Page 31 - Summer 2018

Variable Depth Sonar
In 1948, Longard postulated that the cold intermediate water layer between the two warm layers on the Scotian shelf would form a “sound channel” at the depth of the sound speed mini- mum and that a transducer lowered to this depth should de- tect a target in the same layer at long ranges (Longard, 1993). As an initial test, an existing sonar capsule was fitted with a bridle and cable designed to be lowered from a stopped ship. Trials in 1949 and 1950 demonstrated very long detection ranges with both the transducer and submarine target at the depth of the sound channel. With the concept demonstrated, the focus turned to the engineering required for an operation- al system: the development of appropriate cables and a tow body, transducer, and a display system.
Measurements and theory of cable drag indicated that the length of cylindrical cable required to achieve desirable tow depths and speeds would be too large for practical handling. The solution was to use a faired cable (one with a stream- lined cross section) to reduce the drag and thus the required cable length. The final cable design consisted of a formed rubber section fastened by U-shaped metal clips that turned easily on the cable. The first tow bodies were simply tor- pedo bodies fitted with UK “Type 144 Asdic” (i.e., sonar) transducers. Over six years of development were required to develop a suitable tow body, and the final design based on the Royal Navy (RN) 100-inch sonar dome was known as TRILBY. The experimental “variable depth sonar” (VDS) using the TRILBY body become known as the CAST/1.
The increased operational depth intended for the CAST/1 allowed for a higher power output than the modified steel- quartz transducer from the Type 144 Asdic could provide. Thus, the CAST/1 used the new high-activity low-imped- ance piezoelectric ceramics being developed at the NRE at the time. Small concentrations of cobalt added to barium titanate ceramics resulted in a large reduction in dielectric loss at a high electrical driving power. The optimal mix was determined by Schofield and Brown (1957) and this “NRE- 4” ceramic (the fourth of seven concentrations tested) was used in the CAST/1 transducer.
The electronics and display equipment were designed in conjunction with the Defence Medical Research Laboratory (now the DRDC Toronto Research Centre), who provided “human engineering” advice. The combination of high- quality audio signal replay and visual presentation resulted in “unusually good target classification” (Longard, 1993).
Figure 4. Operational variable depth sonar on the HMCS St. Laurent. Note the Royal Navy (RN) White Ensign (flag) that was the Canadian Naval Ensign at the time. The TRILBY tow body containing the transducer is visible (far right). Water streaming out suggests that it is being recovered. Photo cour- tesy of Canada, Department of National Defence.
In March 1958, the CAST/1X electronics (an experimental version of CAST/1) and TRILBY body were tested by the RCN on the HMCS Crusader, with the resulting recommen- dation that it be accepted with as few modifications as pos- sible to expedite production (Longard, 1993). The CAST/1X demonstrated its superiority in trials when comparing its performance to that of the British experimental medium- range Asdic. As a result, the RN stopped development of their Asdic and adopted the Canadian design. The opera- tional variant of the VDS (Figure 4) was known as SQS-504 by the RCN and Type 199 by both the RN and the Royal Australian Navy and is on display in the Canadian War Mu- seum in Ottawa, Ontario.
Surface Reverberation
When attempting to detect active sonar echoes from a target in the ocean, there are two main sources of interference: am- bient noise and reverberation. Ambient noise consists of all the sounds one is not interested in, and in the case of anti- submarine warfare, it consists primarily of noise from wave breaking, shipping, biological sources, and ice. Reverberation consists of the diffuse echoes arising from large numbers of spatially separated scatterers, primarily the ocean surface and bottom, and scatterers suspended in the water column.
In the modern day, it easy to explain the sources of noise and reverberation, but in the 1950s and 1960s, each of these terms was being examined in detail. At the NRE, Robert P. (Bob) Chapman led many studies using explosive sound
Summer 2018 | Acoustics Today | 29