Indian Defence Review

As infra-red emission is a surface problem, coating ships with low emissivity paint will reduce IRCS significantly. From the law of infra-red emissivity it can be proved that the radiated Intensity (I) is proportional to the Temperature (T) to the power four times. It can be shown that a 10 per cent increase in temperature will result in 46 per cent increase in radiated intensity. Therefore, infra-red stealth is vital in warship design.

Acoustic Signature & Allied Aspects

Ships are full of equipment generating significant acoustic noise which can be detected underwater when the sound is transmitted through the hull. Sound can travel very long distances under water so that ships can be detected by submarines, torpedoes and sonar dipped from helicopter. The greatest threat to a warship is from the silent submarine and sonar. There are two main types of sonar technology in naval use – passive sonar listening to sounds generated from surface vessels and submarines and active sonar which emits pulses of sound energy from a transducer and then listens for returning echoes. The time between transmission and reception of echo enables a sonar operator to establish the range of the target.

For a modern warship, the importance of minimising emitted sound cannot be underestimated. Any piece of machinery incorrectly mounted can provide tell-tale signs of its presence to the experienced sonar operator. In a combat situation, an active pulse can be detected and hence, passive sonar is used instead. Propellers are the noisiest part of a warship. As the propeller blades spin, they create a region of partial vacuum at the trailing edges. Cavitation bubbles, which first grow rapidly in size and then collapse equally quickly due to the surrounding high-pressure water, form in the lower pressure region behind the blades. When the bubbles collapse, a great deal of energy is released in the form of acoustic shock waves which can even generate light through sono-luminescence.

One way to reduce the collapsing cavitation bubble noise as practiced by the US Navy is to inject low pressure air into the partial vacuum behind the rotating blades, which reduces the pressure difference between the bubbles and surrounding water so that the bubbles collapse more slowly and more quietly. Also, there is great emphasis placed on propeller blade design to reduce the effect of singing, blade rate noise and cavitation. Modern propulsion units have reduced cavitation at high speeds with all moving parts housed in ducts so very little sound is radiated directly into the marine environment with a more laminar flow.

The key sources of noise that contribute to a warship acoustic signature include its own active sonar transmissions, machinery both propulsion and auxiliary, flow fluid in distributed pipeline systems and cavitation of propellers. Sonar transmissions are minimised by reducing the time and sonar power levels. However, most of the machinery-radiated noise is provided by ship’s diesel engines, gas turbines which cannot be readily turned off. It is necessary to employ great care to incorporate noise suppression measures to achieve low target noise levels. These include resilient mounts, double mounts, rafts, flexible coupling and quiet propellers. In case of submarines, rubber or polyurethane tiles having broad and narrow band characteristics with multiple layer coatings are tailor-made against enemy sonar. One successful method to silence noisy machinery in the future could be to simply drill holes in the casing that surround it. Work is going on to develop this in France.

Magnetic Signature Aspects

Warships are basically large metal objects which concentrate earth’s relatively weak magnetic field within them and create distortion or magnetic anomalies which potentially have the ability to trigger the detonation of magnetic mines. Warships can now reduce these magnetic distortions to low enough levels so that magnetically triggered mines can not detect them. This can be done by magnetising the ship’s hull in the opposite direction to the earth’s magnetic field, cancelling out the effect. Reverse magnetisation is normally achieved by using hull–embedded electro-magnets. It is also possible to design warships from non-magnetic materials such as GRP, vitally important to mine-hunters.

Deperming is now viewed as a procedure for erasing the permanent magnetism from ships and submarines, in order to camouflage them against magnetic detection vessels and enemy marine mines. Sea-going metal hulled ships will nonetheless develop a magnetic signature as they travel due to interaction with earth’s magnetic field and this signature can still be exploited by magnetic mines or facilitate detection of a submarine by ship or aircraft with Magnetic Anomaly Detection (MAD) equipment. Navies use deperming procedures as a counter-measure against this.

When a warship is close to a magnetic mine or magnetic torpedo, the magnetic field of the warship actuates the firing mechanism causing the mine or torpedo to explode. Degaussing is thus a fitted electrical installation designed to protect warship against magnetic mines and torpedoes. The purpose of degaussing is to counteract the ship’s magnetic field and establish a condition such that the magnetic field near the ship is as nearly as possible the same as if the ship were not there. Degaussing coils are simply large diameter electrical wires which when carrying a large enough current are able to produce an electro-magnetic field.

The US has developed High Temperature Superconducting (HTS) degaussing coil system which works by encircling the vessel with superconducting ceramic cables whose purpose is to neutralise the ship’s magnetic signature. The main advantage of the HTS Degaussing Coil System is its greatly reduced weight and increased efficiency.

