The Australian-developed Nulka anti-ship missile decoy system

By Max Blenkin & Dr Thomas Withington

Almost a decade before an Argentinian Exocet missile speared into the side of British warship HMS Sheffield during the Falklands War, Australian defence scientists had figured there had to be something better than guns, missiles, and chaff to protect ships against sea-skimming missiles.

The May 1982 loss of HMS Sheffield rocked the naval community. And showing it was not a one-off, five years later in the Persian Gulf the destroyer USS Stark was hit by two Exocets launched by an Iraqi jet.

What emerged from the Australian research was Nulka, a ‘soft kill’ hovering decoy which entices an incoming radar-guided missile away from the targeted ship by emulating the ship’s radar return in the missile seeker.

Nulka has gone on to be Australia’s most successful defence technology export, fitted to more than 150 Australian, US Navy and Coast Guard, and Canadian Navy vessels, with more than 1,400 produced. In RAN service, Nulka can be employed from the Canberra class LHDs, ANZAC class frigates, and Hobart class destroyers.

But there is very little on the public record to indicate just how well it works. Though many have been expended in trials and apparently worked as advertised, the most recent operational experience involving real missiles relates to a series of incidents involving US warships off the Yemeni coast in October 2016.

Responding to a pair of missiles, apparently Chinese Silkworm (or variant) sea skimmers fired by Houthi rebels on October 9, the USS Mason fired two SM-2s, an ESSM, and a Nulka. The Mason was not hit, but it’s not clear if the missiles were shot down, lured away, or just missed and crashed into the sea.

Three days later USS Mason was again targeted by a pair of missiles. Again neither hit and a report by the US Naval Institute (USNI) said the ship responded with “unspecified countermeasures”.

On October 15 the Mason was again targeted, this time by five missiles. It replied with a radar decoy, an infrared decoy, and a number of SM-2 missiles, destroying four incoming missiles. The fifth was neutralised by a radar decoy launched by USS Nitze.

But what kind of radar decoy? The Navy Times doesn’t specify, but a US Navy image accompanying the report shows three sailors aboard USS Mason reloading a Nulka round.

It would follow that no navy is going to rely on a system which delivers just marginal protection, and it seems to follow that the US Navy employs Nulka as part of a layered defence.

Nulka had a difficult birth, encountering many obstacles. The concept originated with Australian defence scientists in the 1970s, convinced that countermeasures to emerging sea skimmers, among them the US Phalanx gun system, wouldn’t be sufficient given short warning times and a possible saturation attack.

Curiously, the US approach at that time emphasised development of countermeasures for Soviet-bloc systems such as the Styx missile, rather than ‘sea-skimmer’ systems developed by allies such as the Exocet.

Styx certainly posed a threat. This was the missile which sank the Israeli destroyer Eilat during the 1967 Six Day War, prompting the US development of Phalanx. Styx was also the parent of the Silkworm missiles encountered by the US Navy off Yemen in 2016.

But Australian defence scientists saw emerging sea skimmer missiles as posing a greater threat, which was certainly how it turned out. Sold to anyone with the cash, among them Argentina and Iraq, the UK and US found their ships targeted by these missiles rather than by Styx systems.

What was needed, the scientists concluded, was an electronic soft kill system which initial research showed was a viable concept. But the difficulty lay in how to launch such a system from the ship in a manner which enticed incoming missiles to follow.

Knowing what we know now, the solution would seem to be some sort of UAS. Indeed, early discussions considered helicopters along with balloons, parachutes, gliders, towed autogyros, fixed-wing aircraft, and boats. 

Helicopters looked most promising, but there were a variety of issues, among them the need for instant launch in any weather and sea state. Along the way, scientist Bill Jolley looked at a rocket – specifically a solid fuel thrust-vectored rocket motor. That came to be termed ‘hoveroc’.

Several years of development followed, and a significant challenge was how to develop a rocket propellant which burned slower than anything before.

What was then called Project Winnin was approved in March 1979. The first free (untethered) flight tests occurred in May 1981 and proved a complete success. Thus came about Australia’s signature contribution to Nulka. Along the way, it was appreciated that a partner was needed, and who better than the US Navy. 

According to former Labor Defence Minister Kim Beazley, it took his intercession with US Defense Secretary Casper Weinberger to get the US on board. In Australia’s favour was the success of trials of the hovering rocket, and promise shown by Australian work on the payload. There was simply nothing comparable available overseas.

So in 1986, Australia and the US signed an agreement for full scale collaborative development. 

