental ADI on the Paets van Troostwijk and an experimental active sonar for the detection of sea mines was used to precisely determine the wreck location. With buoys, half of the navigation channel could be made operational again within 24 hours.
After 0:30 PM on August 23, 1954, the KLM DC-6B aircraft ‘Willem Bontekoe’ (PH-DFO), flight KL608 New York-Amsterdam, crashed in The North Sea between Egmond aan Zee and Bergen aan Zee. The ADI on the Paets van Troostwijk was used to locate fragments of the aircraft until November 225. A personal report of this search mission can be found on Our Fleet (in Dutch).
The laboratory model of the sonar was successfully tested onboard HNLMS Paets van Troostwijk on the North Sea, off the French coast near Brest, and in the Mediterranean Sea. The decision was made that the Dutch industry would take over the serial production of the sonar device under the technical supervision by the laboratory. The production prototype (under the name DATO: Detection Device Against Submarines) appeared on board HNLMS Marnix and worked to satisfaction. This was followed by serial production of the PAE-1 by the Van der Heem company. These installations have served for a long time on ships of the Royal Netherlands Navy. This sonar also attracted international interest. This is evident from sales to the German, Swedish and several other foreign navies. This success was not only related to the design but also to some special features. For example, there was a visible indication on an electron beam tube of the Doppler effect, which is the frequency shift that occurs as a result of the movement of a possible target. Moreover, the device directly provided electronic information for the onboard fire control and weapon systems.
In parallel, research was carried out into other piezoelectric materials such as barium lead titanate. This research took place in collaboration with the Central Laboratory of the Dutch PTT.
The active sonar was continuously improved and expanded by the laboratory. In this way a warning version of DATO was developed, the WARO. WARO was commercially produced by Van der Heem as the CWE-1. The successor CWE-10 was equipped with a newly developed, magnetostrictive transducer and a more powerful transmitter. This larger transmitter was driven by the existing CWE-1 transmitter and was therefore placed as an extra module between the original transmitter and the transducer. With that transmitter, the energy output increased from 250 W to 10 kW.
In addition to the active sonar, experiments were also carried out with a Corrective Attack Plot device (“ACP”) that, under favourable conditions, provided an indication of the target’s location if the length thereof was large in comparison with the other dimensions. Improvements in all aspects of sonar and data processing technology were tested and sometimes successful. This resulted in an improvement of the efficiency of the energy transfer between transmitter and transducer, and of automatic gain control. Mechanical arrangements were replaced by electronic circuits and various methods for the visual presentation of the detected sonar signal were tested.
Thanks to the van der Heem & Bloemsma documentation centre, website http://www.vanderheem.com/index.html for some links to the industrial versions of the developed sonar equipment.