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Shallow water operations has been and still is an important facet of Submarine operations. They are capable of effectively denying oceans to hostile surface naval forces, performing covert infiltration of hostile regions, providing intelligence and striking ground targets with missiles whilst operating in the littoral waters. Detection is the first element of Anti-Submarine Warfare (ASW). Airborne ASW is usually performed by either Long Range Maritime Reconnaissance (LRMR), Medium Range Maritime Reconnaissance (MRMR) or ship-borne ASW helicopters. The aircraft have a long endurance, extensive sensor capacity and a relatively fast transiting speed compared to the surface vessels. The extant methods of performing the first and most important ASW element by aircraft includes Radio intercept using Electronic Support Measures (ESM), Radar, Sonar, Sonics, Electro-optic and infrared cameras (EO/IR) and Magnetic Anomaly Detection (MAD). Despite evolution of such sensors for over 100 years, submarines still enjoy credible stealth whilst operating in shallow waters. During the Falkland war, despite the limitations on the Argentine submarine capability of only two being operational, British forces spent a great deal of time attempting to track them, frustrated by the difficulty of conducting sonar operations in shallow water. Barring the developments enhancing the submarine stealth technology, the other factors affecting their detection in shallow waters are numerous and complex.

Shallow waters are generally considered as the coastal waters starting from the coastline to the continental shelf up to a depth of 200 m. Deep waters are generally those with depths greater than 600 ft (182 m). Studies of shallow water noise spectra are also undertaken normally for waters upto a depth of 200m.

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Importance of Shallow Waters to Submarine Operations. Shallow water operations generally undertaken by submarines include but are not limited to Intelligence gathering, sabotage, insertion of Special Forces and land attack. These operations require them to approach the enemy coast in littoral waters which is indeed a challenging affair to the submarines with respect to navigation and safety. However, shallow waters are also a lucrative environment to the submarines which provide them adequate opportunities to hide and lurch their way to achieving their mission.

This service paper aims to bring out the challenges of detecting submarines operating in shallow waters using airborne sensors and propose possible way ahead in overcoming the same.

Performance of Extant Sensors in Shallow Waters
Presently, air ASW platforms worldwide have numerous sensors as standard fit. Both acoustic as well as non-acoustic sensors play vital roles in detecting submarines either surfaced, at periscope depth or submerged. But when the submarines operate in the littorals/ shallow waters, the performance of these sensors are ineffective. The effect of shallow waters on these sensors are highlighted in the succeeding paragraphs.

Non-acoustic Sensors.

Airborne Radars.The radars of today have advanced features and enhanced ranges thereby improving their detection and classification capabilities by leaps and bounds. The airborne radars are able to provide high probability of detection and tracking performance despite high sea states and extreme weather conditions. Presently, radars generally detect large ships (frigates and above) at greater than 150 nm, vessels under 50 m (FACs) and surfaced submarines at greater than 100 nm and snorting submarines at about 40 nm. More accurate figures on performance are classified but are however constantly improving. Since the wavelengths of radio waves used by radars does not penetrate the water surface, only targets on water surface can be detected by radars. With this obvious limitation of radar, only submarines surfaced or at periscope depth would be detected by it. However, detecting a snorting submarine in shallow waters close to the coast is like searching for a needle in a haystack. The littoral waters present an extremely dense environment with numerous small fishing boats, vessels at anchorage, sticks, floating devices and marine life on the surface thereby confusing the radars resulting in exponentially reducing the probability of submarine detection.

ESM.The downside to this sensor is akin to that of a radar and has one in addition. The submarine has to expose its radar mast and has to transmit for an ESM system to come into play for detection. Thus the submarine is undetectable by an airborne ESM system if the former is submerged or has the radar turned off while on the surface or at periscope depth.

Magnetic Anomaly Detection (MAD).This system is generally present only onboard fixed wing LRMR/ MRMR aircraft in a boom-shaped tail. The equipment consists of magnetic sensors, and is used to detect changes in the disturbances in the earth’s magnetic field. The earth’s magnetic field changes considerably when a submarine is present and may be detected by a low flying aircraft operating MAD equipment. Large metal objects will always disturb the magnetic field that is distributed along the North-South axis, and in the case of ASW, the submarine functions as a large body of metal in the water. When the aircraft flies at a low altitude, this disturbance is detected by the sensor, thereby confirming the presence of a submarine underwater. However, practically it has been observed that the disturbance is not significant enough to provide an efficient means of submarine detection especially in the shallow waters which has a complex magnetic field due to the close proximity of vessels acting as huge chunks of metal.

