Stealth technology is traditionally associated with air power but it is just as relevant to the
maritime domain. Naval architects continue to refine their approaches to reducing a ship’s radar cross section.
Warships make a racket in the electromagnetic spectrum and they generate sound. This emanates from their engines, propellers and other machinery and can be mitigated by placing machinery on shock absorbers.
Ships also emit Radio Frequency (RF) energy from their myriad of radars and radios which is mitigated by using low probability of detection/interception waveforms for their radios and radars. These systems can also be momentarily deactivated to cease all RF transmissions from the ship. However, the ship will not be able to use these systems while they are switched off.
A warship is visible to the eye and conventional optronics. Thermal radiation caused by things like a ship’s exhaust allow her to be detected with infrared. Finally, the ship’s physical bulk is visible to radar. This visibility is referred to as her Radar Cross Section (RCS). The RCS is what the radar ‘sees’ when radar transmissions hit the ship and are reflected to the radar’s antenna as echoes. As in the visual domain, make an object smaller and it becomes harder for the radar to see.
A ship’s RCS size is proportionate to her size and shape. Warships are large and typically festooned with a myriad of protrusions, Masts, antennas, weapons, superstructure, in fact, anything that is above the waterline is manna from heaven for radars. They have surfaces upon which radar signals can echo back towards the transmitting radar’s antenna. These echoes will betray details of the ship’s size, speed and bearing. This is invaluable information for an attack.
Understandably, there is an imperative to reduce a warship’s visibility to radar. This is achieved through RCS reduction: “The main goal of RCS reduction is to reflect incoming radar waves in another direction away from the transmitting antenna,” says Hervé Boy, Naval Group’s senior business development manager.
Military aviation is perhaps the most famous application of RCS reduction technology. Low RCS, or so-called ‘stealth’ airframes, are de rigueur for combat aircraft. The US Air Force led the way introducing the world’s first operational stealth combat aircraft, the Lockheed Martin F-117A NIGHTHAWK, in 1983.
The naval world too has embraced low RCS design. Most surface combatants now employ RCS reduction techniques to greater or lesser degrees. Two of the most noticeable examples include the Svenska Marinen (Royal Swedish Navy) VISBY class corvettes and the US Navy’s ZUMWALT class destroyer. Media reports in 2014 spoke of the ZUMWALT class being 40 per cent larger than the US Navy’s ARLEIGH BURKE class destroyers. At the same time, the class has an RCS equivalent to that of a small fishing boat.
Reducing a warship’s RCS rests on shaping the ship’s superstructure so as to reflect incoming radar signals away from the radar antenna. A radar transmits a pulse of RF energy at the speed of light. This pulse collides with an object and is reflected as an echo to the radar. The radar measures the time for a pulse’s round trip from the antenna to the target and back again. By halving this, the radar can determine the target’s range.
The radar will also determine whether the target is moving. This is done by calculating whether the frequency of the echo is higher or lower than the frequency of the transmitted pulse. Think of a radar pulse as a tennis ball. If you drop the ball onto the floor beneath you, it will bounce back to your hands. If you drop the ball onto an angled surface it will bounce away from you.
Low RCS ship design follows the same principle. The imperative when designing a low RCS warship is ensuring that the superstructure has surfaces angled to avoid transmitting echoes back to the radar. This typically embraces the use of flat, angular surfaces rather than curves. Curves are problematic. This is because at least one point on the curve will reflect directly back to the radar.
Radar Absorbent Materials
Radar Absorbing Materials (RAMs) can be employed in conjunction with angled surfaces. Some materials reflect radar transmissions better than others. RAMs work by getting the material to absorb as much of the radar signal as possible, returning as little as possible back to the antenna.
The characteristics and behaviour of RAMs would warrant its own article. In the air domain, these have typically focused on polymer-style materials covering the surfaces of low RCS aircraft. These materials will include ferrite or carbonyl iron coated minute metal spheres. These spheres are suspended in an epoxy paint. Ferrite and carbonyl are two examples of many RAM materials and techniques.
When the radar transmissions hit the paint, they cause the spheres to vibrate. These vibrations create heat which then dissipates as it is released. The paint receives the radar energy and converts it into heat, rather than returning it back to the radar. The aircraft’s airframe acts as a heat sink by absorbing the tiny rise in temperature this process causes.
RAMs are not a cure all. They cannot absorb all radar frequencies, but they can be effective against radar transmissions in specific frequency bands. In the air domain, RCS reduction will be dictated by the radar transmissions most likely to threaten the aircraft. These will almost certainly be fire control and weapons guidance radars used to track a target and guide ordnance to the aircraft.
Such radars tend to use relatively high radar transmission frequencies of X-band (8.5 gigahertz/GHz to 10.68GHz) and above. These frequencies have wavelengths short enough to precisely track a target. This is vital when guiding weapons to their aimpoint. Although used extensively in military aviation, RAMs are employed in warship superstructures to help reduce RCS.
