Passive Countermeasures
Jul 15, 2011

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The development of chaff as a radar countermeasure started in the Second World War by both the British and the Germans – each unaware they shared the same secret. In July 1942, Lady Joan Curran investigated the idea of generating a cloud of false radar echoes by dumping packets of aluminum stripes from an aircraft. The invention originated from the idea by Doctor Reginald Victor (R.V.) Jones in 1937, that a piece of metal foil (Dipole) cut to half the wavelength of the transmitter radar frequency could be used dispersed from aircraft and create false target echos to deceive enemy radar operators. The invented device was codenamed Window by the British, Chaff by the Americans, and Duppel in Germany (named for the Berlin district where the first tests took place in 1942). However, Duppel saw limited use by the Germans during World War II as Field Marshall Goering thought it would invite retaliation. Thus, he ordered subsequent technical records destroyed.

The decision not to use the Window application was a much debated and well-kept secret by the highest levels in Allied Command. It wasn’t until early 1943 that Prime Minister Winston Churchill approved and authorized its use. A couple of weeks later, Window was first used by the Royal Air Force (RAF) during Operation GOMORRAH – the devastating air-raids on Hamburg. During this operation, 90 million aluminized paper strips were dispersed, each measuring 12 by 0.6 inches. Window greatly contributed to the confusion of the Wurzburg radar system and its operators, blinding them almost completely and rendering the German air defence batteries useless. Out of the 791 RAF bombers deployed, only 12 did not return, whereas in previous missions, without the use of Window, more than 10% of the aircrafts had been lost.

For a long time, Window was only used to attack the German Wurzburg radar systems. Quite notably, and along with other deceptive devices, it was later used to provide the two false-target (fictitious) fleets during the D-Day invasion components of Operations Glimmer and Taxable. The success of these operations was greatly contributed to by the Canadian destroyer HMCS Haida, which was designated as the lead ship for trials off the coast of Scotland a couple of months prior to the planned invasion. Haida, along with the Sterling and Lancaster bombers and smaller seaborne vessels, conducted extensive and successful testing trials which led to the enabling and success of these two operations.

Today Window, which is more commonly referred to as Chaff, is used by many modern military forces to distract radar guided missiles off their targets.

Three different types of Chaff Countermeasures techniques are commonly used. These are Chaff C (Charlie) for Confusion, Chaff D (Delta) for Distraction and Chaff S (Sierra) for Seduction. Each are deployed differently to provide the desired deception effect during the different phases of an active radar missile attack with the ultimate aim of creating deception to either the firing unit’s targeting radars or the active radar seeker in the missile head and/or the firing platform’s operators.

Chaff C: Launched before the enemy’s targeting radar turns on. The aim is to provide numerous equally sized false targets- thus creating confusion to the firing unit in its target selection prior to launch. In order for the chaff clouds to be effective after deployment, one must maintain a similar course and speed to that of the true wind (a nautical term calculated on your speed with relative wind speed and direction).
Chaff D: Launched just prior to when the active homing seeker of the in-flight enemy missile is believed to turn on and search for its target based on its pre-programmed firing data. The aim is to distract the enemy missile from its intended target by creating additional false targets. As in Chaff C, it is imperative to manœuvre and maintain a course and speed with that of the true wind.
Chaff S: Launched when the fired missile is locked-on to its intended victim. This is usually indicated by its flight path plus the electronic emission parametric search pattern changes detected by the victim’s Electronic Support (ES) sensor operator. The intent of Chaff S is to walk the missile away from its intended target by fooling the missile tracking sensors and having it track and follow the deployed chaff. However, instead of using true wind, the targeted ship needs to manœuvre to create the desired relative wind and speed to cause separation and lure the missile to the deployed chaff. Because of the shorter timeline, Chaff S may be re-sown at rapid intervals to produce this desired effect.

During any of these Chaff applications or disciplines, timing is of the essence. Of them all, the most critical is deterring a ­missile during the deployment of Chaff S, just before the missile is about to impact. Improvements in effectiveness and the understanding and use of calculable data for a successful Chaff S tactical deployment is the main topic of this article.

Chaff is fired from the Port Bridge wing on HMCS Ottawa during a live fire exercise.

Increasing Effectiveness
The tactical design of a passive countermeasure scenario combines technical information such as the estimated timeframe, the seeker tracking type (Leading Edge and Centroid), the track gate depth/pulse width, cloud design and geometry, and also separation characteristics between the ship and the deployed chaff cloud. Another important consideration is the wind compensation between different chaff firing times of a softkill countermeasure system, in order to increase its effectiveness against a modern seeker which may use a small track gate depth.

