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Dimensions of Submarine Threat in the Littorals –A Perspective

(Published :  “FEATURED | Dimensions of Submarine Threat in the Littorals –A Perspective by RADM Dr. S. Kulshrestha (Retd.), INDIAN NAVY.” IndraStra Global 01, no. 11 (2015): 0408. ISSN 2381-3652,)

Dimensions of Submarine Threat in the Littorals –A Perspective

Abstract

The littorals present a very complex environment in which the platform, weapon and the target interplay is dependent upon the real time and archival understanding of the medium parameters. The article aims to provide a perspective into the extent of the littoral underwater submarine threat and the constraints which hamper its successful prosecution. It also brings out the fact that the Blue water Navy would have to enhance its environmental understanding and modify its approach towards anti submarine operations to reduce likely attritions during littoral conflicts. The article brings out the imperative need to dove tail fundamental environmental research and Indian Naval requirements to tackle the threats in littorals.

 

 “…the very shallow water (VSW) region is a critical point for our offensive forces and can easily, quickly and cheaply be exploited by the enemy. The magnitude of the current deficiency in reconnaissance and neutralization in these regions and the impact on amphibious assault operations were demonstrated during Operation Desert Storm.”

Maj. Gen. Edward J. Hanlon Jr., Director of Expeditionary Warfare, Sea Power, May 1997

A blue water navy’s ability to execute manoeuvre in littorals is severely compromised due to confined sea spaces, lesser depths, heavy traffic, threats due to lurking quiet diesel submarines, coastal missile batteries, swarms of armed boats, deployed mines and threats from the air. The definition of a littoral region encompasses waters close to the shores as well as greater than 50 nm at sea. The Indian Navy, like all the other blue water navies has not been fundamentally positioned for close combat encounters. It is has generally been expected that sea warfare would have standoff distances of at least 50/60 km if not more between adversaries   (outside range of torpedoes and guns). If Carrier groups and anti ship cruise missile (ASCM) cruisers are deployed, the standoff can be up to a couple of hundred kms (ASCM and Air craft limits). However today littorals present an inevitable close quarter engagement situation with CSG remaining well clear of coastal missile batteries and aircraft operating from shore based airfields. In case of countries like China, the CSG may even remain a thousand km away to save itself from a barrage of carrier killer missile like the Don Feng 21 D with a range of over 2000 kms[i].

 Thus littorals have withered away the advantage of the CSG and the big ships as manoeuvring in close quarters is not feasible any more. The lighter ships would have to fight in the littorals with a much larger risk of attrition from the diesel electric submarine, mines, swarm of boats and shore based assets. The blue waters represent large swaths of sea with adequate depths for operations, and much less uncertainties in the sensor – weapon environment. The littorals are confined zones with reducing depths and a very adverse sensor environment. This has drastically compressed reaction times leading to requirements of great agility for the men of war.

 A worthy defender is always considered to be in an advantageous position in the littorals, fundamentally due to the intrinsic knowledge and experience in operating in his home environment. It constitutes what the US DOD calls an access denial area likely to impinge upon the US national interests in the Vision 2004 document this has been articulated as “To win on this 21st Century battlefield, the U.S. Navy must be able to dominate the littorals, being out and about, ready to strike on a moment’s notice, anywhere, anytime[ii] The Indian Navy has in all probability identified areas in Arabian Sea, Bay of Bengal and Indian Ocean where it may have to engage in littoral conflicts either singly or in concert with coalition of navies, should such a contingency arise. On the other hand 26/11 had opened the coasts to attack by terrorists and the Government of India has initiated efforts to tighten its coastal security. As to the plans of defending own littorals against a formidable expeditionary force, nothing much is known in the open domain, in all likely hood it remains a simplistic defensive model due to insufficient focus and the inevitable funding. The fact remains that ocean rim state navies today are focussing more on littoral capability than building a blue water navy. Indian Navy has to consider the littoral capability seriously whilst modernising and achieve a balance, depending upon its current and future threat perceptions. The blue water force has to have an embedded littoral component force so that the IN can operate in littorals far away from her home ports.

 The major under water threats comprise of mines and undetected diesel submarines. However as far as mines are concerned, they are every coastal country’s weapon of choice as they are economic, easy to lay but very hard to detect and sweep. They are the psycho sentinels of defence, since unless their existence is proved it has to be assumed that waters are mine infested and have to be swept before warships can attempt a foray in to the littorals[iii]. The clearing of mines for safe passage is a very time consuming and intensive exercise which introduces significant delays in any operation, while taking away element of surprise and granting time to adversary to plan tactics. Therefore in case delays are not acceptable, littoral operations would have to cater for some attrition on account of mines as well as navigation hazards posed due to sunken or damaged ships on the sea route.

 The aim of this article is to derive a perspective in to the fundamental dimensions of the littoral medium, platform and weapon with respect to the underwater submarine threat which constitutes the most potent hazard to a powerful navy.

