Radar jamming and deception

Mechanical jamming

• Chaff is made of different length metallic strips, which reflect different frequencies, to create a large area of false returns in which a real contact would be difficult to detect. Modern chaff is usually aluminum-coated glass fibers of various lengths. Their extremely low weight and small size allow them to form a dense, long-lasting cloud of interference. This cloud is only effective in the range cell that it occupies. The slow movement of the chaff makes it easily discriminated, based on the lacking Doppler shift. Ships, on the other hand,can benefit greatly from a slow-moving chaff cloud. The cloud is released within the resolution cell of the ship and moves with the wind in one direction. The ship then escapes in another direction. The decoy should have a larger radar cross-section than the target, so the radar tracks it.
• Corner reflectors have the same effect as chaff but are physically very different. Corner reflectors are many-sided objects that re-radiate radar energy mostly back toward its source. An aircraft cannot 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 radar 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 than 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. Decoys also have a deliberately sacrificial purpose i.e. defenders may fire guided missiles at the decoys, thereby depleting limited stocks of expensive weaponry which might otherwise have been used against genuine targets.

  Electronic jamming

• Spot jamming or spot noise occurs when a jammer focuses all of its power on a single frequency. This overwhelms the reflection of the original radar signal off the targets, the "skin return" or "skin reflection", making it impossible to pick out the target on the radar display. This technique is only useful against radars that broadcast on a single frequency, and can be countered by changing the frequency or other operational parameters like the pulse repetition frequency so the jammer is no longer broadcasting on the same frequency or at the right times. While multiple jammers could possibly jam a range of frequencies, this would consume many resources and be of little effect against modern frequency-agile radars that constantly change their broadcasts.
• Sweep jamming is a modification of spot jamming where the 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. Although, depending on the error checking in the device this can render a wide range of devices effectively useless.
• Barrage jamming is a further modification of sweep jamming in which the jammer changes frequencies so rapidly it appears to be a constant radiator across its entire bandwidth. The advantage is that multiple frequencies can be jammed essentially simultaneously. The first effective barrage jammer was introduced as the carcinotron in the early 1950s, and was so effective it was believed that all long-range radar systems might be rendered useless. However, the jamming effect can be limited because this requires the jammer to spread its full power between these frequencies—the effectiveness against each frequency decreases with the number of frequencies covered. The creation of extremely powerful multi-frequency radars like Blue Riband offset the effectiveness of the carcinotron.
• Base jamming is a new type of barrage jamming whereby one radar is jammed effectively at its source at all frequencies. However, all other radars continue working normally.
• Pulse jamming produces noise pulses with period depending on radar mast rotation speed thus creating blocked sectors from directions other than the jammer, making it harder to discover the jammer location.
• Cover pulse jamming creates a short noise pulse when radar signal is received thus concealing any aircraft flying behind the jammer with a block of noise.
• Digital radio frequency memory, or DRFM jamming, or Repeater jamming is a repeater technique that manipulates received radar energy and retransmits it to change the return the radar sees. This technique 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. Electronics, radio equipment, and antenna can cause DRFM jamming causing false targets, the signal must be timed after the received radar signal. By analysing received signal strength from side and backlobes and thus getting radar antennae radiation pattern, false targets can be created to directions other than one where the jammer is coming from. If each radar pulse is uniquely coded it is not possible to create targets in directions other than the direction of the jammer
• Deceptive jamming uses techniques like "range gate pull-off" to break a radar lock.
• Blip enhancement deliberately makes some returns look larger on radar in order to hide their nature. This is used by escort ships to make them look as large as capital ships.

  Noise jamming

Radar burn-through

Inadvertent jamming

Countermeasures

• Constantly alternating the frequency that the radar operates on over a spread-spectrum 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 unsecure radio communication concerning the jamming and its effectiveness is also important. The jammer could be listening, and if they know that a certain technique is effective, they could direct more jamming assets to employ this method.
• The most important method to counter radar jammers is operator training. Any system can be fooled with a jamming signal but a properly trained operator pays attention to the raw video signal and can detect abnormal patterns on the radar screen.
• The best indicator of jamming effectiveness to the jammer is countermeasures taken by the operator. The jammer does not know if their jamming is effective before operator starts changing radar transmission settings.
• Using EW countermeasures will give away radar capabilities thus on peacetime operations most military radars are used on fixed frequencies, at minimal power levels and with blocked Tx sectors toward possible listeners
• Mobile fire control radars are usually kept passive when military operations are not ongoing to keep radar locations secret
• Active electronically scanned array radars are innately harder to jam and can operate in low probability of intercept modes to reduce the chance that the radar is detected.
• A quantum radar system would automatically detect attempts at deceptive jamming, which might otherwise go unnoticed.
• Anti-radiation missile also known as Home-On-Jam missiles: When a target is self-protective jamming, it essentially broadcasts its position. An ARM could be deployed and take out the jamming source. The missile utilizes passive RF homing which reduces its probability of detection. A countermeasure to ARM is not to use self-protective jamming, or have a decoy taking the missile. By towing a decoy/jammer, the decoy maintains a realistic Doppler shift and lures an ARM away from the target.

  Stealth

Interference

Jamming police radar

Jamming in nature