Underwater Acoustics, period 1957 – 1980
Doppler shift (1957 – )
In the early sonar installations, the observer’s hearing has always played an important role, especially when it comes to moving targets that showed a Doppler frequency shift. It was obvious to mimic the frequency-distinguishing characteristic of the human ear with a series of narrow-band filters. This could then be used in plural in panoramic installations. The necessary filters were manufactured around 1957 with the aid of electron tubes; at a later stage, transistors were used herein.
Incidentally, it was not only in analogue circuits that the transistor made its appearance. With transistors, it became feasible to manufacture digital circuits that would require far too much space with electron tubes, not to mention power consumption and heat generation. Thus, a ring core memory was manufactured which cooperated with the aforementioned filter elements. This memory made it possible to visualise the result of a number of sonar transmissions.
In such a memory, the Doppler echoes of four consecutive transmissions as a function of the distance are stored in memory. Only Doppler echoes that give results in all four transmissions are considered as targets. In this way, false echoes are suppressed as much as possible. Submarines approaching or moving away give a frequency shift in their echoes due to the Doppler effect. This frequency shift is audible as a pitch difference. In the ADI, PAE and CWE sonars, this was indicated by the slope of the echo on the indicator screen. However, this reading was not very accurate. A major improvement was obtained by passing the echo signal through a number of electronic band filters with adjacent narrow frequency bands of each 10 Hz wide in the “filter bank receiver”.
The signals from these band filters were displayed on the indicator screen on either side of the central line. After the sonar signal has been transmitted underwater, a light spot passes from left to right across this screen. If an echo comes in, the dot will light up brightly. The location where that echo occurs then gives the distance of the submarine, while the extent to which the echo appears above or below that central line is a measure for the Doppler shift. The black registered signals in the figure are of a real purpose, the red signals are false detections. For a maximum Doppler shift of + and – 400 Hz, 80 filters with a bandwidth of 10 Hz are required. Subsequently, band filters were frequency relays, filter type Q-multipliers using electron tubes, filter Q-multipliers using transistors, and magnetostrictive filters.
Colour sonar display
Early 60’s, the Dutch Navy participated in a NATO exercise in the Mediterranean. A Swedish submarine passed undetected underneath Dutch Navy ship. The cause was that the light green afterglow sonar image was disturbed by sea clutter and reflections from the sea bottom and waves. This caused many light spots on the screen so that the dot of the submarine echo did not stand out in the larger echoes of the environment. The LEOK devised a method to use doppler information combined with colour. The moving submarine could be made visible with a different colour than the cluttered environment of echoes of non-moving objects. The Navy sonar system in use at that time used 36 hydrophones in a circuit around the ship. The direction of the echoes could be determined by the summation of adjacent hydrophone signals from 36 bundles each covering 10 degrees.
These signals were used for the presentation of the sonar image. For colour information, LEOK was able to use doppler filters developed by the TNO Physics Laboratory in The Hague. To display the image permanently, TV presentation was chosen in which the original image had to be converted from revolving to TV-mode. For the colour tube, a brand-new Sony TV was stripped. Only the picture tube and the high voltage parts were used. Digital memory for the TV image was combined with own electronics that provided for the deflection of the electron beam. After a trial run, the Navy was enthusiastic. The Commander of the Navy said: “That system must be installed on all our ships!” This was politically not approved because of agreed budgets cuts. Ultimately, ten copies of the Colour tube were built in-house at the MEB (Navy Electronic department) in Oegstgeest; nine copies for the ships and one for the Navy school in Den Helder.
A panoramic sonar
A panoramic sonar is capable of receiving echoes from different directions simultaneously. In order to know not only the direction and the distance but also the Doppler shift, in 1965 a sonar receiver was developed that could simultaneously display echoes in different sound beams underwater. Spread over two screens, the echoes are displayed on each screen in six adjacent bundles of 10o wide each. Together these bundles cover a sector of 120o. Six bundles are shown on the right screen in white on black. In each bundle, the display of the echoes is identical to that of the prototype Doppler Indicator that was used in the CWE-10, but there the light spots run from the bottom up instead of from the left to the right. The electron tubes for visualisation were custom made for this device by Sylvania, Syracuse, USA around 1965. Each tube had six electron guns. The flat display tube is used in the Doppler indicator.