Operating Littoral Environment

The littorals comprise of different types of zones in which a Navy has to operate. These include continental shelf, surf zones, straits and archipelagos, harbours and estuaries. The main thrust of naval operations hinges upon the underwater acoustics (sonic ray plots) which provide not so accurate measure of effectiveness of Sonars. In the continental shelf not much is known about the tactical usage of bioluminescence, plankton or suspended particles and other non acoustic environmental information. Quantifiable effect on performance of different sensors and weapons under various conditions is also not available to the Commander to help him deploy them optimally. Further predictions about conditions for naval operations in continental shelf areas of interest are at best sketchy and no reliable database exists to provide correlation between various environmental conditions that may be encountered. In the surf zone region (within 10 m depth line till the beach), temporal and spatial environmental data is required for effective planning of naval operations however, there are large variations in acoustic data over short and long term. Archipelagos and straits are subject to; swift changes in currents and water masses due to restricted topography, dense shipping, fishing and human traffic which complicate planning. Most of the harbours are estuarine in nature and present a highly intricate and variable environment (tides, currents, wave amplitudes etc) warranting a holistic approach to understand the same.

Thus it can be seen that carrying out missions in littorals also involve other aspects of environment in addition to the uncertain under water acoustics which have a direct bearing on the missions. These aspects include real time and archival data bases of; meteorological surface conditions required for efficient operation of IR, Electro optical, and electromagnetic sensor and weapon systems; under water topography, accurate bathymetry, bottom composition, and detailed assessment of oceanographic water column environment for under water sensors and weapons.

The availability of overarching oceanic environmental knowledge would provide insight into enemy submarine operating/hiding areas, location of mines and underwater sea ward defences. Currently the Indian Navy does not have the capability to carry out exhaustive littoral environmental scanning let alone field any sensor or weapon system that can adapt to the dynamic littoral environment and carry out missions with conviction. In fact, even for own littoral zones this type of information is not available which would enable effective deployment of static or dynamic defences.

Effect of Environment on Propagation of Sound in Shallow Waters

A brief description of the acoustic environment in shallow waters is relevant at this stage. The main factor affecting acoustic propagation in deep oceans is the increase in pressure of water column with increasing depth as the temperature remains nearly constant. The speed of sound increases with depth and the sound waves finally hit the bottom and reflect upwards. In shallow waters the rays tend to refract, that is bend upward without going to the bottom. This phenomenon takes place when the refracted sound velocity equals the sound velocity emitted by the source. On reaching the region of the source they again refract towards the depths and this process continues.[iv] In very shallow waters the sound rays are reflected upwards from the bottom. The amount of sound energy reflected upwards depends directly on the nature of the sea bottom. Harder the sea bottom better is the reflection and vice versa.[1]In shallow waters it is clear that the sound speed depends mainly on the temperature which in turn depends upon the amount incident solar radiation, wind speeds, wave action etc.

It can therefore be inferred that the acoustic signal in shallow waters is dependent upon factors like temperature, sea surface, nature of sea bottom, waves and tides, in-homogeneities and moving water masses amongst others. These present a very complex effect on the acoustic signal by altering its amplitude, frequency, and correlation properties. Further, multipath reflections from the bottom, as well as surface put a severe constraint on signal processing. The understanding of the underwater sound propagation remains unsatisfactory to this day. The complex interplay of acoustics, oceanography, marine geophysics, and electronics has bewildered Navies searching for submarines or mines in the shallow waters. Two fundamental issues that of beam forming and lining up the sonar are discussed in the subsequent paragraphs.

 Zurk et al[v] in their paper “Robust Adaptive Processing in Littoral Regions with Environmental Uncertainty” have addressed a real time problem in underwater, i.e. the dynamic nature of the sensor, target, medium and the interfering element’s geometry. The moving sensor, target and medium causes difficulties for adaptive beam former sonars which are designed to assume a certain level of stationary conduct over a specified time period . The time period required is dependent upon the number of elements in the array and the coherent integration time. Since large arrays give much better resolution  they have a larger number of elements, leading to a moving source transiting more beams during a given observation period. If target is in motion, the target energy is distributed over many beams weakening the signal and degrading the accuracy of targets location. The arrays with larger volumes thus have larger probability of motion losses. Some techniques to reduce these errors include sub-aperture processing, time-varying pre-filtering of the data, and reduced-dimension processing.

 Naval sonar systems have become more and more complex over time and require expert operators. Optimising sonar line ups has become essential in a littoral environment where the acoustic properties change rapidly over time and space domains. With the sonar automatically determining the optimal line up based upon desired inputs from the operator and the sensor feeds of operating environment, the sonar operator would be able to give his full attention to the task of detection, identification and classification of the targets. The necessity of   autonomous environmentally adaptive sonar control is imperative in littorals because of the tremendously large number of objects which may be present below the water line and skills of the operator would be put to test to sieve out the elusive submarines.

 Warren L.J. Fox et al. “Environmental Adaptive Sonar Control in a Tactical Setting.”[vi]  Have addressed the issue of sonar line up and have recommended neural networks for generating acoustic model simulations required.  Control schemes for Sonars are of two types, namely acoustic model-based and rule-based. Model-based controllers embed an acoustic model in the real-time controller. In acoustic model based controllers, acoustic performance predictions are inputted in to the controller, based upon available estimates of the existing environment, which in turn, gives the feasible sonar line ups. The choice of line up depends upon the chosen parameters for the operation. In the rule based controller, a generic set of operating environmental conditions are defined by the sonar and acoustic experts, which are then subjected to acoustic modelling and the sonar equations to generate the best possible line up. The existing environmental conditions would have to be assessed in real time prior to selection of the best line up available as per design. This approach however may not account for the large number of varying environments that are the hallmark of different littorals, and lead to discrepancies in results. Thus it appears that acoustic model based controller may be a better choice, as it largely takes care of the prevailing conditions at sea, than the rule based one, but it requires much more computational power and time to assess the situation prior to lining up the sonars. Warren L.J. Fox et al[vii] have recommended a method of training artificial neural networks for use in a sonar controller for ships as well as unmanned under water vehicles, to emulate the input/output relations of a computationally intensive acoustic model. Artificial neural networks are much faster and utilise far lesser computational capacity.

