Vol . 8 No. 5 & 6 October/December 2000
The development of underwater sensors is a challenging task mainly due to, unpredictable ocean environment and its unknown factors. Not very many countries have acquired the capability of developing underwater sensors. Sonar (sound navigation and ranging) is the dominating sensor capable of detecting underwater targets. After the successful development of APSOH sonar for leander class of ships, DRDO has not looked back. So far, three types of hull-mounted sonars have been developed for surface ships, i.e., APSOH, HUMVAD and HUMSA. HUMSA is being fitted in all new construction ships. In addition, the .first submarine sonar PANCHENDRIYA has been proved and an airborne sonar MIHIR is in the final phase of acceptance. The development of towed array sonar NAGAN is also in progress. Sonobuoy TADPOLE and its airborne signal processor SIMHlKA and towed torpedo decoy, TOTED have also been developed and are in use.
Ocean data collection, prediction and validation is a continuous process to generate inputs for both the design of a sonar and its operational exploitation. The space-time varying ocean environmental and underwater acoustic conditions critically influence the performance of underwater sensors posing a challenge for the design work. To study ocean behaviour and to develop prediction capability, many experiments are being conducted, encompassing varied fields of basic sciences. Sonar prediction models based on ray, normal mode and parabolic equation approaches have been developed to study the sound propagation characteristics under different oceanic conditions for different frequency regimes. Based on these models, sonar range prediction packages--PRAN for low frequency application and Ray Sweep for high frequency application have been developed and are in use. An oceanographic database comprising marine geophysical, magnetic, physical oceanographic, marine meteorological and underwater acoustic parameters is being developed with the data collected by INS Sagardhwani, one of its kind marine acoustic research ship specially designed and built for DRDO .
Sonar transducer technology covers a wide range of scientific disciplines, such as transduction materials, passive acoustic materials, underwater acoustics, encapsulation, engineering and packaging, and underwater acoustic calibration and installation materials. The transduction materials operate on piezoelectric or magnetostirictive principle. While most of the user hardware is now based on piezoelectric ceramics, like lead zirconate titanate (PZT), many other active materials, like lead titanate, TERFENOL which exhibit giant magnetostiriction properties, piezopolymer composites with different connectivities, piezoelectric polymers and new mechanism like fibre-optic sensors are now emerging in specific applications. In addition, there is a large variety of passive materials used in transducer assembly to add acoustic properties. Material system engineering takes a primordial role because the same basic materials can be engineered to work on acoustic absorptions, reflection or acoustic window materials. DRDO has progressively acquired the technology know-how for transducer design and development including testing capability. Besides well-equipped laboratory facilities, there is a lake test facility at IDDUKI. The Materials & Transducer Simulated Testing (MATS) facility has been recently acquired to test the transducers to meet deep sea requirements.
The detection of underwater targets largely depends on the process by which the received reflected sound signal is converted into a detectable signal level and validated followed by display technique. The signal at the output of the acoustic sensors has the amplitude of the order of a few microvolts. These are corrupted by the ambient noise. Considering that the sensor may be capacitive or inductive or resistive, the input impedance has a wide variation. Achieving low electrical noise and proper matching in case of capacitive transducers requires expertise in front-end design. The front-end electronics to be used with the sensors must withstand the hydrostatic pressure and provide satisfactory performance. The hybrid technology has been widely used in systems to meet the above requirements. The expertise in conduction-cooled PCB development has ensured trouble-free performance of front-end electronics in hermetically sealed containers. The signal coding and data transmission schemes adopted to ensure signal integrity have been successfully developed in various systems. The technology available in front-end signal conditioning can also be exploited for any other type of sensors.
The digital signal processor (DSP)-based developed hardware has laid a
strong foundation for modular
approach to spatial and temporal processor required for sonar. Over the years, expertise
has been developed for time and frequency domain beam forming in both signal and base
bands. The DSP-based signal processors are used in all sonars. DRDO has. expertise in
fixed and floating point processing as well as in design of application specific
integrated circuits (ASICs) for sonar signal processing. The signal processing ASICs have
been developed which reduced the volume of spatial processor and enhanced the capability
of DSP boards. These can be adopted for any type of signal processing.
Power amplifier generates acoustic power through transducers. These have to withstand wide variation in load and power factors and operates in linear and non-linear switch modes. The switch mode power amplifier with high efficiency, high watt/volume and continuous power control has been developed and patented.
Wet-end and onboard installation are the areas where mechanical engineering has a significant role to play. Sensors may be fitted to hull, taken onboard, towed behind a platform, lowered from a hovering helicopter, moored at sea, etc. Therefore, optimum engineering design is required for each wet-end application. For this purpose, DRDO has successfully developed winching and handing systems for towing electromechanical cables, quick disconnect electromechanical and heavy duty connectors, scuttling and pop-up mechanisms, high pressure housings and water sealing, card cages, electronics cabinets, cabinet cooling techniques, encapsulated systems; and embedded hardware.