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MECHANICAL JAMMING

Mechanical jamming is caused by devices which reflect or re-reflect Radar energy back to the radar to produce false target returns on the operator's scope. Mechanical jamming devices include Chaff , Corner Reflector s, and Decoy s.

  • Chaff is made of different length metallic strips, which reflect different frequencies, so as to create a large area of false returns in which a real contact would be difficult to detect. Chaff is often aluminum for weight purposes and is usually coated with something such as a plastic to make it more slick and less likely to bind up in large clumps.


  • Corner reflectors have the same effect as chaff but are physically very different. Corner reflectors are multiple-sided objects that re-radiate radar energy. An aircraft can't carry as many corner reflectors as it can chaff.


  • Decoys are maneuverable flying objects that are intended to deceive a radar operator into believing that they are actually aircraft. They are especially dangerous because they can clutter up a rader with false targets making it easier for an attacker to get within weapons range and neutralize the radar. Corner reflectors can be fitted on decoys to make them appear larger then they are thus furthering the illusion that a decoy is an actual aircraft. Some decoys have the capability to perform electronic jamming or drop chaff.



ELECTRONIC JAMMING

Electronic jamming is a form of Electronic Attack where jammers radiate interfering signals toward an enemy's radar, blocking the receiver with highly concentrated energy signals. The two main technique styles are noise techniques and repeater techniques. The three types of noise jamming are spot, sweep, and barrage.

  • Spot jamming occurs when a jammer focuses all of its power on a single frequency. While this would severely degrade the ability to track on the jammed frequency, a frequency agile radar would hardly be affected because the jammer can only jam one frequency. While multiple jammers could possibly jam a range of frequencies, this would consume a great deal of resources to have any effect on a frequency-agile radar, and would probably still be ineffective.


  • Sweep jamming is when a jammer's full power is shifted from one frequency to another. While this has the advantage of being able to jam multiple frequencies in quick succession, it does not affect them all at the same time, and thus limits the effectiveness of this type of jamming.


  • Barrage jamming is the jamming of multiple frequencies at once by a single jammer. The advantage is that multiple frequencies can be jammed simultaneously; however, the jamming effect can be limited because this requires the jammer to spread its full power between these frequencies. So the more frequencies being jammed, the less effectively each is jammed.


Repeater techniques manipulate received radar energy and retransmit it to change the return that the radar sees. These techniques can change the range the radar detects by changing the delay in transmission of pulses, the velocity the radar detects by changing the doppler shift of the transmitted signal, or the angle to the plane by using AM techniques to transmit into the sidelobes of the radar.


INADVERTENT JAMMING

In some cases, jamming of either type may be caused by friendly sources. Inadvertent mechanical jamming is fairly common because it is indiscriminate and will affect any nearby radars, hostile or not. Electronic jamming can also be inadvertently caused by friendly sources, usually powerful EW platforms operating within range of the affected radar. Unintentional electronic jamming is most easily prevented by good planning and common sense, though sometimes it is unavoidable.


COUNTERMEASURES

  • Constantly changing the frequency that the radar operates on will limit the effectiveness of most jamming, making it easier to read through it. Modern jammers can track a predictable frequency change, so the more random the frequency change, the more likely it is to counter the jammer.


  • Cloaking the outgoing signal with random noise makes it more difficult for a jammer to figure out the frequency that a radar is operating on.


  • Limiting the unsecure radio communication concerning the jamming and its effectiveness is also important. The jammer could be listening, and if he knows that a certain technique is effective he could direct more jamming assets to employ this method.



INTERFERENCE



While not usually caused by the enemy, interference can greatly impede the ability of an operator to track. Interference occurs when two radars in relatively close proximity (how close they need to be depends on the power of the radars) are operating on the same frequency. This will cause "running rabbits", a visual phenomenon that can severely clutter up a scope with useless data. Interference is not that common between ground radars, however, because they are not usually placed close enough together. It is more likely that some sort of airborne radar system is inadvertently causing the interference—especially when two or more countries are involved.

The interference between airbourne radars refered to above can sometimes (usually) be eliminated by frequency-shifting the magnetron.

The other interference often experienced is between the aircraft's own electronic transmitters, i.e. transponders, being picked up by own radar. This interference is eliminated by suppressing the radar's reception for the duration of the transponder's transmission. Instead of "bright-light" rabbits across the display, one would observe very small black dots. Because the external radar causing the transponder to respond is generally not synchronised with your own radar (i.e. different PRFs repetition frequency ), these black dots appear randomly across the display and the operator sees through and around them. The returning image may be much larger than the "dot" or "hole", as it has become known, anyway. Keeping the transponder's pulse widths and mode of operation becomes a crucial factor.

The external radar could, in theory, come from an aircraft flying along side your own, or from space. Another factor often overlooked is to reduce the sensitivity of one's own transponder to external radars; i.e., ensure that the transponder's threshold is high. In this way it will only respond to nearby radars—which, after all, should be friendly!

One should also reduce the power output of the transponder in like manner.


RADAR JAMMING IN THE CIVILIAN WORLD

Radar jamming for the purposes of defeating Radar Gun s is simpler than for military application, although it is often illegal. However, most products called radar jammers on the market do not effectively jam police radar guns.