 The Submarine Threat in Littorals

 The shallow waters pose a serious problem for under water acoustics, they remain unfriendly to current sensors like towed arrays, variable depth sonars and air dropped sonobuoys due to depth limitations, deployment of torpedoes ( both ship and air launched) and depth charges. Shallow waters with close proximity to land also pose difficulties for radars and magnetic anomaly detectors thus providing a relatively safe operating area for small diesel electric submarines. Detection of surface craft by submarines in passive sonar mode is much easier because of their higher acoustic signatures. The surface ships would perforce resort to active sonar transmission as their passive capabilities are degraded in littorals. This in turn makes them more acoustically visible.

 The littoral submarine however has a limited period of quiet operation under water of a couple of days, as it has to either surface or snorkel for recharging its batteries by running its diesel generator sets. The battery capacity drainage is directly proportional to the running speeds, faster the submarine travels quicker is the discharge and hence larger is the discretion rate, which is the charging time required. Interestingly it is this discretion rate, which allows the submarine to be vulnerable to detection. During charging, the radiated noise of diesel generators, the IR signatures and the likely visibility of the snorkel make it susceptible to observation by trained crews. The submarine therefore prefers to lie in wait, barely moving or just sitting at the bottom for the prey to arrive.

 Development of Air independent propulsion technology (AIP) has enhanced the submerged time of submarines by a great extent (from a couple of days to about two weeks). The AIP is dependent upon availability of oxygen on board. The AIP while granting more submerged time to a submarine unfortunately provides the same level of acoustic signature as a snorkelling submarine, thus making it prone to detection.[viii]

 Fuel cell technology has been successfully interfaced with AIP and Siemens 30-50 KW fuel cell units have been fitted in the German Type 212A submarines since 2009, it is said to be much quieter , provides higher speeds and greater submerged time.

 The weapons for the submarines include mines, torpedoes and the submarine launched missiles. The technology ingress in computing, signal processing, hull design and materials have benefitted the submarine, its sensors, weapons and fire control systems. These advances coupled with vagaries of the acoustics in shallow waters have made the diesel submarine a very potent and lethal platform. While many countries have AIP submarines, of interest to India is the acquisition of these submarines by Pakistan[ix] since the Indian Navy does not operate an AIP submarine. The Indian Navy today even lacks the adequate numbers of diesel electric submarines required.

 The Unmanned Submarine (Unmanned Underwater Vehicle; UUV)

 An UUV generally is a machine that uses a propulsion system to transit through the water. It can manoeuvre in three dimensions (azimuth plane and depth), and control its speed by the use of sophisticated computerised systems onboard the vehicle itself. The term Unmanned underwater vehicle includes, remotely operated vehicles, Paravane, sea gliders and autonomous underwater vehicles.

It can be pre programmed to adhere to course, speed and depths as desired by the operator, at a remote location and carry out specific tasks utilising a bank of sensors on the UUV. The data collection can be both time and space based and is referenced with respect to coordinates of the place of operation. It can operate under most environmental conditions and because of this, they are used for accurate bathymetric survey and also for sea floor mapping prior to commencing construction of subsea structures. The Navies use them for detecting enemy submarines, mines, ISR and area monitoring purposes etc.

 The UUVs carry out their routine tasks unattended, meaning there by that once deployed the operator is relatively free to attend to other tasks as the UUV reaches its designated area of operation and starts carrying out its mission, be it survey, search, or surveillance.

Compared with many other systems, UUVs are relatively straightforward, with fewer interoperable systems and component parts, facilitating reverse-engineering of any components that might be restricted in the commercial market place. All of these factors, however also increase the likelihood that even a low tech littoral adversary could easily field offensive, autonomous UUVs, this in turn leads to seeking rapid developments in UUVs by major navies.

 UUVs are on the verge of three developments which would accelerate their induction into modern navies. First is the arming of UUVs to create Unmanned Combat Undersea Vehicles (UCUVs). This is virtually accomplished with UUV designs incorporating light weight torpedoes as weapons of choice. Heavier UUVs are contemplating missile launchers and/or heavy weight torpedoes as weapons in their kitty. However these appear to be interim measures, as a new class of weapons specific to unmanned vehicles are already under advanced development. These include much smaller and lighter missiles, torpedoes and guns firing super-cavitating ammunition.

 A second potential technology development is radically extended operational ranges for these armed UUVs. Already, the developed countries have invested in programs to create long-range underwater “sea gliders” to conduct long-range Intelligence Preparation of the Operational Environment (IPOE) missions[x]. While the technologies enabling the “sea glider” approach probably do not provide the flexibility and propulsion power to enable armed UUVs, such programs will significantly advance the state of UUV navigation and communications technologies. Leveraging these advancements, other nascent technologies such as Air-independent-propulsion (AIP) or Fuel Cell propulsion or perhaps Aluminium/Vortex Combustors, could provide the propulsion power necessary to effectively deploy armed UUVs even well outside of the operating area limitations of conventionally powered submarines.

