Visualisation of sonar targets (the 1960s)
In the early 60s, the Dutch Navy participated in a NATO exercise in the Mediterranean. A Swedish submarine passed undetected underneath a 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, a 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 its 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 budget 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. 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 by the Doppler indicator.