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The anti-submarine warfare (ASW) capability in an ocean wide detection system that robs submarines of their chief asset is its concealment. Conventional detection of submerged vessels involving both acoustic and non-acoustic techniques is highly effective in sector location. However, over the past several years, acoustic techniques are the mainstay for detection and tracking of submerged bodies like submarine and AUV’s vehicles. Military sonars are meant for detection and tracking of underwater contacts like dived submarines in the oceans. Long range detection of such contacts will be possible only by making use of sound channels or paths like surface ducts, convergence zones, or bottom bounce, which support long distance propagation of sound. A ‘surface duct’ is a natural near-surface sound channel in the oceans. In tropical oceans like the Arabian Sea and the Bay of Bengal, strong surface ducts form during winter, but disappear by summer. To be more effective in the case of hull mounted sonars, the surface ducts need to be a few tens of meters thick. Submarines avoid surface ducts by diving deeper (below the ‘layer’) thus making detection more difficult for an adversary. Presence of permanent thermoclines is a major oceanographic feature in tropical oceans. Large sound speed gradients in the thermocline region result in substantial downward refraction of sound, which severely affects long range detection capability of sonars. The most difficult target depths for hull mounted sonars lie just below the layer, where a prominent geometrical shadow zone could exist beyond a couple of kilometers from the sonar. Even in the case of targets within the layer, detection could be hampered by in-layer shadow zones. The technology for making submarines quieter, and for hiding them amid background noise, has kept pace in the past two decades with advances in acoustic sensors. The development of quieter submarines with many noise reduction techniques like precisely balancing rotating parts to minimize vibration, mounting machinery on sound absorbing platforms, and covering submarine with anechoic materials which absorb sound, the noise signatures of submarines reduce significantly and are likely to be reduced further. Other concerns are the limited range of acoustic transducers and the effect of ambient noise in the oceanic environment. Also acoustic sensors may be confused by the decoys fired form enemy platforms which generates similar sounds of submarine. Moreover, acoustic detection is a piecemeal affair, relying upon large numbers of individual sensors scattered throughout the oceans. Over a wide area, acoustic detection requires the extensive deployment of sonar buoys or towed arrays and the data fusion of their responses to provide a surveillance picture. Another limitation of wide area surveillance is concerned with the revisit time, i.e. the period between successive surveillance of the same area. For these reasons, traditional acoustic methods are not likely to provide the detection to allow simultaneous attack on all the present submarines at sea. The movement of surface and submerged vessels produces vortices in the water excited by its motion, thereby forming wake of disturbed water. The track left in the water by a moving vessel, the wake, is an important clue in the non-acoustic detection. The ocean wake structures can be categorized as (i) turbulent wake stretched out directly behind the vessel, (ii) Kelvin waves generated by the passage of vessel and propagating outward from the vessel track, (iii) narrow V-shaped wakes generated by the hydrodynamic processes along the ship’s hull and (iv) internal wave wakes generated under conditions of shallow stratification. Theoretical possibilities of detecting minute changes on the ocean’s surface caused by a submarine or wake it leaves underwater have been widely recognized. Several contemporary oceanographic remote sensing techniques, ranging from direct detection of the vessel structure using laser light, to indirect detection using analysis of the effect the vessel has on the surrounding marine environment, have potential for application in this area. A number of non-acoustic observables are being considered for non-acoustic detection of these objects such as Magnetic Anomaly Detection; Biological disturbance: bioluminescence, mammal behavior; Surface waves due to submerged moving objects; Chemical and radioactive effects: trace elements; Temperature changes due to reactor; disruption of thermocline; Optical reflectivity or absorption: laser; Effects of turbulent wakes on surface; Internal waves due to movement of underwater object. The emerging techniques based on the remote sensing data from satellite data show great promise in the detection of surface targets. Recently, majority of research on non-acoustic methods focuses on the detection of internal wave signatures created by the movement of underwater target using satellite data, which according to several studies seems to be the most promising physical effect to exploit for the detection of submerged vessels over a wide area. In the present day, there is a need to extend this concept towards the detection of underwater targets where satellite images can be analyzed for expected patterns. Space borne Synthetic Aperture Radar (SAR) sensors have demonstrated their ability to observe ocean features related to dynamical processes including internal waves, currents, eddies, fronts and the presence of bathymetric features in all weather, day or night and has wide area coverage with swaths up to 500 km across. This availability and coverage gives SAR satellite data the potential to enhance acoustic environmental assessment for naval applications. The signatures often visualized in SAR data are mostly linked to reflect in the optical range or emitted in thermal bands and back scattered from the ocean surface in the microwave range of the electro-magnetic spectrum. In nutshell, the combination of non-acoustic methods of detection of underwater targets and the traditional acoustic methods employed in Sonar’s would lead to a focused approach towards a better underwater surveillance technology.
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