 Finally, “autonomy” for these armed, long range UUVs will allow them the flexibility to conduct operations far away from the home port. Artificial intelligence (AI) based autonomous control systems are being developed at a frenetic pace, fuelled principally by demand for improved UAVs. Such developments will directly contribute to UUV autonomy, but in fact, are not actually necessary for the majority of “sea denial” missions envisioned for UCUVs. Even with current state of missile seeker technology, UCUVs would only need enough autonomy to navigate to a known area of operations (a port, choke point, or coastal location) and launch, and the missile would do the rest. For more complex missions, weapons could be guided by an on-site observer, for instance on a trawler or even ashore, in real-time or near-real-time. In short, there are a remarkably small number of “hard” technology barriers standing in the way of the long range, autonomous, armed and capable UUVs. There is little reason to think that this capability will be limited to high end, navies only. Thus networked operations of unmanned vehicles with PGMs are going to become the lethal weapon combo for the future.

 A request for information has been floated by the Indian Navy to meet its requirement for at least 10 autonomous underwater vehicles (AUVs). These AUVs are to be developed and productionised within four years of contract finalisation. The Navy has opted for a special category MAKE for the armed forces under the Indian Defence Procurement Procedure for high technology complex systems designed, developed and produced indigenously .Modular payload capability of the AUVs have been asked for, where in  payloads like underwater cameras for surveillance reconnaissance and high definition sonars can be mounted.

  UUVs in various configurations and roles such as communication and navigation nodes, environmental sensors in real time or lie in wait weapon carriers are going to be the choice platform in the littorals. These are expendable if required, economically viable, and offer flexibility in design as being unmanned they can have much lesser degree of safeties.

 Weapons

 “The Navy’s defensive MCM capabilities in deep water are considered fair today, but they are still very poor in very shallow water (VSW) – not much better in fact than they were some 50 years ago.” [xi]

Milan Vego.

 The naval mine is a relatively cheap, easy to employ, highly effective weapon that affords weaker navies the ability to oppose larger, more technologically advanced adversaries. The mere existence of mines poses enough psychological threat to practically stop maritime operations, and thus deny access to a desired area at sea. Further they can be used as barricades to deter amphibious forces and cause delays in any naval operation in the littoral. Thus, a mine doesn’t have to actually explode to achieve its mission of access denial. North Koreans were able to deter and delay arrival of U.S Marines sufficiently to escape safely, by mining Wonsan Harbour in October of 1950 with about 3000 mines.

 Mines are classified based upon their depth of operation, methods of deployment or the way they are actuated. The versatility of deployment can be gauged by the fact that mines can be laid by majority of surface craft, submarines, crafts of opportunity and aircrafts/ helicopters. Mines have been used by countries and non state actors alike with dangerous effects and thus continue to pose a credible threat to Navies as well as merchant marine.

   Types of mines are based upon the depth at which they are deployed. As per the  21ST Century U.S. Navy Mine Warfare document[xii] the underwater battle space has been divided into five depth zones of, Deep Water (deeper than 300 feet), Shallow Water (40-300 feet), Very Shallow Water (10-40 feet), the Surf Zone (from the beach to 10 feet) and the Craft Landing Zone (the actual beach). Mines are of three basic types namely, floating or drifting mines, moored or buoyant mines and bottom or ground mines.

  Drifting mines float on surface and are difficult to detect and identify because of factors like visibility, sea state and marine growth etc. Moored mines are tethered mines using anchoring cables to adjust their depths. These can be contact or influenced based mines. Bottom mines are most difficult to locate as they can also get buried under sediment layer which cannot be penetrated by normal sonars.

Mines can be actuated by contact, influence, and by remote or a combination thereof. With modular Target Detection Device (TDD) upgrade kits, the older contact mines can be easily upgraded to actuate by influence methods. The influence needed for actuation could be pressure, acoustic or magnetic or a desired combination. In addition ship counters and anti mine counter systems are also being incorporated in to the mines to make them much more potent and lethal.

Mine technology has kept a step ahead of the ships designs for low acoustic and magnetic signatures, and many countries are engaged in development and production of naval mines. Non metallic casings, anechoic coatings, modern electronics and finally reasonable costs have made mines a choice weapon for poor and rich nations alike. It is estimated that about 20 countries export mines while about 30 produce them. Sweden Russia, China and Italy are the leading exporters. Mine MN 103 Manta from SEI SpA of Italy is one of the most exported mines in the world with about 5,000 Mantas in inventories throughout the world.

 It is estimated that China has in its inventory about a hundred thousand mines of various vintages and from the WWI simple moored contact mines to modern rocket-propelled mines with advanced electronic systems for detection and signal processing. [xiii]

 The submarine torpedoes are an embodiment of a synergetic mix of engineering disciplines ranging from mechanics, hydraulics, electronics, acoustics, explosive chemistry etc. to sophisticated software and computing. Their development has therefore involved differences in propulsion designs from steam engines to electrical motors to thermal engines and rocket motors. The control and guidance systems have also evolved from simplistic mechanical/ hydraulic to sophisticated electronic and onboard computer based systems. The guidance has further diversified in to self guided and wire guided varieties. The simple straight runners have given way to active passive homers and wake homers to attack moving targets. The warheads have moved from minol based to TNT/RDX/Al and now on to insensitive explosives with a life of over 40 years. The warheads over the years have been fitted with simple contact exploders, to acoustic influence and magnetic influence proximity fuses. The diameter of the torpedoes has ranged from 324mm to 483mm to 650mm, and before settling for internationally acceptable 533mm. Interestingly with the advent of microelectronics space has never been a constraint for the torpedo and electronic/ software updates always get comfortably accommodated in the torpedo.

