December 13, 2003

WITHIN the last 10 to 20 years, electronics have steadily become a vital part of our lives. More so in the case of military applications where the trend of being technologically superior to the enemy fuels the race towards ever more sophisticated weaponry.

Fulfilling this desire is the use of cutting edge electronics in weapons, communications and detection systems.

Just as Isaac Newton said that every action has an equal and opposite reaction, in the same way the proliferation of electronics in the military has given rise to more electronics which seem to counter the effect of the previous.

Electronic use in military application ranges from communication systems for maintaining a direct link between command and units to detection systems like radars.

The key to winning battles and wars is to deny the enemy to use its means of communications and detection while keeping one’s own systems operational at peak performance level.

Basic electronic jamming works on the principle that all electronic equipment emits electromagnetic waves. It is the most fundamental concept of modern electronics and communications. The EM wave has a velocity equal to that of the velocity of light and thus is a very effective way to communicate and is also used in detection equipment like radars.

Pick up any dictionary and the usual definition of electronic jamming is given as: “deliberate radiation or reflection of electromagnetic energy for the purpose of disrupting enemy use of electronic devices or systems”.

To jam means to block or deny. Electronic jamming or “Blocking of electronics” can be of a number of different types. I will be discussing three of the most important.

 

Radar

Air defence radars work on the principle of reflected or echoed electromagnetic energy which is emitted by the radar itself in the first place. It is the to and fro oscillation of charge (electrons) in the metal structure of the radar which gives rise to an Electro Magnetic wave being emitted from it. The EM transverse (Electric and Magnetic portions perpendicular to each other and to the direction of motion) wave propagates outwards from the metal structure of the radar at the speed of light and is deflected upon encountering an obstacle of reasonable size and reflectivity. Some portions of the EM wave are reflected back towards the radar and thus are picked up by the receiver station at the radar site which “translates” the echo to determine the nature of the obstacle and its distance, depending on the strength of the received signal.

The amount of energy, in the EM wave, reflected by the obstacle depends on the size and reflectivity of that object; which both determine the object’s Radar Cross Section (RCS); the smaller it is the better.

The active use of the electronic jamming aircraft, the EA-6B Prowler, by the United States military demonstrates the importance of electronic warfare in this day and age.

The jamming of radars by electronic means constitute the non-lethal SEAD (Suppression of Enemy Air Defences) operations of a military. This is usually done by first analyzing the frequency and power of the EM wave which is being emitted by the enemy radar. Once this is determined an emitter which emits EM waves of its own can bombard the radar with EM waves which are of the same frequency as those emitted by the radar. Thus the radar will register much more echoes than it normally would get and end up displaying false information on the radar screen, this is known as noise jamming.

In the case of pulse radars, jamming can be done by emitting EM waves which are at a higher pulse rate (occurring more frequently) so the radar can be “range jammed” because it will be getting more return pulses making it look as if there are more aircrafts at different distances than there actually are and the radar will thus be ineffective into translating the distance of the object being tracked.

 

Communications

Effective communications in times of conflict between command and units of a military can turn the tide of a war. Instantaneous and up to the minute communications are necessary for a military commander to stay on top of the situation and guide his/her troops and/or units effectively according to changing situations on the battlefield.

The communication between the command centers and regional commands are usually very secure and done through the use of fiber optic cables, which are immune to non-invasive jamming. However communications between battlefield and other temporary commands is usually done by using radio communications which either use the RF (Radio Frequency) range of the frequency spectrum or microwaves.

The transmitted signal consists of two portions. A message signal and a carrier signal. The message signal (information part) e.g. voice is modulated with a carrier signal. Modulation means that some characteristic of the carrier signal is altered by the message signal. For example if the message signal changes the frequency of the carrier signal then it is known as Frequency Modulation or FM.

The carrier signal has much higher frequency than the message signal. The message (e.g. voice) is carried in the bandwidth portion of the transmitted signal and the higher the frequency of the signal, the bigger the bandwidth thus allowing more information to be transmitted.

