Radar jammer

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 (PRF) 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.
• Interrupted-sampling repeater jamming (ISRJ) provides a coherent-jamming mode against wideband radars. By ISRJ, radar jammers can reduce the sampling rate, and the transmit-receive isolation enable designers to reduce the number of antennas. A single-antenna jammer working under ISRJ mode periodically samples, i.e. interrupted sampling, and repeats a fraction of the intercepted signal. The coherent-jamming signal generated by ISRJ form multiple verisimilar false targets at the victim radar receiver, and some false targets can precede the real target.
• Deceptive jamming uses techniques like "range gate pull-off" to break a radar lock.
• Signature Augmentation is a technique of a (radar) ECM system to deceive high-resolution radars (HRR) by creating complex false targets that resemble the radar signature of another platform. A specific form of this technique is Blip enhancement which deliberately makes some radar returns look larger in order to hide their nature. This is used by escort ships to make them look as large as capital ships.