Significance As targets become increasingly complicated, image detection with high sensitivity, high resolution, and high precision has become essential. However, owing to their high cost and large size, monostatic radars fail to meet the abovementioned requirements. To overcome this problem, we propose a distributed coherent-aperture-imaging radar (DCAIR). DCAIR is a novel radar system that uses multiple spatially dispersed small-aperture unit radars for cooperative detection and imaging. By employing the signal-level coherent fusion of the unit radars, the DCAIR obtains target images with high signal-to-noise ratios (SNRs). Thus, it is an important means to deal with long-range, low radar cross sections (RCS), or small threats. Additionally, DCAIR has many advantages such as flexibility, high survivability, low cost, and strong maintainability, making DCAIR an important development in the direction of imaging radars. Further improving the detecting resolution requires DCAIR to generate and process high-frequency broadband signals. However, traditional DCAIR realized using purely electronic technologies suffers severely from the “electronic bottlenecks” , making it difficult to generate and process high-frequency broadband-radar signals directly. Moreover, conventional time and frequency synchronization technologies fail to strike a balance among the transmission distance, stability performance, and synchronization precision. Microwave photonics has been considered a promising solution to these bottlenecks. Because of the broad bandwidth, flat response, low loss transmission, and multidimensional multiplexing of photonics devices, microwave photonic technologies have merits in high-frequency broadband signal generation, transmission, and processing. Combined with microwave photonic technologies, DCAIR exhibits better performance in terms of range resolution, velocity resolution, angular resolution, and SNR gain. With the funding of major programs of the National Natural Science Foundation of China, many achievements have been made. This paper highlights the achievements of DCAIR based on microwave photonic technologies proposed by researchers at Tsinghua University.

Progress In this paper, the international developing status of the three key modules is briefly reviewed, including generation of a dynamic reconfigurable waveform, optical fractional Fourier domain receiver front-end, and high-precision fiber-optic time-frequency synchronization network (OTFSN), and the the achievements funded by the major program of the National Natural Science Foundation of China are highlighted. From these achievements, the first experimental X-band distributed coherent broadband imaging radar system using microwave photonics was built and the staged experimental results were obtained. To generate multichannel orthogonal waveforms and achieve dynamic switching to coherent waveforms, a generation method for dynamic reconfigurable radar waveform using photonics-based broadband is proposed. We use the phase-coded linear frequency modulated waveform (PCLFMW) as the orthogonal waveform and the linear frequency modulated waveform (LFMW) as the coherent waveform. Here, two PCLFMWs in X-band with a bandwidth of 3.5 GHz are generated and the orthogonality between the waveforms reaches about 29 dB ( Fig.3). The proposed scheme achieves arbitrary generation and dynamic reconfiguration of the waveform. Furthermore, an optical FrFD receiver front-end is proposed to eliminate ghost targets produced by multiple echoes that are overlapped in both the time and frequency domains. The received broadband LFMW echo signals are projected on the optimal fractional Fourier domain formed using the photonic rotation of the time-frequency plane. By controlling the fractional Fourier transform spectrum, the proposed receiver front-end cancels ghost targets in multitarget circumstances. Experimental results show that the proposed receiver front-end can adapt to multiple noncooperative target environments and is immune to ghost targets under optimal working conditions ( Fig.6). An all-optical stable quadruple frequency dissemination scheme using photonic microwave phase conjugation is presented over a fiber-optic loop link. The relative frequency stability of 10 ^-16 at 1000-s averaging time can be obtained at every remote site located at a 20-km fiber loop link ( Fig.8). Moreover, a fiber-optic two-way time transfer method based on the time-frequency domain transforms (TFDT) is proposed. The TFDT directly obtains the two-way transmission delay by chirp frequency mixing and time-frequency analysis. With the proposed method, the time offset fluctuation and TDEV can reach 5.6 and 0.36 ps at 10000 s, respectively ( Fig.10). Further, using the stable frequency transfer technology, a three-node time-frequency synchronization network over a 20-km fiber loop link is presented. At 1000-s averaging time, frequency stability levels reach 10 ^-16 and time deviations reach 0.5 and 0.8 ps for two sites, respectively ( Fig.12). Two X-band two-unit microwave photonic DCAIRs based on the static and dynamic OTFSN are demonstrated. From the static OTFSN, the SNR gain ratio relative to the coherence-on-transmit mode is ~5.98 dB and that for the full coherence mode is ~8.6 dB. The range and cross-range resolution of 3.4 cm and 4.3 cm, respectively, are achieved in the experiment for rotating-target imaging ( Fig.14). From the dynamic OTFSN, the fully coherent SNR ratio gain can be increased by 8.1 and 7.9 dB, respectively, for the two-unit radars ( Fig.16). Thus, weak targets can be imaged and probed using the mutually coherent operation, while they are undetectable using the single radar.

Conclusions and Prospect This paper introduces the achievements of high-frequency broadband DCAIRs using microwave photonics technologies proposed by researchers of Tsinghua University. Combined with microwave photonic technologies, DCAIR realizes high-resolution and high-precision imaging. The abovementioned achievements will promote the development of DCAIR. The microwave photonics-based DCAIR has a wide application potential in both civil and military fields.