Impact of Biological Noise on Sonar Performance in the IOR
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Issue Vol. 30.2 Apr-Jun 2015 | Date : 22 Nov , 2015

The deployment of nuclear powered submarines is a game changer. However, that comes with its own challenges. The deployment of the erstwhile INS Chakra way back in the year 1988, had experienced major operational difficulty, due to Snapping Shrimp noise, while being deployed off the East coast of India. Snapping Shrimps are found in very large groups in the tropical littoral waters and their acoustic signals are high intensity sound that can swamp the sonar display. The vast oceans/seas away and closer to our shores will require massive efforts and mass participation that may not be feasible by only research organisations. The user-academia partnership has been a weak area in India so far, particularly when it involves field experiments, but the requirement of such collaboration is inescapable now.

As the third largest body of water on the Earth, the Indian Ocean is an important resource providing the major sea routes connecting the Middle East, Africa, and East Asia with Europe and the Western hemisphere. Driven by Asia’s economic rise, the Indian Ocean is surpassing the Atlantic and Pacific as the world’s busiest and most strategically significant trade corridor. One-third of the world’s bulk cargo and around two-thirds of world oil shipments now passes through the Indian Ocean. An estimated 40 per cent of the world’s offshore oil production comes from the Indian Ocean. South Asia ranks among the world’s most densely-populated regions, containing almost 1.6 billion people – about a quarter of all the people in the world.

The stability and security in South Asia has been in flux over the last half century as the region went through a wave of democratisation that is yet to be completed. Wars in this region have been fought over territory, resources and religion. The combination of all these factors provides a view of relative instability and insecurity. Thus, the security of this region has great economic and political implications. Regional cooperation to ensure the safety and security of these vital trade routes will become more important over coming decades.

Snapping Shrimps had nearly jeopardised the deployment of a critical strategic submarine…

The geo-strategic scenario compels India to play a very critical role in the Indian Ocean Region (IOR). The constructive engagement that India has to indulge itself in terms of dealing with its neighbors and many other strategic partners, she also has to strengthen her maritime forces to leverage on the possibility of any conflict situation by providing deterrence. A strong maritime force in the IOR will facilitate economic growth and political advantage for India and boost its journey from a developing nation to a recognisable global power.

The maritime strategy post independence in India continued to remain limited due to the continental outlook of our policy makers. However, things have changed in the last decade and we see significant maritime activity on varied aspects right from infrastructure development to augmentation of the maritime forces. Focusing on the maritime forces, it is estimated that by the year 2015, the government of India would have spent $80 billion in capital outlays for military modernisation. Of the total spend, 16 per cent would have been dedicated to acquisitions for the Indian Navy.

A major naval modernisation programme has been underway since 2002, aimed at promoting the Indian Navy from the fifth to third largest fleet in the world. The aircraft careers and submarines have received significant attention in the program. More recently, the Indian Navy’s Maritime Capabilities Perspective Plan was tabled in the Indian Parliament in the last week of April 2012 by the Standing Committee on Defence that seeks to dominate the Indian Ocean Region by acquiring blue water operational capability while effectively countering current and emerging threats closer to the coastline.

The push for a ‘blue water’ navy entails that the two categories of platforms – aircraft carriers and submarines, are high valued and critical assets that demand specialised underwater capabilities for their protection and operational deployment. The IOR with its tropical littoral waters presents unique challenges for operational deployment of sonar for tactical as well as strategic purpose. The dependence on effective underwater sensor is even more crucial and requires a completely different approach. The induction of INS Chakra and the launch of INS Arihant demand a new perspective towards ASW and its preparedness.

The tropical and sub-tropical littoral waters that is the precise description of the IOR…

The last strategic submarine deployment (INS Chakra leased from Russia in late 1980s) and the lessons learnt need to be revisited and future studies and approach evolved to meet the challenges of ASW in the IOR. The strategies adopted by other nations need to be reviewed and incorporated in our planning process. The specific reference here is to the impact of biological noise with respect to Snapping Shrimps in the tropical waters on the sonar deployment. Vice Admiral R.N Ganesh (Retd.), the Captain of the first INS Chakra (Project 670A Skat – Charlie-I Class Nuclear Submarine) in his firsthand account titled “How Shrimp Nearly Wrecked India’s First Brush with a Nuke Sub” had given a detailed account of how Snapping Shrimps had nearly jeopardised the deployment of a critical strategic submarine.

In his post script he had written, “This story is based on a real experience in INS Chakra in 1988. The noise was generated by the denizens of large shrimp beds in the general area of Kakinada. From later reading I learned that the culprit was the Snapping Shrimp, which thrives in tropical waters near the coast. A one and a half-inch crustacean almost foiled a 5,000-tonne nuclear submarine!”