A major technical feature that sets apart a torpedo from a missile is the fact that a practice torpedo is recoverable for reuse; this enables excellent weapon capability assessment, crew training as well as analysis of vital firing geometry. Some of the noteworthy heavy weight torpedoes are the American Mk 48 Adcap, the Italian Blackshark, the German DM2A4 and the Russian 53-65 K oxygen torpedo.

The torpedo has been evolving with leaps in technology but some characteristics towards which the heavy weight torpedoes are headed include; faster speeds (~over 60 Kts), Quieter signature, better reliability in detection, enhanced ranges of operation (>100 Kms),smarter electronics, and increased lethality.

The cruise missile owes it origins to the German V1/V2 rockets and mainly to the fact that manned aircraft missions had proved to be very expensive during the wars (loss of trained fighter pilots as well as expensive aircraft). Unfortunately the cruise missile development until the 1970s resulted only in unreliable and inaccurate outcomes which were not acceptable to the armed forces. Cruise missiles overcame their inherent technical difficulties and owe their tremendous success and popularity to some of the technological advances in the fields of; firstly, propulsion, namely small turbofan jet engines which resulted in smaller and lighter airframes; secondly miniaturisation of electronic components, which led to much smaller on board computers thus to much better guidance and control abilities and finally,  high density fuels and much better explosives and smaller warheads.

  Cruise missiles have become weapons of choice at sea because of their ability to fly close to the sea surface at very high speeds (sub-sonic/supersonic), formidable wave point programming and lethal explosive capabilities. These make the missiles very difficult to detect and counter at sea. Some of the naval cruise missiles worth mentioning are the Brahmos, the Tomahawk, the Club and the Exocet family.

It appears that the trend towards hypersonic scramjet cruise missiles will continue to gather momentum and such missiles could be in the naval inventories by 2020. Coupled with hypersonic missiles would be real time target data updating and guidance by extremely fast computers and satellite based systems. The kinetic energy of hypersonic cruise missiles would be a lethality multiplier against targets at sea and therefore such a missile would be a formidable weapon without a credible countermeasure as on date. The costs continue to increase with new developments; however maintenance requirements appear to be reducing with canisterised missiles.

As Far as weapons are concerned the Indian Navy has a fairly reliable capability in mines, torpedoes and cruise missiles, however the numbers appear deficient for the defensive role in own littorals.

The submarines today can launch such missiles from their torpedo tubes or from vertical launchers which can also be retrofitted. In fact the submarines can launch torpedoes, missiles, UUVs and also lay mines comfortably. Thus making them, the toughest of platforms to counter.

Conclusion

“The marriage of air independent, nonnuclear submarines with over-the-horizon, fire and forget antiship cruise missiles and high endurance, wake homing torpedoes . . . [means that] traditional ASW approaches, employing radar flooding and speed, are not likely to be successful against this threat.”[xiv]

                                                                                    Rear Admiral Malcolm Fages

 The dimensions of submarine threat in the littorals, discussed briefly above encompass the underwater acoustic environment, the developments in submarine technology, the underwater unmanned vehicle, and the weapons. The discussion has brought out the potent danger an undetected submarine in littorals presents to the aggressor.

The Navy today faces a deficiency in an all inclusive understanding of the undersea environment in the coastal areas due to which it is difficult to counter the diesel electric submarine threat in the littorals. This deficit would not allow correct positioning and deployment of sensors for timely detection of the underwater peril. Research and development is also needed in the quality of sensors such that they are embedded with real time environmental information and can calibrate themselves accordingly for best results. As far as other environmental sensors are concerned, like those dependent on zoo plankton behaviour or bioluminescence fundamental research needs to be initiated with naval needs in focus. The acquisition of submarines and UUVs should be fast tracked if Indian Navy wants to be a credible littoral force.

 


[1] The speed of sound is given by the equation (available in text books):-C(T,P,S)=1449.2+4.6 T+0.055 T2+1.39 (S−35)+0.016 D(1)

Where: C is in m/sec, T in ° Celsius, D in metres and embodies density and static pressure effects. S in parts per thousand.

The speed of sound is dependent upon temperature and depth because the S, the salinity is nearly constant at 35 ppt for sea water.

