Follower Jamming

The core mechanism involves a Digital Radio Frequency Memory (DRFM) system that continuously samples the electromagnetic environment. Upon detecting a transmission, the jammer instantaneously tunes its transmitter to the active frequency and retransmits a corrupting signal. This creates a parasitic signal that follows the target's frequency hops in near real-time, making it highly effective against Frequency Hop Spreading (FHSS) systems that rely on agility for protection.

Often called a repeater jammer, this technique fundamentally relies on receiving, processing, and retransmitting the victim's own signal. The jammer captures the legitimate waveform, optionally modulates it with noise or deceptive data, and amplifies it for retransmission. This architecture creates a distinct vulnerability: the jammer must operate in a half-duplex or isolated full-duplex mode to avoid self-interference, which can be exploited by advanced Electronic Counter-Countermeasures (ECCM).

Follower jammers are categorized by their signal processing depth:

• Non-Coherent Repeater: Simply amplifies and retransmits raw received noise and signal, creating a barrage-like effect on the active channel.
• Coherent Repeater: Uses DRFM to capture and store the signal's precise phase and frequency characteristics before retransmitting a modified, highly deceptive copy. Coherent techniques enable sophisticated deceptive jamming that passes receiver authentication but corrupts data payloads.

To avoid jamming itself, a follower jammer must periodically cease transmission to look through and re-acquire the target signal. This creates a duty cycle where the jammer alternates between a receive window and a transmit window. The ratio of jamming time to look-through time is a critical performance parameter. A low duty cycle creates gaps in coverage that frequency-hopping radios can exploit by transmitting short, bursty packets during the jammer's silent periods.

A primary defense against follower jamming is to transmit packets shorter than the jammer's reaction time plus propagation delay. If a frequency-hopping radio completes its transmission and hops to a new channel before the jammer can process and retransmit on the original frequency, the jamming energy arrives on a now-vacant channel. This technique, known as burst transmission or fast hopping, directly defeats the follower's reactive architecture by exploiting its inherent latency.

Follower jamming is fundamentally different from sweep jamming. A sweep jammer blindly scans a predefined frequency range regardless of signal activity, wasting energy on vacant channels. A follower jammer is signal-activated; it conserves power by only transmitting on frequencies confirmed to be active. This makes follower jamming more energy-efficient and harder to detect by spectrum monitoring systems until a legitimate transmission begins, at which point the attack is immediate.

The broader category to which follower jamming belongs. A reactive jammer remains in a passive listening state until it detects a transmission, then activates to corrupt only the active data packets. This conserves power and makes detection more difficult compared to continuous barrage jamming. The jammer's effectiveness depends on the ratio of its reaction time to the target's packet duration.

A primary Electronic Counter-Countermeasure (ECCM) against follower jammers. AFH dynamically modifies the pseudo-random frequency hopping sequence based on real-time link quality metrics. When a channel is detected as jammed, it is removed from the hop set. This forces the follower jammer to continuously reacquire the new frequency, creating a window for clean transmission before the jammer can react.

A class of transmission techniques designed to prevent a follower jammer from detecting the signal in the first place. Without detection, the jammer cannot react. Core LPI strategies include:

• Direct Sequence Spread Spectrum (DSSS) to bury the signal below the noise floor
• Ultra-wideband (UWB) pulses with extremely low power spectral density
• Power management to transmit at the minimum necessary level

A defensive technique to physically locate a follower jammer using distributed sensor networks. Methods include:

• Time Difference of Arrival (TDOA) using synchronized receivers
• Angle of Arrival (AOA) via interferometric antenna arrays
• Received Signal Strength (RSS) triangulation Once located, the jammer can be physically neutralized or bypassed via spatial filtering with adaptive null-steering antennas.