The University of California, Division of War Research at the US Navy Radio and Sound Laboratory San Diego, California had prepared a detailed study titled “Underwater Noise and the Distribution of Snapping Shrimp with Special Reference to the Asiatic and the Southwest and Central Pacific Areas” way back in April 1946. It was a highly classified study that was de-classified and made available in the public domain in October 1967. The main aim of the study was to analyse the impact of snapping shrimp noise on sonar performance. The US Navy was redefining its role post the World War II and at the peak of the Cold War, it was important for them to generate significant data that could ensure effective underwater sensor performance globally. The report identified that Snapping Shrimp beds existed in tropical and sub-tropical littoral waters and observed that the characteristics of noise generated by them overlaps with the military sonar frequency band.

The report highlights the following aspects that are relevant to the maritime strategic thinking in India today:

(a) The characteristics and the dominant intensity level of Snapping Shrimp noise that can mask the received signal of any military sonar deployed for ASW mission.

(b) The abundance of Snapping Shrimp beds in the tropical and sub-tropical littoral waters that is the precise description of the IOR.

(c) The Navy-Academia collaborative model for undertaking study on such a relevant issue for national security and maritime strategy.

The Asian Seas International Acoustics Experiment (ASIAEX) included two major field programmes conducted in the spring of 2001, one in the South China Sea and the other in the East China Sea (ECS). The program was conducted primarily under the auspices of the US Office of Naval Research and the Natural Science Foundation of China along with multitude of research organisations from US, China and Korea. The programme was a great example of academic collaboration being initiated and supported by the US Navy to understand the tropical littorals far away from its mainland where its naval platforms could possibly be deployed in the future.

Strategists have to evolve a way to foster partnerships between the academia and the Indian Navy…

The present paper attempts to highlight the importance of ambient noise characterisation specifically the biological noise component pertaining to Snapping Shrimps for facilitating effective sonar deployment in the IOR. The complexities of sonar deployment in the tropical littoral waters of the IOR is presented and the way ahead discussed to effectively exploit and monitor the vast undersea zone, relevant to our maritime interest. The gains in the shift of the naval deployment plan from conventional submarines to strategic submarines could be further leveraged with enhanced sonar performance by overcoming the limitations caused by the uncertainties of the biological noise component in the underwater medium. Significant and meticulously planned R&D efforts are required to overcome the limitations of the underwater medium in the tropical littoral waters of the IOR. Collaborative models for such R&D efforts have been discussed in this paper as brought out in the subsequent sections.

Sonar Operations in the IOR

The significant thrust on maritime strategy faces unique challenge of ensuring effective sonar performance in the IOR. No sonar deployment can be optimum, if the medium uncertainties are high and the received signal at the sonar deployment location undergoes random fluctuations.

The medium characteristics are largely reflected by the sound velocity profile that influences the propagation characteristics of the sonar signal underwater. Fig. 1 shows a typical sound velocity profile undersea.

The deep water propagation of sound is characterised by refraction of the sound around the sound axis with minimal interaction with the surface and the bottom of the sea. Fig. 2 presents deep water propagation. In case of shallow waters the sound axis, either does not exist in the available water column or partially available to force multiple interaction with the surface or the bottom or even both. Fig. 3 presents typical shallow water propagation.

Post World War II, significant efforts were made to minimise the uncertainties of the underwater medium and the ambient noise on the sonar performance.1,2 The sonar deployment during this period was in deep waters specifically to ensure engagement of the adversary far away from the mainland. The shift in the naval operations post the Cold War era from the deep waters to the littorals has resulted in a significant change in the naval operational strategy and the challenges.3The multiple interactions with the surface and the bottom of the littoral sea while propagating underwater ensures significant influence of the local conditions on the sonar signal. Local condition here refers to the characteristics of the surface parameters and the bottom type/profile that are unique and site specific.4 The generalisation of the channel behavior is not possible unlike the deep water sonar operation. However, the site specific underwater channel behavior has limited the usefulness of studies undertaken in the deep waters post World War II and resulted in sub-optimal performance of the sonar systems in the littoral waters.5

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The views expressed are of the author and do not necessarily represent the opinions or policies of the Indian Defence Review.

About the Author

Vice Adm DSP Varma And Cdr (Dr) Arnab Das

Vice Adm DSP Varma Adviser to DRDO and Cdr (Dr) Arnab Das Commander in the Indian Navy.

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One thought on “Impact of Biological Noise on Sonar Performance in the IOR

  1. This is the best article I have read in this web site so far. Majority of Naval officers are interested in increasing the number of weapon carrying vehicle rather than giving importance to its safety. Till last year all our warships. and frigates were working without Active Towed Array Sonar (ACTAS) system.

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