LRASM/Long-Range Anti-Ship Missile. Image: Lockheed Martin
Millimetric wave radar seekers for anti-ship missiles are concentrating the minds of naval soft-kill countermeasures experts and triggering debates.
Guest post by Dr. Thomas Withington
Millimetric wave (MMW) radar refers to any radar transmitting in frequencies of 30 GHz and above. MMW gets its moniker from the wavelength of its frequencies; a 30-GHz frequency has a wavelength of 9.99 millimeters, for example. Radars transmitting in such frequencies are increasingly used by anti-ship missiles (AShMs): The Islamic Republic of Iran’s Kowsar and the People’s Republic of China’s YJ-7/C-701 series anti-ship missiles are thought to use Ka-band seekers.
There is a rule of thumb in radar engineering that the higher the frequency, the more detailed the radar imagery. The detailed imagery generated by an MMW radar means an AShM thus equipped receives a very detailed picture of its target. The radar picture generated by the AShM’s radar seeker as it homes in on its target can be precisely matched with a pre-loaded picture. The missile’s radar can make a detailed match of the target it has detected with the loaded picture. This is important when the missile may be looking for a single target in crowded seas with other vessels nearby.
Chaff and Radar
Since its first use during the Second World War, chaff has been one of the preferred means of jamming a hostile radar. Put simply, chaff is the collective term for thousands of metallic fibers, known as dipoles, cut to precisely one-half or one-quarter of the wavelength of the radar they are intended to jam. For an antenna to receive or transmit a particular frequency, it must be one-quarter or one-half of the wavelength it is using. Chaff is typically dispersed into the air between the target and the radar. A warship may receive warning of an inbound AShM from its radar, optronics or electronic warfare (EW) systems. AShMs typically follow a sea-skimming trajectory as they approach their target. This helps the missiles stay below their target’s radar coverage for as long as possible. A missile will typically be detected at a range of about 14 nautical miles (26 km) from the ship. This calculation assumes the ship’s radar and its EW system antennas, the latter of which detects the AShM’s radar transmissions, are around 40 meters (131 ft) above the waterline. Once the missile is detected, the ship will use maneuvering and launch soft- and hard-kill countermeasures to avoid impact.
Part of the soft-kill response may include chaff. Chaff will be dispersed to either mask the target or present a more lucrative, but ultimately false, target to the missile’s radar. The latter’s transmissions will hit the chaff. The radar transmissions cause the chaff to resonate and reflect the signals back to the missile’s radar. For all intents and purposes, the missile’s radar now has two or more targets to deal with thanks to the cloud of chaff. In fact, the chaff may be so effective that the missile’s radar finds the chaff cloud to be a more attractive target, or gets confused, and thus misses its prey.
Chaff has remained a favorite soft-kill countermeasure for AShM radar seekers transmitting in X-band frequencies of 8.5 GHz to 10.68 GHz. X-band dipoles were relatively easy to produce. For chaff to be effective against 8.5-GHz radar seekers, dipoles would need to be between 17.63mm and 8.81mm long. Chaff effective against 10.68-GHz radars would need dipoles between 14mm and 7mm long. Things start to get more challenging for MMW radar frequencies. For example, for chaff to be effective against MMW frequencies of 30 GHz, the dipoles must be between 4.99mm and 2.49mm long. For frequencies of 40 GHz, they will need to have a length of between 3.74mm and 1.87mm.
The Unfolding Debate
Speaking during the recent Euronaval exhibition held in Paris, officials from countermeasures specialists Etienne-Lacroix disputed chaff’s utility against MMW-radar-guided AShMs. Their argument rests on the practicality of cutting chaff precisely to such small lengths to ensure its efficacy. They argue that other countermeasures like radar corner reflectors (RCRs) are more effective against these threats. RCRs are inflatable dodecahedron structures launched from a ship into the air above. Their sharp angular shape scatters much of a radar’s incoming transmissions away from the radar’s antenna. Only a small part of the energy is reflected to the radar, hopefully causing it to lose track of the target.
It may be premature to write off chaff as an effective MMW-radar-guided AShM countermeasure. “Chaff tailored to the MMW threat is one of several capabilities capable of being used alone or in tandem with other [soft- and hard-kill] countermeasures,” says Andy Hogben, managing director of Chemring Countermeasures. Hogben concedes that the “manufacture of MMW chaff requires greater precision … to generate as fine as possible dipoles which then have to be cut to optimize MMW radar return frequencies.” Nonetheless, it is possible, and Chemring “currently manufactures MMW chaff for air countermeasure applications, with the technology readily available to use in naval countermeasures.”
It is entirely possible that both Chemring’s and Etienne-Lacroix’s philosophy toward the MMW-radar-guided AShM threat are correct and that both chaff and RCRs are effective. This will certainly afford warships more soft-kill options when countering these weapons in the future.
Dr. Thomas Withington is an award-winning analyst and writer specialising in electronic warfare, radar and military communications. He has written widely on these subjects for a range of specialist and general publications. He also works as a consultant and adviser in these areas for several leading government and private sector clients. Furthermore, Dr. Withington provides regular commentary on security and defence aspects of electromagnetic spectrum use for major media organisations around the world. @tomwithington
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