————–

 [i] G D Bakshi China – Dong Feng 21-D: A Game Changer?

http://www.globaldefence.net/portals/aviation/21579-china-dong-feng-21-d-a-game-changer.html?showall=1    ( Accessed 20 Oct 12)

[ii] VISION | PRESENCE | POWER 2004, A Program Guide to the U.S. Navy, Chapter 1 http://www.navy.mil/navydata/policy/vision/vis04/vpp04-ch1.pdf ( Accessed 17 Oct 12)

[iii] US Navy DOD document, Concept for future naval Countermeasures inlittoral power projection 1998. http://www.fas.org/man/dod-101/sys/ship/weaps/docs/mcm.htm ( Accessed 23 Oct 12)

[iv] Robert J. Urick, Principles of Underwater Sound 3rd Edition Peninsula Pub (August 1, 1996)

[v] Lisa M. Zurk, Nigel Lee and Brian Tracey, “Robust Adaptive Processing in Littoral Regions with Environmental Uncertainty” in Impact of Littoral Environmental Variability of Acoustic Predictions and Sonar Performance, ed. Nicholas Pace and Finn Jensen [Bruxelles, Netherlands: Kluwer Academic Publishing, 2002], 515.  http://web.cecs.pdx.edu/~zurkl/publications/saclant_2002.pdf ( Accessed 30 Oct 12)

 [vi]  Warren L.J. Fox et al. “Environmental Adaptive Sonar Control in a Tactical Setting.” in Impact of Littoral Environmental Variability of Acoustic Predictions and  Sonar Performance, ed. Nicholas Pace and Finn Jensen [Bruxelles, Netherlands: Kluwer Academic Publishing, 2002], 595

 [vii] Ibid.

 [viii] Benedict, Richard R. “The Unravelling and Revitalization of U.S. Navy Antisubmarine Warfare.” Naval War College Review 58, no.2, (Spring 2005), http://www.jhuapl.edu/ourwork/nsa/papers/art4-sp05.pdf ( Accessed 27 Oct 12)

  [ix]  Rajat Pandit, Pak adding submarine muscle as India dithers, The Times of India Apr 11, 2011.

 [x] K. L. Mahoney and N. D. Allen Glider Observations of Optical Backscatter in Different Jerlov Water Types: Implications to US Naval Operations, Research paper, 2009. Naval Oceanographic Office, Stennis Space Center, MS 39522 USA

 [xi] Milan Vego. “Future MCM Systems: Organic of Dedicated, Manned or Unmanned,” Naval Forces 26, no.4,(2005).

[xii]  21ST Century U.S. Navy Mine Warfare, http://www.navy.mil/n85/miw_primer-june2009.pdf

[xiii]  Scott C. Truver. TAKING MINES SERIOUSLY Mine Warfare in China’s Near Seas, Naval War College Review, Spring 2012, Vol. 65, No. 2

 [xiv] Malcolm I. Fages, Rear Adm., USN; remarks at Naval Submarine League Symposium, June 2000, as published in Submarine Review (October 2000), p. 34.

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21. Naval Systems:Naval Radars and Multi Mission Combat Systems

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(Published in Defence and Security of India, May 2013)

Naval Systems:Naval Radars and Multi Mission Combat Systems

 

The discovery of reflection of radio waves from solid objects was studied by Heinrich Hertz in 1886; subsequently Huelsmeyer in 1904 was the first to patent a detector (“Telemobiloscope”) which could detect metallic objects at a distance using radio waves. In 1922 Taylor and Young showed that range and bearing of ships can be determined using radio waves even in low visibility and darkness, but the US Navy did not accept their novel idea. Many patents were granted and work carried out in secrecy in a number of countries during the ensuing period till in 1934 the British decided to use it very effectively to detect and thereafter shoot down hostile German aircraft  by using a prototype made by Watson Watts. Automatic tracking of aircraft in azimuth and bearing and subsequently in range was also accomplished during the WWII itself. Though it took over 45 years since the initial discovery for the Radar to prove its tremendous utility in military and civil applications, Radar has continued its dominance as a formidable sensor for the past eight decades. Post WWII a major improvement was to introduce moving target indicator (MTI) function by using Doppler Effect, by which it was possible to discriminate between a stationary and a moving target. This was followed by the Phased array antenna technology which involved dynamic beam forming by combined operation of a number of individual transmitting elements. Strides in digital signal processing led to development of the synthetic aperture radar and consequently to high resolution imagery.

The aim of this article is to provide the discernible reader with  a bird’s eye view of the naval radar world covering major relevant aspects, in as simple terminology as feasible.

Classification of Radars

A radar system comprises of a transmitter, a receiver, a signal processor and an antenna. Radar is used in a range of diverse applications in the civil and military field. The applications include, weather sensing, air traffic control, navigation, target detection, acquisition and tracking, missile and gun direction, air borne systems, research and so on.  Military radars can be classified in many ways; for example based upon the type of platform, i.e. land based, ship borne or air/space borne; or mission based for example, early warning, tracking, fire control, weather etc; or they may be classified based upon radar characteristics like wave form, frequency used, type of antenna etc. Most prevalent classification is according to frequency or wave form utilised.

