A Novel Nonlinear Frequency Scanning SAR Imaging Mode

Frequency scanning (F-SCAN) synthetic aperture radar (SAR), as an advantageous choice for SAR systems operating at higher carrier frequencies, can achieve high performance in terms of swath width, azimuth resolution, and signal-to-noise ratio (SNR). In this article, a novel nonlinear F-SCAN (NF-SCAN) SAR imaging mode is proposed. Compared to the F-SCAN SAR, NF-SCAN SAR has two main advantages: one is to change the distribution of the transmit bandwidth over the swath by precisely tuning the beam scanning response to frequency. The other is to adapt the SNR distribution by adjusting the beam scanning response to time. First, this article derives the system parameters for NF-SCAN SAR. Second, methods for designing nonlinear beam scanning responses (NBSRs) to frequency and time (NBSR-F and NBSR-T) are proposed, by which a well-balanced ground range resolution and adaptive SNR are acquired. Third, a waveform design method dedicated to NF-SCAN SAR is given and the corresponding signal model is derived in detail. To reduce the data rate of NF-SCAN SAR with an incompletely compressed echo window, an improved data subsampling algorithm (IDSSA) is proposed, where a precise filter is designed, thus avoiding the loss of effective bandwidth. Finally, simulation experiments are conducted to verify the superiority of NF-SCAN SAR imaging mode. The proposed NF-SCAN SAR can be viewed as an important candidate for high-performance spaceborne SAR.