In 1988, AWA Defence Industries – now part of BAE Systems Australia – was contracted to perform Nulka engineering development, while ADI was sub-contracted to develop and manufacture the rocket motor at its Mulwala facility. US company Sippican – now part of Lockheed Martin – was to develop the all-important electronic payload. Australia and the US began joint production of Nulka decoys in 1996.

TECHNICAL ASPECTS
BAE Systems’ Nulka decoy protects warships against radar-guided Anti-Ship Missiles (AShMs). 

At the heart of its design lies an elegantly simple principle – a Radio Frequency (RF) repeater mounted atop of a rocket. When an incoming AShM is detected, the decoy is launched, and the rocket propels the decoy some distance from the ship. 

Once it reaches an optimum range and altitude from the vessel the RF repeater starts to transmit on a frequency designed to lure away the incoming missile. In short, the RF is transmitted in such a way as to appear as to make the decoy appear as a more tempting target than the vessel under attack. 

Before a modern AShM is launched, its radar will have been programmed to ‘recognise’ its target. The missile’s radar will have a radar picture of the target, more properly known as the Radar Cross Section (RCS). When the radar encounters its target, it will match the RCS it is ‘seeing’ with the loaded RCS. If the match is good, the missile will understand that it is heading towards its intended target. 

Nulka fools a missile’s radar by transmitting a similar RCS but exploiting it to appear as a more tempting target. 

Understandably Nulka’s attributes are highly classified. Nonetheless, open sources provide some indication of the approaches that the countermeasure might use.

Deception of the missile’s radar is achieved using a Digital Radio Frequency Memory (DRFM). Once launched and aloft, Nulka’s DRFM will detect outgoing transmissions from the incoming AShM’s radar. It is then that the decoy starts to work its magic. 

It is likely that Nulka uses standard deception techniques like range gate and velocity gate pull-off. 

Range gate pull-off is a technique designed to fool the missile’s radar on the range to its target. The radar will contain a range gate which is designed to only process radar echoes a set distance from the missile. 

Suppose a destroyer detects an enemy ship at a range of 75nm (139km) travelling at 30 knots. At that point it will take a radar pulse 0.25 milliseconds to make a round trip from the radar’s antenna to the target and then back to the radar. 

Now let us suppose our AShM travels at 1,000 knots (1,852km/h). It will take the missile 145 seconds to reach its target. Within those 145 seconds, the targeted ship can move a little more than one nautical mile or two kilometres in any direction. Thus prior to launch, the missile’s radar will have been programmed that its target is between 73.8nm (136.7km) and 76.2nm (141.1km) from the destroyer. 

After launch, the missile’s navigation system will continually record the missile’s position relative to the launch vessel and, as the missile approaches its target, its radar will be activated to search for the target. 

Now, let us suppose that the radar is activated when the missile is 60nm (111.1km) from the target – this is where the range gate comes into play. Taking into account the distance the missile has already travelled, the range gate will tell the radar to hunt for its target which will now be at a range of
between 13.8nm (25.6km) and 16.2nm (30km) from the missile. 

Obviously, these range thresholds are continually reducing as the missile approaches the target. The point of the range gate is to improve the missile’s accuracy, and it tells the radar to only concentrate on potential targets within these set ranges. Other targets are ignored to avoid distraction or collateral damage in a potentially crowded maritime environment. 

The decoy’s range gate pull-off technique works by the DRFM sampling the missile’s incoming radar pulse which is then transmitted back to the incoming missile’s radar, thus mimicking the echo the missile radar would expect to receive from the target. 

The DRFM will keep sampling the incoming pulses from the missile, but it will subtly change the time delay of each echo, the idea being to create the illusion that the target is progressively moving out of the missile’s range gate. The logic is that once the target appears to be outside the range gate it will no longer be considered a target. 

The crucial aspect of this approach is for the DRFM to capture the missile radar’s attention to convince it the echoes it is receiving are from the target. This is achieved by placing the decoy at a certain position relative to the ship and the missile, with the latter detecting the ‘echoes’ from the decoy so the deception can begin. 

Techniques such as range gate pull-off are seldom used exclusively, as modern missile radars equipped with electronic countermeasure (ECCM) techniques may be able to detect this tactic and discount it. 

Thus, other ruses such as velocity gate pull-off may be likewise employed. The missile’s radar will have a velocity gate which ensures that the missile only engages a vessel moving at a particular speed – for example a warship at circa 30 knots rather than a trawler slowly dragging its nets. 

Velocity gate pull-off works like range gate pull-off but, rather than the DRFM exploiting the time delay of the returned echo, velocity gate pull-off exploits the doppler shift. 

