A scheme of high-resolution inverse synthetic aperture radar (ISAR) imaging based on photonic receiving is demonstrated. In the scheme, the linear frequency modulated (LFM) pulse echoes with 8 GHz bandwidth at the center frequency of 36 GHz are directly sampled with the photonic analog-to-digital converter (PADC). The ISAR images of complex targets can be constructed without detection range swath limitation due to the fidelity of the sampled results. The images of two pyramids demonstrate that the two-dimension (2D) resolution is 3.3 cm × 1.9 cm. Furthermore, the automatic target recognition (ATR) is employed based on the high-resolution experimental dataset under the assistance of deep learning. Despite of the small training dataset containing only 50 samples for each model, the ATR accuracy of three complex targets is still validated to be 95% on a test dataset with the equal number of samples.

The fragmentation of a shell filled with explosive is quite valuable for evaluating the killing ability of improvised explosive device. A typical explosive device with Q235 steel shell and TNT explosive is chosen to study the fragmentation of thin-walled structure. With sand recovery method, experiments are conducted to get fragments with the mass above 0.1g. G-S formula shows a better agreement with experimental results when predicting average fragment mass than Mott formula. Further researches show that shell thickness and length-diameter aspect ratio have great effect on the fragmentation of shell. Average fragment mass increases exponentially and total fragment number decreases exponentially when shell thickness increases, while average fragment mass remains almost unchanged and fragment number grows linearly when length-diameter aspect ratio increases. These results are very helpful for whenever academic study or real application.

No abstract

The frequency offset caused by the Doppler shift and the oscillators instability degrades the performance of satellite-based demodulators for automatic identification system (AIS) signals unless proper synchronization algorithms are used. A noncoherent demodulation algorithm (NDA) based on cyclic redundancy check (CRC) error correction is proposed in this paper, which is able to tolerate large frequency offsets, thus avoiding the use of an explicit frequency synchronizer. To reduce the complexity, the number of CRC register states for error correction can be limited. Simulation results of the proposed algorithm are presented and compared with other demodulation algorithms in the literature in terms of bit error rate (BER) and packet error rate (PER). KEYWORDSCRC error correction, complexity reduction, Doppler shift, NDA, satellite-based AIS, Viterbi algorithm INTRODUCTIONThe automatic identification system (AIS) is primarily developed for vessel collision avoidance and usually works within the range of very high frequency band. 1 Coverage of coastal stations is about 40 nautical miles, and the communication between a vessel and a coastal station is impossible if the vessel is outside the radio visibility. Therefore, it is difficult to obtain a real-time supervision of all ocean-going vessels. To ensure a global coverage, a good solution is to receive the AIS messages from a constellation of low Earth orbit satellites. 2 However, the AIS standard has not been designed for such a use, and several technical issues arise at the receiver in case of a reception from a low Earth orbit satellite. 3 Doppler shift, message collisions, path delay, and low signal-to-noise values are the main issues degrading the demodulation performance. 3Studies on improving the demodulation performance of AIS signals received by a satellite are still ongoing. Based on maximum likelihood estimation and Viterbi algorithm, an innovative coherent detector is proposed for satellite AIS signals in Gallardo and Sorger. 4 However, the performance of coherent detection relies on accurate parameter estimation and tracking. Especially when a message collision exists, it is difficult to recover and track the carrier phase, thus resulting into a degradation of the coherent detection performance. Noncoherent detection is an attractive alternative, because it is very robust to the oscillators' instability. In Burzigotti et al, 5 a scheme based on the noncoherent sequence detection algorithm of Colavolpe and Raheli 6 is proposed, using three zonal demodulators that process different (but overlapping) frequency bandwidths to increase the system diversity. Since in AIS the cyclic redundancy check (CRC) is used at the transmitter side, the receivers in previous studies 4,5 compare the checksum computed from the received data with the frame check sequence (FCS) to detect transmission error bits. Colavolpe et al 7 replaced the algorithms for synchronization, detection, and postprocessing in Burzigotti et al 5 with new ones obtaining a significant performance i...