Around 1965, the Royal Netherlands Navy, unfortunately, felt that the Netherlands would be too small to develop its own active and passive sonar systems. Instead, a connection would be sought with other (larger) countries. This also included the circumstance that, especially for passive sonars, there was only very little or no sailing time available for testing. After this decision, the emphasis of the Underwater Acoustics group of the TNO laboratory was therefore on the study and research of those parts of the sonar that determine its quality, such as the transducer, the signal design (for active sonars), the signal processing and the belonging presentation. By maintaining the expertise in this way, one was able to provide advice to the Navy on many aspects of sonar. This was especially important when the sonar equipment of the “Van Speijk” class of frigates was specified and technically tested at a later stage.
Sound propagation in seawater
The quality of a sonar is not only determined by the technical characteristics of its components. With the improvement of these properties, sometimes up to the limit of the physically possible, attention increasingly focused on the properties of the propagation of the sound in (sea)water. Apart from disturbances, sound propagation is also a limitation of the sonar possibilities. Around 1970, for that reason, the first cautious steps were taken in the unknown field of “medium research”. The unknown was not only in the research itself, but also in the methods for obtaining large numbers of measurement values, the computer processing thereof, and the interpretation of the results.
From the onset on, it was certain that the work would concentrate on a few specific points that were considered important and had not yet been sufficiently investigated. In particular, consideration was given to the factors that cause distortion of the sonar signal in any way. A number of circumstances have encouraged the smooth start of this work. For example, the Royal Netherlands Navy supported this work with an assignment and through the provision of facilities. The latter concerns the AFAR (Azores Fixed Acoustic Range) where the first extensive series of measurements were performed. Also, sailing time was made available for the new oceanographic vessel Hr.Ms. Tydeman (A906). In good cooperation, this research vessel was, equipped with a number of facilities especially for researching sound propagation in seawater. The smooth start is also largely due to the fact that the necessary expensive electronic equipment could be purchased. Some parts of it (transducers, hydrophones and electronic circuits) that were not for sale were manufactured in-house by TNO. It is clear that the contacts with foreign laboratories, which had already started such research several years before, were of great importance in this sound propagation research. Thanks to the activity in this new area, a start could be made with a new possibility to predict the reach of a sonar system. The use of special sound paths was also studied.
In order to overcome the need for two ships to perform sound propagation experiments, a special raft was made. This raft was able, on the open sea and unmanned, to perform the task of one ship: controlled by radio commands, selected sound signals were produced underwater. These signals were then received and processed remotely at the measurement ship. This raft, called NEREUS (Netherlands Experimental Raft-Suspended Electromagnetically controlled Underwater Sound-source), which was unique in its kind, was a great saving in needed ship time.
A so-called non-linear effect is present in the propagation of sound in water. This effect, relatively small and therefore previously neglected, came to the attention because of the special possibilities. In this context, the laboratory has made a special transducer with which the properties of “non-linear acoustics” are studied. This knowledge allowed for great potential in the design and manufacture of sonar transducers and hydrophones, which is also used by civilian authorities. Considerable experience has also been gained with the design and manufacture of electronic circuits, both for sonar and for measuring and auxiliary equipment. Finally, with propagation studies and experiments, a new area was entered that also requires new activities both theoretically and technically. The previously acquired experience forms an indispensable element in this.
Around 1980, a new principle of submarine detection was started. The submarines were no longer tracked by actively transmitting sonar signals, but by listening with sensitive hydrophones to the noise caused by the submarine. These hydrophones were then towed far behind the ship in a hose (length of 150 metres) attached to a long cable (1200 metres). This allows one to listen to submarines at very large distances and keeps the array’s hydrophones away from the ship’s own noise sources. However, this method has the disadvantage that if one detects a submarine, one does not know whether it is on the right or the left side. To solve this problem, not one hose was dragged but two hoses next to each other. The sound strikes one hose earlier than the other> In this way, one can determine whether the sound source is at the left or the right. Towed array sonar, however, only works well by virtue of advanced signal processing capabilities.
In one study, 32 acoustic sensors were installed at equal distances in the hose supplemented by twelve non-acoustic sensors. Each sensor is connected to the deck unit with a wire connection of more than 1200 metres. Because this large number of connections made the system vulnerable, a solution was required: the transfibre system. At the end of the hose, a multiplexer unit (“pod”) was installed which supplies the sensor signals with a high sampling frequency in time-division multiplex to a single optical glass fibre that transfers the signal to the ship via the 1200 metre-long towing cable, where a deck unit converts the received signal back into a set of individual sensor signals.