High-resolution wide-swath (HRWS) has always been a primary demand for synthetic aperture radar (SAR) remote sensing. Multiple-input multiple-output (MIMO) is a feasible scheme for achieving HRWS and is a hot research topic among scholars. However, in contrast to classic single-input multiple-output (SIMO) SAR, MIMO-SAR schemes require the isolation of transmitted waveforms. The transmitted waveforms and the corresponding echo separation methods are essential techniques in MIMO-SAR. Orthogonal frequency-division multiplexing (OFDM) chirp waveforms and an OFDM-beamforming (OB) method have been proposed, but have not been validated experimentally. In this letter, an airborne X-band DBF-SAR system is used to demonstrate MIMO-SAR with improved OFDM-chirp waveforms for the first time. Experimental results of frequency-and spatial-domain methods demonstrate that the mixed echoes can be effectively separated by OB scheme. The utilization of bandpass filters (BPFs) in OB scheme can conserve computation resources. Signal-to-interference (SIR) ratio is employed to test the performance of BPFs, where the indicator shows that the SIR loss is within 0.2dB.

Jamming for synthetic aperture radar plays a important role in electronic countermeasures. Currently, there are many jamming methods, but each method has its own issues. Barrage jamming has the drawbacks of requiring high power and being easy to detect, while deceptive jamming faces issues such as inadequate real-time performance and the need for multiple parameters estimated with high precision. An intra-pulse modulation jamming scheme with a novel jamming effect is proposed to ameliorate these issues. Firstly, in the sidelobe, jamming parameters and optimal jamming weights are calculated by estimated parameters combined with the backscattering coefficient of the region of interest (ROI). Secondly, based on the jamming parameters, multiple intra-pulse modulations are simultaneously applied to the received ROI echo. Then the modulated signals are transmitted with corresponding optimal weighting power and superimposed with the original echo, leading to the damage of the ROI imaging results. Finally, the utilization of relatively low-power Gaussian noise for suppression resulted in complete covering of the target in the ROI. This scheme improves the real-time performance of generating jamming signals and significantly reduces the power required for noise for suppression. It analyzed that even in the presence of forwarding power errors and inadequate real-time performance, there will still be a significant jamming effect. Results of simulations and measured data validate that the proposed scheme can effectively destroy image information.

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) is a promising scheme for high-resolution wide-swath (HRWS) imaging. After echo separation processing, a MIMO-SAR system can provide many equivalent phase centers (EPCs) in azimuth. However, EPC duplication occurs for traditional monostatic systems with uniform antenna arrays, leading to system resource waste. Moreover, range ambiguity suppression is a necessary process for wide-swath SAR systems. In this paper, a novel MIMO-SAR echo separation and range ambiguity suppression processing framework is proposed for HRWS imaging. A set of transmission delays is introduced to the transmit channels to displace the repetitive EPCs. The transmission delays can also be used to flexibly control the performance of echo separation. A wide-null beamformer is employed to accomplish echo separation and ambiguity suppression simultaneously. The proposed framework is designed for real-time processing and therefore does not require frequency-domain operations. Finally, the proposed framework is verified through point target and distributed scene simulation experiments.

Frequency diverse array synthetic aperture radar (FDA-SAR) has been recognized as a potential technique for high-resolution wide-swath imaging, and employs a slight frequency increment to the transmitting sub-apertures/channels to form a range-dependent beam pattern for swath widening. The key challenge of FDA-SAR is echo separation at the receiver. Former studies have employed quasi-orthogonal waveforms with low cross-correlation energies as the transmitted signals, which can not ensure reliable echo separation, especially for distributed scatterers. In this letter an FDA-SAR processing framework is proposed based on the segmented phase code waveform. An encoding scheme and a notch-expanding receiving beamformer based on convex optimization are used for precise echo separation. Then, using the degrees of freedom on transmitting, a beamformer is applied to resolve range ambiguities. The proposed scheme avoids the performance deterioration caused by the echo separation issue, even for distributed scatterers, while allowing other multiple-channel techniques to be integrated. Simulation experiments have validated the effectiveness of the proposed scheme.

Geosynchronous orbit (GEO) satellites are widely used in communications and navigation fields due to their special properties, such as comprehensive ground coverage and fixed subsatellite trajectories. The safety of GEO satellites can be ensured by using space-borne inverse synthetic aperture radar (SBISAR) to achieve high-precision observations or monitoring of uncatalogued targets near the GEO satellites. However, the amount of echo data is significant due to the slow relative motion between the GEO targets and SBISAR during long coherent processing intervals (CPIs). In this work, a variable sampling start echo receiving method is proposed to reduce the amount of redundant echo data, and the timing which combining the radar tracking with imaging is designed. First, the Earth center inertial (ECI) coordinate position vectors of the GEO target can be obtained through the monopulse method and coordinate transformation. Second, the initial orbit parameters of the GEO target are established by the Gibbs method. Third, the slant range of the GEO target is obtained by the interacting multiple model (IMM). Finally, the slant range is used to design the sampling start of the echo window and the timing of tracking imaging integration. Taking GEO satellite imaging as an example, experiments based on simulation data verify the effectiveness of the proposed method.

Lightweight multi-rotor unmanned aerial vehicles (UAVs) have shown great potential in flexible Earth observation, but they impose strict restrictions on payload, volume, and power consumption. Traditional pulse-Doppler synthetic aperture radar (SAR) systems offer high imaging performance but suffer from high peak power and large volume, making them unsuitable for lightweight UAV platforms. To meet the low-power demand, most existing lightweight UAV SAR systems adopt frequency-modulated continuous-wave (FMCW) schemes, which are compact and low cost yet limited by a low range resolution, poor anti-interference ability, and single imaging modes. Therefore, it is urgent to develop an SAR system that combines the high performance of pulse radar with the lightweight advantage of FMCW radar. To this end, this paper proposes a compact, low-power Ka-band pulse-Doppler SAR system for multi-rotor UAVs. With 1.2 GHz bandwidth and highly integrated RF and antenna design, the system achieves miniaturization and low power consumption while maintaining high-resolution imaging capability. Furthermore, two-step waveform error correction and a signal predistortion method are presented to compensate amplitude and phase errors and improve the purity of the transmitted signal. Experimental results show that the proposed system can obtain clear SAR images with a resolution better than 0.3 m, providing a practical high-performance pulse-SAR solution for lightweight UAV platforms.