Frequency Based Classification. The frequencies that have been longest in use are in the band 3MHz to 300MHz. Over the horizon radar (OTH), i.e. radars which can detect targets beyond the radar horizon and the early warning radars use the high frequency (HF) band 3MHz to 30MHz (eg. Russian Woodpecker and US Navy’s AN/TPS-71 Relocatable OTH radar). The accuracy in this type of radars, however gets comprised while gaining the range advantage. Very long range early warning radars use the very high frequency (VHF) band in the range of 30MHz to 300MHz, or the ultra high frequency (UHF) band 300MHz to 1GHz, this band is very useful in detection and tracking of ballistic missiles. Frequency band 1GHz to 2GHz (L band) is used in naval applications of long range air surveillance. The SMART-L naval radar has a phased array with 24 elements; it has a maximum range of 400km against patrolling aircraft and 65km against an incoming missile. The band 2GHz to 4GHz (S band) is used for Air Borne Warning and Control Systems (AWACS), Boeing E-767 AWAC aircraft uses the AN/APY-2 Pulse Doppler radar, it can determine the velocity of the target as well as distinguish between airborne and maritime targets from ground interference and sea clutter. The band 4GHz to 8GHz (C band) is used for weapon guidance; these are small but highly precise radars. An example is the TRS -3D naval radar for weapon guidance and surveillance, it uses a phased array in 3D for simultaneous detecting and tracking of multiple targets up to a range of 200km. It is designed for detecting sea skimming missiles and attack helicopters also. The band 8GHz to 12.5GHz (X band) is used for maritime navigation and airborne radars. The naval Active Phased Array Multifunction Radar (APAR) works in this frequency band, it is capable of automatic detection and tracking of low level sea skimmers up to 75km and is designed for carrying out terminal guidance requirements of ESSM and SM-2 missiles. The higher frequency bands from 12.5GHz to 40GHz are subject to very high attenuation and therefore are limited to very short ranges and have applications in civil/police/research requirements.

Wave Form Based Classification. The radars can also be classified as continuous wave (CW) or pulsed radars. As the name depicts, CW radars emit continuous waves and therefore have distinct transmitting and receiving antennas. CW radars can determine the radial velocity and bearing of the target accurately and are used for missile guidance. The pulsed radars use a sequence of modulated wave pulses and can be further classified in to; high & medium pulse repetition frequency (PRF) radars used for velocity determination, and low PRF radars used for range finding purposes.

At this juncture it would be appropriate to briefly discuss some features which affect the detection, range, bearing and display of targets in naval radars.

Features Affecting Detection, Range, Bearing and Display of Targets 

The maximum detection range of the target is affected by the frequency of operation, since lower frequencies attenuate less, therefore lower the frequency higher the range of the radar. It depends also upon the peak power output, i.e. more the power higher the range. Further longer the pulse length, more the power radiated on to the target hence longer the range. Since the difference between two transmitted pulses should be more than the maximum designed range of the radar to avoid echoes from different targets from overlapping, the maximum measurable range of the radar is also a factor of the pulse repetition rate (PRR). In addition the range is  affected by target characteristics & aspect presented (large or small, material, RCS etc), and the antenna rotation rate (lower rotation rate increases chances of smaller targets to be detected at maximum range). Radars with longer wavelengths suffer lesser attenuation at sea and therefore can detect targets at a range longer than radars with a shorter wave length.

The minimum detection range of radar is dependent mainly upon its pulse length and is ideally given by half of the pulse length of the radar (150 meters per microsecond of pulse length). It is however affected by inherent electronic circuit delays, side lobe echoes, sea clutter and the design of the vertical beam width. The range accuracy is dependent upon the precision in measuring the interval between the transmitted pulse and the received echo. Further it is dependent upon the fixed inherent errors, constancy of voltage, variations in transmitted frequency, incorrect calibration and interpretation of the target. Range resolution or the clarity, in lay terms, of separation between two targets on same bearing but nearby in range mainly depends upon the length of the transmitted pulse. The targets cannot be distinguished if they are on the same bearing unless they are separated by a distance more than half the pulse length. For example radar with pulse length of 0.05 microseconds would be able to distinguish targets which are more than 7.3 meters apart only. The target separation on the display also depends upon the proper adjustment of the gain of the receiver unit, size of the CRT spot, and range scale selected.

The bearing accuracy is dependent mainly upon how narrow the horizontal beam width is, which in turn provides clearer definition of the target. It also depends upon the size of the target, its rate of movement, parallax errors etc. Bearing resolution is the ability of the radar to separately present two targets at the same range but on different but close bearings. The bearing resolution is dependent upon the width of the horizontal beam, as the target blip on the display suffers an angular aberration up to the size of the horizontal beam width. Further two targets at the same range must be angularly separated by more than the horizontal beam width to be clearly discernible on the display. Thus for a 2 deg beam width, targets at a range of 8km should be separated by at least 320 meters. Hence a narrower horizontal beam provides a better bearing resolution.

Some Specific Types of Radars

Stealth Radars – Low Probability of Intercept Radars (LPI).     LPI radars transmit weak signals which are difficult to detect by an enemy intercept receiver. This capability is attained by the use of specific transmitter radiated waveform, antenna & scan patterns and power management features. The LPI radars are continuous wave, wide bandwidth radars emitting low power signals. This makes LPI radars difficult to detect by passive radar detection systems. Such radar is used in Super Hornet aircraft of the US Navy.

2D, 3D and 4D Radars.      A 2D radar provides range and azimuth information about the target. 3D radar, in addition provides the elevation information. These are of two types namely;  Steered beam radars, which steer a narrow beam through a scan pattern to generate a 3D picture, for eg. AN/SPY-1 phased array radar on Ticonderoga class of guided missile cruisers; and the Stacked beam radars which transmit and receive  at two different angles and deduce the elevation by comparing the received echoes, for eg. The ARSR-4 radar with a range of over 250 miles.