Electro Optic/ Infra-Red (EO/IR) Sensors.FLIR or Forward Looking Infra-Red is one of the latest sensor fit on most of the modern military aircraft including fixed wing, helicopters and Remotely Piloted Aircraft (RPAs). The obvious downside to this sensor is that the submarine has to be surfaced if the camera/ IR sensor has to classify and provide further details to the aircrew. Because of this simple fact, the modern cameras and IR sensors are of little use when it comes to actually searching for a submerged submarine.

Acoustic Sensors.Acoustically, the shallow water/ littoral environment enhances the submarine’s ability to operate covertly as the extant acoustic sensor performance has been sub-optimal to poor. The effect of shallow water on airborne sonars and sonic systems are elucidated below:-
Sonars (Passive Mode).Shallow waters present extremely noisy environment to sonars due to its proximity to the coast, concentration of traffic, confluence of rivers to the seas, etc. Submarines typically radiate in the same frequency band as that of ships i.e. less than 1 KHz. The use of passive mode of an airborne sonar is hence limited in terms of signal-to-noise ratio (SNR) and the acoustic aperture or the antenna size to be commensurate for airborne use. In case of active mode, the sea environment distorts a transmitting pulse when it travels to and from the target.
Sonars (Active Mode).The range of an active ASW sonar system is determined by environmental conditions, by the operating characteristics of the sonar, such as power-level and beamwidths, and by the operating frequency. The general rule of thumb is, the lower the operating frequency, the longer the sonar’s range. However, a lower operating frequency in the past has required a larger and heavier sonar transducer. Since the ASW helicopters have limitations in space and carrying capacity, traditionally a compromise would have to be made choosing as low as possible operating frequency. However with respect to employment of active sonars in shallow waters, the limitations arise from the inhomogeneity of the medium as well. The transmitting pulse of an active sonar interacts in a complicated manner with the sea surface, sea bottom, sub sea bottom structure and water sound speed profile. The performance of active mode in shallow water is therefore heavily dependent upon the acoustic propagation conditions of the environment; specifically sea depth, state of the sea, sound speed profile, reverberation and noise. The active transmissions bounce off the bottom and returning confusing signals back to the operator. Sounds are also refracted and reflected by the different layers in the water column that are more clearly distinguished from each other in coastal waters. Further, fresh water, differences in temperature, turbulence and currents in the littorals will create layers in the water, which complicate all sorts of acoustic searches, no matter how efficient they are. These factors are extremely dynamic and stochastic thereby making the active mode of an airborne sonar sub-optimal in comparison to its effectiveness in deep waters.
Sonics.As far as acoustic propagation in shallow waters is concerned, the passive as well as active buoys will face the same dynamics as that of an airborne sonar and hence would be limited in performance. However, the advantages of sonics over sonar is that multiple buoys can be deployed in the area creating a multistatic environment where the possibility of triangulation exists. But the dropping of sonobuoys from an aircraft in accordance with the tactical buoy field patterns formulated for reaping optimum performance is next to impossible due to traffic density near to the coast. In addition, the buoys are susceptible to swamping in such an environment.
Airborne ASW in shallow waters is hence a complicated business. The aircrew needs to understand this complex environment they are working in and they need to have a working knowledge of the particular equipment they use and how well it might function. Mobility, flexibility to change operating areas quickly and efficiently and the ability to deploy sonobuoys are the primary advantages of an air ASW platform. But as the submarines get quieter and quieter, the airborne ASW sensors are lagging behind, and moving the operational area into littoral waters certainly complicate matters for the ASW aircraft. The extant ASW sensors do not perform optimally in shallow waters close to the shore as they might do at deep waters and in order to stay on top of the game, alternate methods for submarine detection in shallow waters need to be explored.

Way Ahead
The solution to the above mentioned challenges posed by shallow waters is being explored by navies worldwide and advanced air ASW sensor prototypes are being developed for employment in shallow/ littoral waters. Undoubtedly, sound waves propagates the best in water when compared to EM waves. However, despite the existence of sonars and the technology maturing for more than 100 years, there has been little credible success in submarine detection at the littorals. The other alternative to sound in water which has shown encouraging potential in air ASW is ‘light’. On evaluation of advancements in sonars and other sensors being developed for employment in shallow water ASW, the proposed way ahead for overcoming the extant challenges are enumerated in the succeeding paragraphs.
Twin Inverted Pulse Sonar (TWIPS).As per information posted in open source, research at Southampton University in the UK has begun to challenge the relative security submarines currently enjoy in littoral waters, with the development of a system known as TWIPS based on the natural sonar of dolphins.  Scientists there have shown that a dual stream of underwater pulses can penetrate bubbles more effectively than conventional sonar and provide a significantly better detection rate – at least under laboratory conditions.  Although the work is still in its infancy, it could herald the end of easy invisibility for submarines amid turbulent coastal seas.

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