Deep learning algorithms have been widely utilized for synthetic aperture radar (SAR) target detection. Nevertheless, the traditional feature extraction methods and deep learning methods achieve improved ship detection accuracy at a cost of increased complexity and lower detection speed. As detection speed also is meaningful, especially in real-time maritime rescue and emergency military decision-making applications, we propose a new framework of faster region-based convolutional neural network (R-CNN) detection method to handle this problem. A new lightweight basic network with feature relay amplification and multiscale feature jump connection structure is designed to extract the features of each scale target in the SAR images, so as to improve its recognition and localization task network. Moreover, the K-Means method is used to obtain the distribution of the target scale, which enables to select more appropriate preset anchor boxes to reduce the difficulty of network learning. Finally, RoIAlign instead of region of interest (RoI) Pooling is used to reduce the quantization error during positioning. Experimental results show that the detection performance of the proposed method achieves 0.898 average precision (AP), which is 2.78% better than the conventional Faster R-CNN and 800% faster detection speed.

Due to the employment of frequency increment across the array elements, frequency diverse array (FDA) produces a jointly range-angle-time-dependent beampattern, which finds applicabilities in improving the target detection and imaging resolution performance. By exploiting the multi-frequency transmission across elements, this paper proposes an FDA-based space-time-frequency deceptive jamming approach against spaceborne synthetic aperture radar (SAR) imaging. We analyze the range-azimuth distribution of false targets generated by the FDA jammer and reveal the controlling ability of the number and locations of false targets through adjusting the frequency increment, azimuth offset and the number of FDA elements. Moreover, the allowable minimum/maximum frequency increment, maximum number of elements and minimum/maximum azimuth offset are derived via ambiguity function together the Doppler frequency analysis. The proposed approach is verified by extensive numerical results.

In this paper, we investigate frequency diverse array (FDA) antenna for physical-layer security and analyze the system performance (i.e., average secrecy capacity and secrecy outage performance) over independent but not necessarily identical distributed Rayleigh fading channels. A single-antenna multiple-channel receiver structure is proposed to address the time-variance property of frequency diverse array antenna. The transmitter allocates part of its power to send artificial noise (AN) so as to ensure the security of communications, such that the channels other than the desired receiver’s are degraded. Specifically, we derive the closed-form expressions for average secrecy capacity, probability of nonzero secrecy capacity, and secrecy outage probability, respectively. The correctness of the proposed analysis is corroborated via simulations. Furthermore, the corresponding asymptotic expressions for average secrecy capacity and secrecy outage probability in high signal-to-noise ratio (SNR) regime are also provided, respectively, to have deep insights into the performance of the considered system. Numerical results show that the FDA communications significantly outperforms the phased-array (PA) scheme in range dimension for secure communication.

Frequency diverse array (FDA) produces an angle range dependent and time-varying transmit beampattern due to the small frequency increment across its array elements, which provides potential applications for secure communications and radio frequency stealth. In this article, we study the low probability of intercept (LPI) properties of FDA transmitted signals under typical passive reconnaissance techniques. First, the passive reconnaissance property of direction-of-arrival estimation for FDA transmitted signals under phase interferometry measurement is evaluated, together with the corresponding Cramér–Rao bound analysis. Second, the passive reconnaissance property of time difference of arrival and frequency difference of arrival estimation for FDA transmitted signals are also evaluated for moving platforms. Both theoretical analysis and numerical results demonstrate that FDA transmitted signal indeed achieves significantly better LPI performance than phased-array transmitted signals, which is particularly useful for military applications.