4D radar is Pulse-Doppler radar capable of 3D functions and determines a target’s radial velocity as well. This type of radar has great applicability in defence, since it can detect targets by removing hostile environmental influences such as electronic interference, birds, reflections due to weather phenomenon etc. In addition a 4D radar uses much less power and thus helps in stealth function. TRS-4D surveillance radar with Active Electronically Scanned Array (AESA) technology is in use by the German Navy.

Multi Mission Combat Systems.            

Multi mission combat systems have been designed for operation from naval ships as a total weapon system from detection to final destruction of the threats emanating at sea from air, surface and subsurface. Such a system is expected to search and track multiple targets and also carry out weapon direction by use of a sophisticated radar/sonar and competent decision making component.  By use of this design a naval ship can effectively carry out anti-air, anti-surface and anti-submarine operations concurrently. The most effective and proven multi mission combat system which deserves mention here, with reference to the usage of sophisticated radar, is the Aegis weapon system.

“… and among them went bright-eyed Athene, holding the precious aegis which is ageless and immortal: a hundred tassels of pure gold hang fluttering from it, tight-woven each of them, and each the worth of a hundred oxen.”

Iliad, Homer, Martin Hammond’s Translation

 

Aegis weapon system comprises of the AN/SPY-1 type radar, the Command and decision suite, Mk 99 Fire control system and Standard Missile family of weapons. The  AN/SPY-1 type radar, is an advanced multi function phased array, auto track and detect  radar which can track more than 100 targets at over 190km and also carry out missile guidance functions. It communicates with the Standard Missile for mid course correction using an RF link, terminal guidance is carried out by the AN/SPG 62 radar. The target is handed over by the AN/SPY-1 type radar to the AN/SPG 62 radar, which is a continuous wave, illuminating I/J Band fire control radar. The Aegis  is fitted on many of US Naval cruisers, destroyers and frigates. The current modifications include Commercial Off the Shelf (COTS) networking system infrastructure and Multi mission signal processor. This would also simultaneously address ballistic missile defence (BMD) along with anti-air warfare against multi mission threats. The Aegis BMD is capable of neutralising short to intermediate range ballistic missiles with Standard Missile-3 (SM-3), further it can destroy short range ballistic missiles in terminal phase with Standard Missile -2. Equivalent anti-submarine capabilities are also part of the Aegis weapon system.

Radars with Indian Navy.

Indian Navy has various types of indigenous and imported radars. Among the indigenous radars, it has L Band surveillance radar RAWL MK II &III, F Band combined warning and target indication radar RAWS 03 Upgrade, 3D surveillance radar Revathi and navigation radar  APARNA etc. Among the imported radars, it has a mix of radars from both the east and the west. Some of the imported radars are; MF-Star 3D phased array radar,MR-760 Fregat M2EM 3-D,MR-90 Orekh fire control radar, Signaal D Band radar,MR-310U Angara air surveillance radar, MR-775 Fregat MAE air surveillance radar, Garpun-Bal fire control radar, MR-352 search radar etc. The P8i Maritime patrol aircraft recently delivered to India will be operating AN/APY-10 multi function, long range surveillance radar, capable of operating day and night under all weather conditions. It provides mission support for ISR, anti-surface and anti-submarine warfare. It has both Synthetic Aperture Radar (SAR) and Inverse SAR capability, the Inverse SAR can detect, image and also classify surface targets at long ranges.

Future Trends

       New technologies are being developed rapidly in the commercial sector for low cost manufacturing processes of RF and microwave devices due to very heavy penetration and demand of smart mobiles and broad band in the public arena. These are likely to impact the defence sector and soon such mass produced devices (albeit manufactured to stricter specifications) would be available for defence use. Thus the trend is a reversal of defence requirement based technology development to mass commercialisation driven innovation. A wide range of Gallium Arsenide (GaAs) Monolithic Microwave Integrated Circuits (MMICs), RF power amplifiers and other RF devices already developed in the commercial sector  have direct applications in Radars and other RF devices in defence.

Cognitive Radar.      The term cognitive radar implies a radar that has tremendous transmit/receive  adaptivity and diversity along with high performance inbuilt intelligent computing. With the inclusion of environmental dynamic database and knowledge aided co-processor it is feasible to  add new sources of information which facilitate additional adaptivity. Currently new generation cognitive radars are at the design stage.

Quantum Radar.      Quantum illumination has been tested up to a distance of 90 miles and it is believed that soon it will be possible to establish much longer ranges utilising this principle of bouncing photons off a target and comparing them with their unaltered twin. It has been observed that the amount of information so gathered is much more than that available through conventional RF beam reflection from objects. Since energy quanta behave both as a wave and as a particle it would be possible to design quantum radar. It is expected that such quantum radar would provide a many fold increase in information parameters and data about the target than has been feasible till now. Quantum radar is currently at the concept stage.

Conclusion

Radar technology has by and large kept pace with the miniaturisation as well as digitisation of electronic components. It has also been possible for the radar designers to meet the changing multi mission requirements of modern naval warfare post the cold war. Today, using single multi function radar, a ship can track and attack emerging fast cruise missile and aircraft threats, ballistic missile attacks, swamp attacks by fast small craft in littorals and also carry out various missile & gunfire surface warfare functions.The versatility and adaptability of the radar technology has thus ensured its continued relevance to the naval designers and war fighters.

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