The doppler shift phenomenon is at the heart of pulse doppler radars and, while the shift is slight, a noticeable change in frequency occurs in the echo of radar pulse if a target is moving towards or away from the radar. 

In short, the frequency of the echo will decrease as the target moves away from the radar antenna because the echoes take longer to return to the radar as the target moves away, or less time if the target is moving towards the radar. 

The oft-quoted example of doppler shift is this: as the police car approaches the tone of the siren appears to rise. As the police car drives away the tone appears to lower. 

The same phenomenon of the siren of a police car moving towards a stationary pedestrian being a higher pitch than that of one moving away from is exploited by radars. The echo frequency will continually increase as the target approaches the radar and decrease as it moves away so, by calculating the range of the target and the doppler shift, the radar determines the target’s speed.

For velocity gate pull-off, the decoy’s DRFM will sample the incoming radar pulse, generate an echo and continually alter the doppler shift in such a way to convince the missile’s radar that the target is moving at a speed outside of the radar’s velocity gate. Once outside the velocity gate, the target
will no longer meet the missile’s criteria and will be ignored. 

One can see that combining techniques like range and velocity gate pull-off results in a heady jamming cocktail to intoxicate a nefarious radar. Even when deceived through these techniques and the missile’s radar begins a new search for its target, the DRFM will repeat procedures such as these for as long as the missile remains a threat. 

Given that Nulka is arguably the leading naval active RF decoy in service today, it is all but certain to have yet more sophisticated jamming techniques at its disposal. The decoy will also likely contain elaborate algorithms to overcome the ECCM techniques routinely used by radar-guided AShMs. 

Another potential capability for Nulka could be the means to deceive Millimetric Wave (MMW) AShM radars. 

Radars transmitting in frequencies of 30GHz and above are in vogue, as such frequencies have short wavelengths compared with lower frequency radar transmissions. For example, a 30GHz radar signal will have a wavelength just shy of 10 millimetres which allows the radar to ‘see’ its target in sharp detail. 

This is particularly important for the radar to correctly discern its target from other ships with different RCSs nearby, or in a littoral environment. MMW transmissions also help a radar discern physical chaff or radar corner reflector countermeasure decoys from the target. Also, MMW antennas are small which favours their installation in space-constrained environments like AShMs. 

But the downside of MMW radars is their shorter range compared with their lower frequency brethren. However, as AShM radars are typically used during the end-game when the weapon is relatively close to its target, this may not be such a problem. 

Modern AShMs will tend to use their GPS/INS (Global Positioning System/Inertial Navigation System) to get them to the target’s neighbourhood and within radar line-of-sight range, at which point the radar takes over. 

The GPS/INS is essential. AShMs typically follow a sea-skimming flight profile to avoid detection from radars over the horizon, meaning the missile will only become visible to the target’s radar in the final few moments before it hits. 

A number of AShMs are already thought to use MMW radars. Iran’s Kowsar series is thought to have a Ka-band (33.4GHz to 36GHz) radar seeker, while open sources state that China’s YJ-7/C-701 air-to-surface missile – upon which the Kowsar is reportedly based – has a Ka-band or K-band (24.05GHz to 24.25GHz) radar. Both lie within the MMW or just below bandwidth.

FUTURE DEVELOPMENT
So, having the wherewithal to jam MMW radars will be imperative for Nulka.

But Nulka is not resting on its laurels. In March 2021, BAE Systems Australia was awarded a five-year contract for development, procurement, and sustainment of Nulka to be brought together into a single contract construct for the ADF and for the US, Canada, and the UK. 

“The new contract will ensure that the next generation of Royal Australian Navy warships have the most effective anti-ship missile defence and that we continue to keep Australian and allied nations’ servicemen and women safe, both here and abroad,” BAE Systems Australia Managing Director Defence Delivery Andrew Gresham said in a company release.

“This contract will enable Australia to secure and strengthen its position as the world leader in the evolution of technologies at the heart of Nulka,” he added. “As a long-term partner of the Commonwealth in this program, the new contract underpins our shared aspirations for the future development of this sovereign capability.”

In a separate release, Defence Industry Minister Melissa Price said, “Nulka is undoubtedly one of Australia’s most significant Defence exports. This contract will see the next generation of Nulka capability fitted to the Royal Australian Navy’s new platforms, such as the Hunter class frigates.

“The Nulka program provides warships with a highly effective, all-weather defence against anti-ship missiles, utilising cutting-edge hovering rocket, autonomous system and electronic technologies,” she added. 

This article appeared in the July-August 2021 issue of ADBR.