Bio-luminescence Aspects

The night time wakes of the ocean going ships are frequently observed for their bio-luminescence flow fields. Ship-wake bio-luminescence is well known and in fact, the last German U-boats detected in World War I was sunk because it created a significant bio-luminescence footprint. Bio-luminescence can give away submarine position to a vigilant enemy. As such, the bio-luminescence flow simulated light emission is now considered a real threat to maritime stealth operations. Marine bio-luminescence is produced by a vast number of creatures including bacteria, dino-flagellates, radiolarians, jellyfish, hydrozoa, sea pens, sea pansies and comb jellies. Bio-luminescence is especially abundant in warm coastal regions where nutrients are abundant and marine life thrives.

Wake Effect Aspects

A ship on surface will generate a wake which not only persists for a long time but also easy to see at high altitude and even on satellite imagery. In incompressible liquids such as water, a bow wake will be created when a warship moves through the medium. As water cannot be compressed, it is displaced instead, resulting in a wake. The wake spreads outwards from the source until its energy is sufficiently dispersed. The wake bubbles are a real problem which can persist for up to a kilometre behind the warship.

Radar systems now mounted on satellites are sensitive enough to detect ship wakes. The US Navy has recently patented a technique that may make warship harder to detect by eradicating the bubbles as soon as they appear. In a test to remove bubbles, several transducers injected 1 MHz acoustic waves into the water and the bubbles floated to the surface. It is opined that active production of 1MHz acoustic waves into water is perhaps not a practical stealthy solution.

Extremely Low-Frequency Signature Aspects

Galvanic currents flowing in the hull and in the water around the hull can generate underwater electrical potentials. Under certain conditions, these can cause ELF electrical fields to be radiated into the water. Detection of ELF signatures can be prevented by certain basic electrical design measures. Recent research combining ELF electrical signals recorded along with those of hydro-acoustic detection is even now producing a greater level of sophistication in target detection and identification.

Likely Future Cross-Sections in Stealth Technology

Maritime sensing could be revolutionised in the future with developments in ultraviolet spectrum and ELF waves. Moreover, warships may incorporate more natural or synthetic materials such as multiple wavelength coatings that are becoming increasingly necessary notably of visible, infra-red and microwave wavelengths to provide optimum response across all threat wavelengths. Active RAM could be an attractive possibility for the future or a surface skin could act as a large area living ear to listen for long range search radar pulse characteristics and then re-radiate low amplitude but out-of-phase pulses to cancel the enemy radar echoes.

In the future, technology may also utilise the abundance of water in the maritime environment for various activities such as directed walls of water for close range missile defence, mist sprays to defeat the electro-optics of future sophisticated guided missiles and to provide ice packs on Arctic Patrol vessels to insulate heat from external thermal imagery detection. It is likely that satellite tracking of ships in visible, near infra-red, radar and thermal bands will become more common place over the next decade.

Conclusion

Overall electronic stealth design has become extremely sophisticated, encompassing numerous signatures and cross-sections. The ongoing one upmanship between offensive and defensive capability will continue to encourage this trend, requiring design of further stealthier warships. The ideal of a warship which cannot be detected is unlikely to be achieved across all energy bands concurrently. Warships will be less likely to be detected if signatures are minimised. This also increases the probability that decoys and electronic counter measures will be more effective against enemy threats.

In most cases, the monitoring of emissions is simple, comprising easily applied safety issues which can readily counter any unwitting ignorance such as use of mobile phone on the quarter deck in combat situation. At the same time, sophisticated electronic systems are quite capable of generating fake or pre-recorded radio emissions to saturate the airways and saturate the enemy’s ability to handle the communication traffic.

With so many possible different types of sensor emissions from a warship, be they active signatures or passive cross-sections, it would be imperative to consider the impact of communications equipment used onboard and the overall coordination of all of the warship’s sensors and communications to have a transmission control policy in place at all times to ensure that all emissions are kept as low a level within operational constraints. A warship’s stealth should not be compromised by operator error.

Providing stealth features in the design of a warship will entail additional costs. It has been reported that Visby stealth Corvette costs 50 per cent more than a conventionally built Corvette of the same dimensions. The projected cost of the future US Navy DD(X) Zumwalt is $2.8 billion which represents a significant increase. However, the DD(X) will offer full-spectrum signature management to cloak it from a variety of detection and targeting methods. In spite of the higher costs, stealth features are now considered amongst the key requirements around which a warship will be designed and built.