Suppression Jamming
Suppression jamming transmits signals that, when received by the radar, reduce the radar's ability to process echo returns. Generally, suppression jamming uses noise modulation, although other modulation forms may be used to suppress specific radar operating modes. Suppression jamming fills the radar display with clutter, making it difficult to discern echo signals. The example shown is a plan position indicator (PPI) display; similar outputs on other radar displays are caused by suppression jamming.
Suppression jamming generates background clutter that makes it difficult or impossible for the radar to extract required information from the received signal.
Barrage (Broadband) Jamming
Barrage jamming is the simplest form of suppression jamming. The jammer transmits noise across a wide frequency range that covers the target radar's operating frequencies. An advantage is that barrage jamming does not require precise knowledge of the radar's parameters. The disadvantage is relatively low jamming effectiveness. Since the radar receives energy only within its receiver bandwidth and ignores pulses outside its receive gate, much of the transmitted jamming power is ineffective. Jamming effectiveness is defined as the ratio of the interference power actually received by the target radar to the jamming transmitter power.
Barrage jamming continuously radiates power across a wide frequency band. This is inefficient because the affected radar can only detect jamming within its own receiver bandwidth and only when echo returns arrive.
Spot (Targeted) Jamming
If the noise jammer narrows its transmitted band to a small range around the target radar's operating frequency, jamming effectiveness improves. However, this requires monitoring to ensure the radar has not changed frequency.
Spot jamming transmits narrowband signals that cover the affected radar's operating frequency.
For optimal effectiveness, the jamming bandwidth should be only slightly larger than the radar's operating bandwidth. Due to manufacturing constraints, practical spot jammers often have much wider bandwidths, typically in the 3–20 MHz range.
Swept-Frequency Jamming
Swept-frequency jamming tunes a narrowband noise source across the possible operating frequency range of the target radar. When the sweep covers the radar's operating bandwidth, this technique can be highly effective, but the jamming duty cycle is less than 100%. For continuous-wave radars, some radar pulses may not be affected, allowing the radar to receive some echo returns.
Swept-frequency jamming covers only part of the radar's operating band at any instant, but it can scan across the entire band over time.
To jam multiple radars operating in different frequency bands requires complex RF switching. Multi-site spot jamming is the most effective approach when addressing radars on multiple bands.
Range Gate Pull-Off (RGPO)
The time-domain structure of echo pulses is shown in the figure. Range gate pull-off (RGPO) increases transmitted power and retransmits enemy radar pulses with slightly increased delays, thereby increasing pulse delay. The delay may increase following a parabolic or exponential profile.
RGPO transmits delayed pulse trains with greater power and a larger number of delayed pulses to simulate the target moving away from the radar. This shifts the arrival time of the echo pulses on the enemy radar display, making the target appear farther from the radar. The delayed pulses enter the radar receiver gate and cause the radar range-tracking circuitry to compute a much larger distance than actual.
RGPO transmits delayed, amplified echo pulses to increase power in the radar's rear range gate, forcing the radar to estimate a range beyond the actual target. After the interference pulse delay increases to a maximum, it quickly returns to zero and repeats, preventing the radar from performing reliable range tracking. Another form of this process is called range gate deception.
When jamming noncoherent radars, a transponder can automatically perform range gate deception by retransmitting a delayed RF pulse for each received radar pulse. Note that if the radar switches to a leading-edge tracking mode that locks onto the front edge of the echo pulse, the delayed jamming pulses may be ignored and the radar will continue to track the real echo; in that case, other jamming techniques are required.
Range Gate Pull-In (RGPI)
This method targets leading-edge tracking. The jammer predicts the arrival times of subsequent pulses using the pulse repetition frequency (PRF) tracking system and transmits a higher-power pulse before the echo arrives.
Range gate pull-in (RGPI) transmits strong pulses that initially coincide with the echo pulse, then generates increasingly numerous pulse trains that simulate the target moving toward the radar. The jamming-induced timing shift starts at zero and increases parabolically or exponentially, making the radar estimate a shorter range than actual. RGPI requires calculating future pulse arrival times. This is possible when the radar's pulse repetition interval is fixed or staggered, but it is not possible when the PRF is random or jittered.
Velocity Gate Pull-Off
The figure illustrates received power versus frequency for continuous-wave Doppler radar. Because of terrain features and relative motion, multiple Doppler frequency components can appear.
Doppler radars present Doppler frequency components corresponding to the relative line-of-sight velocities of terrain and target aircraft. A velocity gate is placed around the tracked target. If a high-power signal enters the velocity gate, it can trigger the radar's frequency-tracking function; if that signal is far from the true echo frequency, the radar's reported target velocity will differ from the real velocity, disrupting velocity tracking. This technique can be used against pulse-Doppler radars.
Inverse-Gain Jamming
Non-single-pulse radars determine target azimuth and elevation from the amplitude profile of echo pulses versus time. For example, conical-scan antennas detect changes in echo signal energy over time.
When the radar antenna is not pointed directly at the target, the echo pulse amplitude is smaller. Inverse-gain jamming transmits larger-amplitude pulses at these points. The echo signal power varies sinusoidally as the antenna scans; the maximum occurs when the beam is nearest the target and the minimum when it is farthest. By manipulating the scan so the target sits at the conical-scan center and rotating toward the maximum amplitude direction, and by transmitting higher-power, synchronized pulses at the sinusoid minima, the radar receiver sees a composite amplitude profile that can cause the tracking system to register a phase reversal. If the radar uses a relatively narrowband tracking filter to obtain correct guidance signals, sudden amplitude bursts may be filtered out, causing the radar to misinterpret the phase. Shifting the scan center away from the target rather than toward it can disrupt the radar's angular tracking. This technique is effective against many scanned-antenna radars but does not work against monopulse tracking radars.
Automatic Gain Control (AGC) Jamming
AGC jamming transmits high-power, narrowband, low-duty-cycle pulses. Radars rely on automatic gain control to manage required dynamic range, and modern AGC circuits typically feature fast attack and slow decay. Jamming pulses excite the radar AGC, reducing front-end gain and preventing the radar from detecting amplitude variations caused by antenna scan. The following figure illustrates conical-scan radar operation.
AGC jammers emit high-power narrow pulses that trigger the receiver AGC, compressing the amplitude profile of scan-induced echo pulses. Note that the schematic second trace may be exaggerated; in practice, the reduction in received signal is often sufficient to entirely mask the sinusoidal scan signal, illustrating a reduced scan amplitude.
