Yijiang Nan

3D architectures in flexible electronics provide an effective route to decouple mechanical compliance from functional performance, which is particularly important for shape‐adaptive radio‐frequency systems on curved platforms. However, flexible radio‐frequency devices remain susceptible to deformation‐induced variations in resonant frequency, impedance matching, and radiation efficiency. Here, we present a mechanically guided assembly strategy for flexible 3D air‐gap phased arrays that integrates conformal adaptability with deformation‐stable electromagnetic performance. Buckling‐guided assembly and a shape‐locking scheme form freestanding 3D air‐gap patch antennas on multilayer flexible printed circuit boards through vertical interconnection, providing strain‐isolated radiators with locally tunable stiffness. The air‐gap dielectric reduces dielectric loading and loss, while the out‐of‐plane geometry helps preserve radiator planarity and mitigate deformation‐induced variations in resonant frequency and impedance matching. The resulting antenna elements exhibit stable impedance characteristics under different bending directions and curvatures, with good agreement between full‐wave simulations and measurements. At the array level, integrated inertial measurement units enable shape reconstruction of the deformed aperture, and phase compensation recovers planar‐like beamforming under convex, concave, and hyperbolic deformations. This mechanically and electromagnetically co‐designed architecture provides a potential route toward lightweight, conformal, and shape‐adaptive flexible phased arrays for future wireless systems.

Aerostats, operating persistently in near‐space, have garnered attention in remote sensing, communication relay, and disaster monitoring applications. Integrating large‐area conformal antenna arrays onto their envelope surfaces can enhance electromagnetic functionality while reducing the aerodynamic and structural penalties of externally mounted rigid antennas. However, deformation of the flexible envelope may alter the antennas’ geometry, input matching, and radiation characteristics. A deformation‐tolerant flexible conformal antenna array for near‐space aerostats is developed and mechanically and electromagnetically characterized under wind loading, cyclic bending, controlled curvature, and aerostat inflation. The sixteen‐element antenna array is integrated onto four aerostat models (scale, dolphin‐shaped, jellyfish‐shaped, and disc‐shaped). Across the four platforms, the largest within‐platform variation in measured |S ₁₁ | among the tested inflation states is 1.12 dB. Radiation‐pattern measurements reveal configuration‐dependent changes in gain, sidelobe level, and half‐power beamwidth after platform integration. Ground and low‐altitude flight tests demonstrate orientation‐dependent microwave signal reception, showing the potential of the array for near‐space communication and directional microwave sensing.

Ka-band carrier-frequency-agility (CFA) synthetic aperture radar (SAR) employs pulse-to-pulse random wide-range frequency hopping to enhance anti-interference capability. However, the random hopping disrupts the azimuth phase continuity, and the millimeter-wave wavelength of the Ka band makes the imaging quality extremely sensitive to motion errors. To address these challenges, this paper proposes an auto-focusing imaging framework and performs a performance analysis for Ka-band CFA SAR. First, a back-projection (BP)-based imaging model is derived to restore the coherent phase history from the hopped echoes. Second, to compensate for the residual phase errors inevitable in high-resolution millimeter-wave imaging, an auto-focusing framework is developed. This framework incorporates a dynamic sub-aperture strategy and an adaptive spectral notching mechanism to ensure precise phase error estimation in complex scattering environments. Furthermore, the imaging performance under different frequency-selection modes is analyzed to provide a guideline for the parameter selection of the Ka-band CFA SAR. Experiments with a vehicle-mounted Ka-band SAR system demonstrate that the proposed method achieves well-focused images with 5 cm resolution.

Sub-band errors are inevitable in synthetic bandwidth synthetic aperture radar (SAR) systems due to differences in signal paths and frequency responses of the components used for different sub-bands, which degrade imaging performance if not properly compensated. In this paper, a prominent-reflector-based sub-band error estimation method is proposed for synthetic bandwidth SAR. Based on the analysis of the sources and impacts of sub-band errors, the proposed method estimates and compensates the errors in three steps, corresponding to time-delay error, amplitude error, and phase error. By leveraging the stable reflective properties of prominent reflectors in the scene, the proposed method directly derives sub-band error estimates from focused sub-band images in the time domain. Compared to existing methods, the proposed method achieved robust, high-accuracy performance while requiring less execution time. The effectiveness and efficiency of the proposed method are validated using real data collected by a Ka-band synthetic bandwidth SAR system.

