Military Technology 04/2021

Producing chaff to counter anti-ship missile millimetre wave radars missiles brings a unique set of challenges. Is this countermeasure still relevant? Millimetric Wave (MMW) radar is proliferating throughout the military domain. It is particularly attractive for Anti-Ship Missiles (AShMs). MMW uses very narrow radio waves to depict targets in detail. All radio waves have a wavelength. This is the distance between each crest and trough in a wave. The British Broadcasting Corporation’s (BBC) World Service radio transmits on frequencies of 1.413 megahertz/MHz to 15.75MHz. These frequencies have wavelengths of 212.1 metres/m (696 feet/ft) to 19m (62ft). Wavelengths shorten as frequency increases. MMW radio waves stretch from 30 gigahertz/GHz to 300GHz. In contrast BBC World Service these transmissions have wavelengths of 9.99 millimetres/mm to 0.99mm. MMW radio waves are routinely used in fire control radars because of their unique attributes. Their very short wavelengths mean that radars transmitting these frequencies can discern the tiniest of move- ments: A Texas Instruments publication looking at MMW attributes says that a radar transmitting in frequencies of 76GHz to 81GHz, which has a wavelength of around four millimetres is sufficient can detect movements a fraction of a millimetre. The small size of a MMW radar antenna means they can be housed in space-constrained environments. Moreover, MMW transmissions perform well when the atmosphere has contaminants like mist, fog, smoke or rain particles. These may degrade some transmis- sions in lower frequencies. These attributes have made MMW radars attractive for Anti-Ship Missile (AShM) designers. Firstly, AShMs are space-constrained platforms. This makes any high-performance radar which can fit within their confines at- tractive. Secondly, the exceptional detail of a target discernible with an MMW radar is a major benefit. The missile can be loaded with a detailed radar picture of the targeted vessel it should target. The sharpness of the MMW transmissions will let the missile’s radar signal processor discern the targeted ship from other vessels. This will ensure that the latter are not engaged. Accuracy is improved making it more likely the AShM will hit its intended target first time. The need for additional shots will thus be avoided. The oft-inclement weather of the maritime environment can see the air thick with moisture. This may degrade radar transmissions in other frequencies. One disadvantages of MMW transmissions is that they may lack the longer ranges of lower frequencies. For an AShM this may be less of a problem. The missile’s global navigation satellite signal and/or iner- tial navigation system will get it to a suitable distance from the targeted vessel. At this point the radar takes over providing highly detailed imagery helping the missile accurately execute the end game. It is little surprise that AShM developers are harnessing MMW radars for their wares. This is a concern for North Atlantic Treaty Organisation (NATO) and allied nations. They may have to face contemporary Russian- or Chinese-origin AShMs in future conflicts. These weapons may be equipped with MMW technology: The Islamic Republic of Iran’s Kowsar AShM series may have a Ka-band (33.4GHz to 36GHz) radar seeker. It is thought that this was the missile fired from the coast of Lebanon on 14 July 2006 by the Hezbollah Shia Islamist movement. The weapon hit the Israeli Navy’s INS Hanit ‘Sa’ar-5’ class corvette. It killed four sailors. Likewise, the China Haiying Electromechanical Technology Academy’s YJ-7/C-701 air-to-surface missile has a radar seeker believed to transmit in either Ka-band and/or K-band. Meanwhile Russian MMW ingenuity has been used in MKB Radgua’s Kh-59 Ovod (NATO reporting name AS-13 Kingbolt) AShM series. Specifically this technology is thought to be in the Kh-59MK and Kh-59MK2 variants. Protection Warships have several soft kill methods available to degrade the ef- fectiveness of radar-guided AShMs. These include manoeuvring a ship under attack so as to reduce her Radar Cross Section (RCS) visible to the radar. A ship will also attempt to jam the radar using an Electronic Countermeasure (ECM). This can mask the ship from the radar’s gaze using electromagnetic noise. Alternatively, it can create the illusion of a more tempting target away from the ship; a process known as seduction. Physical decoys can be launched to have a similar effect. These include radar corner reflects launched into the air or on the surface. These giant dodecahedrons have similar masking and/or seductive effects. Chaff can be used for a similar effect. Nonetheless there is now a lively debate re- garding the efficacy of this countermeasure given the embrace of MMW seekers for AShMs. Chaff Chaff was first used during the Second World War. The counter- measure exploits an elegantly simple principle. It uses thousands of di- poles cut to precisely half or one-quarter of the wavelength of the radar frequency it is intended to jam. Dipoles are made from metal or glass fibre as these materials conduct. Typically, dipoles are half the length of the frequency they are intended to engage. Take an X-band frequen- cy of 8.5GHz. This has a wavelength of 35.2mm. Half this wavelength is 17.6mm. Chaff dipoles intended to jam this frequency would need to be this length for a radar transmitting on 8.5GHz. This would ensure the radar’s transmissions would be reflect by the chaff as echoes. It is this process of reflection which gives chaff its power. Specialising in radar, communications, electronic warfare and all things C4I, Thomas Withington is a defence journalist, writer and regular contributor to MilTech . Thomas Withington The Final Cut? A Royal Air Force (RAF) Avro Lancaster heavy bomber disperses bundles of chaff from its weapons bay during a mission. The countermeasure was first used by the RAF during the Second World War in 1943. (Photo: UK MOD) f C4ISR Forum MT 4/2021 · 69 Iran is pouring investment into AShM technology and is thought to have taken a keen interest in MMW radar for these weapons. (Photo: Iran State Media)