Interrupted-Sampling Repeater Jamming Suppression for Frequency-Agile Radar Based on Time–Frequency Analysis and Multi-Domain Filtering

Interrupted-sampling repeater jamming (ISRJ) produces coherent false targets after pulse compression and degrades the reliability of frequency-agile radar sensors, particularly at low signal-to-noise ratio (SNR) and high jamming-to-signal ratio (JSR). Existing single-domain suppression methods often provide insufficient separation between jamming and target echoes and may attenuate target-bearing components. This paper proposes a joint time–frequency analysis and multi-domain filtering method for radar waveforms employing intra-pulse and inter-pulse frequency agility. A forwarded jamming segment retains the carrier frequency of the sampled sub-pulse while overlapping a subsequent sub-pulse with a different carrier frequency, producing a joint displacement in time and frequency relative to the genuine target echo. Based on this property, a sub-pulse-wise two-dimensional feature extraction strategy is developed in the short-time Fourier transform (STFT) domain. Empirical probability density criteria are derived from projections along the time and frequency axes to determine adaptive rejection thresholds. The resulting time-selective and frequency-selective rejection regions are combined through logical intersection to construct a two-dimensional notch filter. The retained components are reconstructed using the inverse STFT and processed by segmented pulse compression. Simulations covering direct, periodic, and cyclic ISRJ show that the proposed method suppresses coherent false targets while limiting the loss of pulse-compression gain. It achieves higher peak signal-to-jamming-plus-noise ratios than the reference method over the tested SNR and JSR ranges and remains effective under adverse conditions. The proposed method improves the interference resilience of frequency-agile radar sensors and provides more reliable target echoes for subsequent detection and parameter estimation.