Frequency scanning (F-SCAN) synthetic aperture radar (SAR) offers unique advantages over digital beamforming (DBF) SAR for high-performance imaging. However, F-SCAN SAR introduces several new pitfalls, such as reduced effective average power, potential increase in data rate, and performance constrained by available bandwidth, which require more rigorous mathematical analysis. In this paper, we first introduce four performance ratios derived mathematically to compare the system performance between the two imaging techniques, then propose an optimization strategy for design of system parameters. Our analysis shows that F-SCAN SAR should not be considered a superior alternative for DBF SAR, but rather an important complement, as the former may result in degraded performance in specific scenarios. With a large available bandwidth, F-SCAN SAR would be well-suited for applications requiring very wide swaths with medium to low range resolutions.

Spaceborne radar imaging is the key means to obtain the fine-resolution radar image of space targets. However, the nonuniform relative rotational motion caused by the high-speed orbital state leads to the degradation of the radar imaging performance. To address this problem, a fine-resolution spaceborne radar imaging method of space targets with parametric second-order scaling and high-order spatial-variant phase compensation is proposed in this article. First, an accurate echo signal model that has higher precision than the existing methods is derived, which considers the spatial-variant second-order migration through range cell (MTRC) and high-order phase error (SVPE). Then, the problem of the relative rotational motion parameters estimation is transformed as the optimization solution in the Doppler domain, and an efficient rotational motion estimation method with the time-frequency ridge extraction and Levenberg-Marquardt (L-M) algorithm is proposed. On this basis, a parametric second-order scaling (PSOS) operation is proposed to correct the spatial-variant second-order MTRC. Afterward, a parametric high-order SVPE compensation function is introduced to achieve the azimuth focus. Finally, a fine-resolution spaceborne radar image of the space target can be obtained. The results of the simulated and measured data verify the advantages of the proposed method.

In this paper, a novel sinogram decimation fast back-projection (SD-FBP) algorithm for strip-map synthetic aperture radar (SAR) imaging is proposed. We first review the back-projection (BP) and the fast factorized BP (FFBP) algorithms, then analyze the cause of the FFBP imaging degradation theoretically based on the combination of SAR imaging and the sinogram concept of computed tomography, illustrating that the degradation is much more severe for strip-map mode. Based on the SAR sinogram, the SD-FBP algorithm is proposed to mitigate the FFBP degradation for strip-map SAR imaging. The complexity of the SD-FBP is analyzed and compared with that of the FFBP and the Cartesian factorized BP (CFBP). The superiority of the SD-FBP algorithm is validated using the simulation and real spaceborne strip-map SAR data, e.g., Sentinel-1, Gaofen-3, and LuTan-1. The results show that the SD-FBP can tackle the FFBP imaging degradation for strip-map SAR, and runs faster than the FFBP and the CFBP.

Range ambiguity is a technical challenge due to the deterioration of the image quality in the multistatic SAR constellations. This paper proposes an effective range ambiguity suppression scheme based on multi-echoes coherent processing. First, a general signal model impacted by range ambiguity is built up based on the geometry of the multistatic SAR constellation, showing the different phase characteristics of the desired and ambiguous signals between the satellites. Then, an effective processing scheme for range ambiguity suppression is proposed based on the different phase characteristics, and the corresponding ambiguity suppression performance is analyzed accordingly. This scheme can achieve a coherent summation of the desired signals by compensating for the corresponding phase difference, while the ambiguous signals are summed incoherently. Therefore, the range ambiguities can be suppressed without increasing the SAR instrument complexity. Finally, the system and imaging simulation results are provided to validate the theoretical analysis of the ambiguity suppression performance and demonstrate the effectiveness of the proposed scheme, respectively.

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 paper, a novel nonlinear frequency scanning (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 paper derives the system parameters for NF-SCAN SAR. Second, methods for designing nonlinear beam scanning responses 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 sub-sampling 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.