Jamming these kind of signals needs an external emitter to produce signals of the corresponding frequency albeit at higher power (higher amplitude wave) and with garbage in the bandwidth. Thus the deliberate introduction of noise in a signal so as to stop the effective message signal from reaching its source is known as jamming.

For this reason the communication systems have rolling frequencies which keep on changing to avoid staying on one frequency for too long lest it should be jammed.

Another way that communication systems avoid jamming is that they change their frequency as soon as the noise level goes above a certain threshold, since it could be a result of jamming by the enemy.

 

Guided missile

Blocking or jamming an incoming missile from an airplane is one of the most critical aspects of electronic warfare. Since there is very little reaction time and the failure to effectively jam an incoming missile might end up in the destruction of the airplane.

An effective defence against missiles constitutes jamming the radars of the launching vehicles e.g. enemy airplanes prior to the launching of any missiles.

Towed ECM decoys behind the wingtip rails(Photo courtesy Gripen.com)

Under any circumstance, once the missile is fired, the emphasis goes onto defeating the incoming missile.

Missiles are guided by “seekers” which are usually mounted on the front tip of the missile. The seeker is the eye of the missile and could be of a number of different types.

Jamming missiles which are using RF (Radio Frequency) seekers is done more or less on the same principle as in the case of radars, which is an emitter on the target airplane emits EM waves of the same frequency but with modulated noise and higher power in order to throw the missile of its course and lose its target acquisition.

The emitters are also usually contained in towed decoys which are attached to the airplane by cables or glass fibre cables. The type of EM waves which the towed decoy emits can be actively controlled from on board the aircraft depending on the type of threat it is facing, through the use of these glass fibres.

On the other hand active radar guided missiles, for example the AIM-7 Sparrow, need to be guided actively (continuously) or intermittently by the airplane which launches it. The airplane has to keep “painting” the target airplane with its onboard radar and uplink the information to the launched missile in order to guide it towards the target.

The two ways to jamming this missile would be either to neutralize the radar of the guiding aircraft or by trying to disrupt the link between the aircraft and missile by the use of a higher powered EM wave of the safe frequency.

The third and more difficult to jam missile are the heat seeking missiles which use IR (Infra Red) seekers which home in on to the heat signatures being emitted by the exhaust of the aircraft. Usually missiles which have Infra Red seekers are for engagement of aircraft which are in relative close proximity. Best examples of Infra Red seeker missiles are the shoulder launched man portable missiles (Man pads) like the American Stinger or the state of the art Pakistani made Anza.

Flares are one way of defence against these missiles but since the article is only about Electronic jamming so I will stick to the topic. To defeat IR seeker missiles the effective way is to use an appropriate wavelength laser directed towards the missile which can “blind” the IR seeker, which is located on the tip of the missile. The aircraft usually has an RWR (Radar Warning Receiver) which gives a warning to the pilot as soon as it comes into contact with EM waves, but in the case of shoulder launched “fire and forget” type of weapons the RWR doesn’t even come into play since they are usually launched by a person who visually identifies the target and launches the missile, without the aid of a radar.

The ideal way is to use a “smart” jammer which can first identify the incoming missile and then shoot an in-band jamming signal towards the seeker of the missile in order to blind it and break lock on the target aircraft. This is of course easier said than done and even till now flares remain the most widely used form of defence against these short range missiles.

Perhaps the 1999 conflict in the Balkans serves well to show the importance of Electronic Warfare when a total of around 700 Surface to Air Missiles (SAMs) were fired on the NATO aircrafts by the Serbs. But due to the excellent electronic counter measures deployed by the Americans, only two planes ever got hit.

Suffice to say that Electronic Counter Measures (ECM) also known as Electronic Jamming is here to stay in the realm of military conflict with countries developing ever more sophisticated weaponry every day. It is turning into a game of one-upmanship by missile, radar and communication system developers and by the people who have the job of jamming them.

The writer is majoring in Telecommunication Engineering from Northern Savo Polytechnic in Kuopio, Finland

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