Passive countermeasures, through the use of Chaff Sierra, are well-known engagements and have proven their effectiveness in many international trials. Improving effectiveness requires a thorough understanding of many parameters, such as the defensive and manœuvrability abilities of your own ship, and a detailed knowledge and understanding your Radar Cross Section (RCS) – how will the attacking missile see you?

The RCS diagram below shows that significant deviations between roll angles will occur, sometimes as much as 10 times higher. This diagram ignores additional types of radar propagation or ducting effects (which are highly influenced by environmental conditions), therefore it can serve only as an indication of how the attacking missile may see you. To have a more precise view, additional data of your own ship’s RCS distribution, the missile and seeker data, and the environmental data are essential.

Compiled intelligence may provide you with the missile’s known transmitting or operating values or ranges for: Frequency, Pulse Width, Pulse Repetition Frequency, and Scan time; the missile’s speed; polarization; beam width; and launch ranges, including its attack profile. Advance intel may also provide the recommended countermeasure options.

This shows a burst of decoy rounds during a live fire exercise.

Known or estimated radar values can parametrically be reprogrammed into one’s Electronic Support (ES) system, thus cueing its recognition and increasing the response timeline. Environmental data (sea state, wind, ship course and speed, and the desired turning rate) will assist in your countermeasure response. Using this data, a tactical decision to introduce a more accurate chaff countermeasure can be initiated, including different course-and-speed-alter­ations to minimize the RCS signature of your own ship, and optimize the separation of the deployed chaff cloud which should foil the missile from its intended target.

Placing the chaff cloud at the right time and place can spoof the cloud into the ­missile’s radar seeker range gate in order to present a fictitious, but valid target to the incoming missile, thus deceiving it.

The diagram above shows an example of an RCS pattern diagram (for a given ship model) in different roll angles, at a transmitter/receiver elevation of 0.28° using a frequency measurement of 9.0 GHz (horizontally polarized). The measured RCS values show significant deviations between the 0° roll angle (blue line) and the +2°/–2°roll angles (red and green lines). In some aspect angles (i.e. 290°), the RCS at the 0° roll angle can be up to 10 times higher than in the +2°/–2° roll angles.

Using passive countermeasures in the Chaff-S mode limits the effective time that chaff can be deployed and achieve it potential mimicking bloom or equivalent RCS pattern, however, in most cases, an appropriate Electronic Support sensor can detect this phase via changes in the missile’s scanning techniques (changing from a fast sector scan to an audible fixed or steady scan tone). The distance from which lock-on can be attained depends on the missile type, and may vary, somewhere between 5 and 8 nautical miles, or in the case of Stand-Off Anti-Ship missiles, possibly well outside either ship’s Air Search radar range.

The attack height of the missile, the missile seeker’s transmitting frequency, its polarization, the ship’s pitch and roll angle aspect to the missile while in a manœuvre, as well as the calculated environmental data, will all impact the available time for an effective seduction to take place, which, one can assume, will be very short with or without pre-detection or intelligence.

The use of 3D RCS modelling (as shown above) is one of the best possible methods of calculating this available data to enable one to effectively deceive an anti-ship missile during a seduction countermeasure.

This 3D RCS model displays the availabilities to increase the effectiveness of a Chaff S countermeasure. This example shows three of many roll angles with aspect angles between 180 and 280°, this model is calculated for distances between 15,000 and 0 meters using a horizontally polarized radar frequency of 9.0 GHz. The missile attack height is assumed at 5 meters above sea level, in a sea state 3 (wave height 0.5 to 1.25 meters) weather environment. It includes radar propagation effects and environmental data. The resulting RCS values of the used ship model are ­displayed through different colors: Ranges from 10 dB = 10 m2 (dark blue) up to 50 dB = 100,000 m2 or more (red). The diagram shows different timeframes available at different roll angles. Due to a high dynamic pitch and roll of the ship, the timeframe analyses of different roll angles is always searching for a positive ‘trend’ and not for a single static solution. (read more details online)

It has been almost 70 years now since the first deployment in 1943 and, many mechanisms have been created to increase the ability to effectively calculate and deploy Chaff. Although the composite make-up of the Chaff product has not changed much since then, it is still effective in producing the same desired effect, and remains increasingly important in defeating today’s complicated threats – in seconds.
Dipl.-Ing. Peter Huber is an engineer based in Bischofswiesen, Germany. He is a retired Captain of the German Armed Forces and has more than 10 years of industrial experience in the development of tactical algorithms for passive countermeasure systems. Current activities focus on his dissertation at the “Universität der Bundeswehr” in Munich with the topic: “Mathematical Optimization for positioning of decoys in Anti Ship Missile Defence,” as well as on the development of the ASM simulation software ASMD-CAT.
© FrontLine Defence 2011