A 1994 academic article in the US Navy’s Naval War College Review by Captain John McGillvray entitled ‘Stealth Technology in Surface Warships’ stated that ship RCS reduction does not exist in a vacuum. It works with other self-protection techniques like chaff. Chaff is the collective noun for the thousands of dipoles dispersed into the atmosphere to outfox an incoming radar-guided anti-ship missile. A dipole is a thin piece of metal or glass fibre cut to precisely half or one-quarter of the wavelength of the radar frequency it is jam.
For example, an X-band frequency of 8.5GHz will have a wavelength of 35.2mm. Half this wavelength is 17.6mm. Chaff dipoles would need to be this length to jam a radar transmitting on 8.5GHz. This would ensure the radar’s transmissions would be reflect by the chaff as echoes.
A radar signal will hit the dipole. This will make the dipole resonate. As the radar waves keep hitting the dipole, the resonance will continue. These resonances are returned to the radar as echoes. The echoes are detected by the radar which will perceive them as targets. If thousands of dipoles are thrown into the atmosphere, then the radar will perceive these as thousands of targets. Although the target is still there, it is now masked by a dandruff of interference.
For a warship, RCS reduction needs to make the vessel’s RCS smaller than that of the chaff cloud. This will make the chaff cloud a more appealing target to the radar than the ship.
As naval architects will attest, a holistic approach must be taken when reducing a ship’s RCS. This begins with ensuring that every component above the waterline has as low an RCS as possible. Everything from gun mounts to antennas, deck lockers and helicopter hangers will have a radar cross section. The RCS of each of these must be reduced as much as possible. Just one deck component without a low RCS can render other RCS reduction efforts redundant. Mr. Boy calls this a “flashlight effect” where one component can strongly reflect a radar signal and risk betraying the ship.
One can observe in sleek designs like the VISBY and ZUMWALT classes the efforts taken to reduce the amount of superstructure clutter. Capt. McGillvray’s article says that even a trash can or bucket left unobscured on the deck can reflect significant RF energy back to the radar: “We are masking all the outside equipment as much as possible,” says Mr. Boy.
For example, rigid inflatable boats routinely used for boarding operations are kept behind steel doors. These doors are designed to close flush to the superstructure. This ensures that gaps between the doors and superstructure cannot reflect echoes to the radar. Mr. Boy adds that similar approaches must be taken for every external system and fitting on the superstructure.
The trend towards the adoption of integrated masts over the last two decades highlights this approach. Integrated masts package a ship’s radars, communications, electronic warfare and optronic sensors in one structure. This eliminates the need for separate housings for these systems. The integrated mast can be appropriately designed and use RAMs to help reduce a ship’s RCS.
Harder to Detect
RCS reduction techniques do not make a warship invisible to radar. Instead, they make her harder to detect. One source shared an illustrative anecdote with the author. They were serving as the captain of a large surface combatant in a NATO navy. The ship was approaching port and preparing to receive a pilot.
The pilot’s boat was equipped with a standard marine navigation radar. The pilot radioed the warship to ask for their location as the only vessels their radar could detect were trawlers. RCS reduction techniques had provided the warship with an RCS equivalent to a small fishing boat. The ship was able to ‘hide’ amongst the large number of trawlers out at sea that night.
Furthermore, it might not be necessary to design a warship to present a low RCS to every radar she may encounter. The wide range of frequencies and radar transmissions in this section of the radio spectrum could make such an approach nigh-on impossible.
Instead, a ship’s RCS may be configured to ensure she presents a low RCS to the frequencies most likely to be used by naval surveillance radars for detection. Such radars typically transmit in S-band (2.3GHz to 2.5GHz/2.7GHz to 3.7GHz) and C-band (5.25GHz to 5.925GHz). She will also be configured to present a low RCS to fire control radars. These typically transmit on frequencies of X-band and above.
Computer Aided Design
Computer Aided Design (CAD) has helped immeasurably in RCS reduction. Every part of the superstructure is measured to ascertain its radar cross section: “After measuring the RCS of each of these components, we add these to the global RCS of the ship,” observes Mr. Boy. “This gives us a complete RCS measurement of the vessel.” He adds that RCS reduction is factored into the ship’s design from the outset well before the first piece of steel is cut.
In addition to CAD approaches to RCS reduction, the radar cross sections of components can be measured in anechoic chambers. Dockside systems can also be used to measure the ship’s overall RCS during construction.
Since debuting with the French Navy`s LA FAYETTE class frigates in the mid-1990s, RCS reduction techniques have proliferated around the world. To date, they have mainly been used for frigate and destroyer-sized combatants. As shown by Sweden’s VISBY class, this technology is migrating to smaller ships like corvettes.
Over coming decades, RCS reduction techniques may be adopted by Offshore Patrol Vessel (OPV) sized ships or amphibious assault platforms. The Argentine Navy`s BOUCHARD GOWIND class OPV is indicative of this trend.
Mr. Boy says that RCS reduction is one of Naval Group’s main research and development activities. He notes that the company is looking at ways to change as well as lower a ship’s RCS. This could be useful in making the vessel appear as a different type of ship altogether. For example, a frigate could have her RCS altered so she appears to have a similar RCS to a ferry.
Mr. Boy believes that this approach could be possible in the future using avantgarde materials. These maybe able to modify their shape and physically alter a warship’s appearance to create the desired RCS.
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