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Most traditional multi-aircraft flight path optimization methods are oriented toward area coverage, use static optimization models, and face the challenge of model mismatch under complex dynamic environments. Therefore, this study proposes a flight path optimization method for dynamic area coverage based on multi-aircraft radars. First, we introduce an attenuation factor to this method to characterize the actual coverage effect of airborne radar on a dynamic environment, and we take the area coverage rate under the dynamic area coverage background as the optimization function. After integrating the constraints of multi-dimensional flight path control parameters to be optimized, we built a mathematical model for dynamic area coverage flight path optimization based on multi-aircraft radars. Then, the stochastic optimization method is used to solve the flight path optimization problem of dynamic area coverage. Finally, the simulation results show that the proposed flight path optimization method can significantly improve the dynamic coverage performance in dynamic areas compared with the search mode using preset flight paths based on multi-aircraft radars. Compared with the traditional flight path optimization method oriented to static environments, the dynamic coverage performance of our proposed method is improved by approximately 6% on average.

Distributed soft target refers to nonrigid target or a target group with wide distribution range, time-varying spatial distribution, or internal relative motion. This type of target is currently attracting considerable interest in the radar field, and the research on its radar characteristics and sensing technology is a typical interdisciplinary problem. To help the radar technicians better understand the related technologies, this study introduces the dynamics, scattering/transmission, radar characteristics, detection, and parameter retrieval of this type of target in continuous and discrete forms, as regards the positive and inverse problems. Considering the aircraft wake vortex as an example, the radar characteristics and sensing technology of this type of target are illustrated, which can serve as a good reference for the development of related radar detection technologies.

There is an urgent need for radar-measured data to tackle key technologies of radar maritime target detection. The ‘‘Sea-detecting X-band Radar and Data Acquisition Program’’, proposed in 2019, aims to obtain data through radar experiments and share them publicly. In 2020, the program continued to advance and conducted several experiments in three aspects, namely, Radar Cross-Section (RCS) calibration of radar targets, detection of sea clutter and target under different sea conditions, as well as detection and tracking of maneuvering targets in sea. The measurement data of the stainless steel sphere calibrator at different distances in radar slow-scanning mode, sea clutter in radar staring mode in different directions, sea target in radar staring mode, and marine engine speedboat in radar scanning mode are obtained. In addition, wind and wave data, data from the Automatic Identification System (AIS) of targets, visible/infrared data, and other associated sensor data are synchronously obtained.

Regarding the target characteristics in marine radar detection, this paper introduces classic radar target characteristics and models, and the main problems associated with the measurement and computation of these target characteristics. From three perspectives, i.e., the target, environment, and sensor, we discuss the target characteristic that have attracted much attention in the field of marine target detection. We discuss the diversity of marine target characteristics, the variety and complexity of the marine environment, coupling effects between the target and the environment, and the main requirements of the typical marine radar in target detection applications. The techniques used in the measurements and computation of target characteristic are also introduced. We propose a multidimensional representation of the target characteristics and briefly discuss its applications.

To meet the radar data requirements of target detection technology research and address the lack of publicly available sea-detecting radar data, a data-sharing program for sea-detecting radar is proposed herein. The aim of the proposed data-sharing program is to conduct sea detection experiments using an X-band solidstate phase-coherent radar and other multi-type radars to obtain the target and sea clutter data under different sea conditions, resolutions, and grazing angles. Moreover, the marine meteorological and hydrological data, target position, and track data are simultaneously obtained using the proposed data-sharing program to help achieve the standardized management of radar-measured data. The proposed data-sharing program aims to promote the open sharing of data sets, serve as the basis for research on sea clutter characteristics, and facilitate the research on sea clutter suppression and target detection technology.

In complex marine environments, sea clutter greatly affects the detection performance of maritime targets. Because the influencing factors of sea clutter are numerous and the mechanism is complex, there are great difficulties in feature description and sea clutter suppression, and it is necessary to carry out long-term, systematic, continuous, and in-depth research. Carrying out sea clutter measurement experiments and obtaining measurement data under the influence of different parameters is an important prerequisite for supporting this research. This paper mainly focuses on the sea clutter measurements that have been carried out. First, typical experiments in various countries such as Canada, South Africa, Australia, the United States, Spain, and Germany are categorized and summarized from the aspects of shore-based experiment and airborne experiment. Then, sea clutter measurement experiments with wave tank conducted by the United States and Japan are reviewed, and domestic sea clutter measurement experiments as well as the construction of the maritime target detection experimental center in Yantai are briefly introduced. Finally, the future research directions that should be emphasized are projected: more systematic and continuous sea clutter measurement experiments need to be conducted; experiment and data analysis under explicit task background need to be strengthened; and sea clutter and target datasets that meet the requirement of intelligent radar applications need to be urgently constructed.

Distributed Coherent Aperture Radar (DCAR) utilizes multiple separated antenna apertures to emit signals in the same space area, realizing spatial coherent synthesis of electro-magnetic waves. Such a flexible radar system has advantages such as higher resolution, greater radar power, and lower cost. Combined with microwave photonic technologies, which have merits in wideband signal generation, transmission and procession, the DCAR has a comprehensive and better performance. This paper introduces a microwave photonics-based high-resolution distributed coherent aperture radar that was proposed by researchers of Tsinghua University. Taking advantages of microwave photonic technology, a group of wideband orthogonal phase-coded linear frequency modulation waves is generated in the coherence-on-receive mode, with the frequency ranging from 8.5 GHz to 11.5 GHz, all with phase coding under a bit rate of 0.5 Gbps. The orthogonality of the signals is nearly 30 dB, and the range resolution is better than 0.05 m. While in the full coherence mode, the transmitted waveforms can be flexibly switched to the wideband coherent linear frequency modulation waves, and the full coherent synthesis can be realized. The waveforms generated by the proposed system can meet the waveform requirements of the DCAR in different operation modes. In the experiment, full coherence is achieved with two sets of radars, resulting a signal- to-noise ratio gain of 8.3 dB.

This paper presents a radar working mode based on multi-pulse combination and coding, and the basic concept and theoretical model of pulse-coded radar are established. Using multi-pulse combination and the modulation of the time, frequency, and phase parameters, the pulse signal is coded in time domain, frequency domain, or time-frequency domain. Based on multi-pulse combination and coding, a new working method and theoretical basis are provided to solve the problem of coupling constraints among parameters of traditional pulse and continuous wave radars, which limits the performance of the radar system. Based on the fabricated Synthetic Aperture Radar (SAR) and its tests, the pulse coding, target signal recovery method, and pulse coding performance were studied. The study and test results show that the sampling rate of radar signals could be reduced to break through the limitation of Nyquist sampling theorem and simplify the difficulty of system implementation using frequency division pulse coding method. A 5 GHz bandwidth signal was sampled with 4.8 GHz and was recovered without distortion in the radar system, and an SAR imaging resolution of 0.03×0.03 m was realized. By using the time domain pulse coding method with an increase of duty cycle, the signal-to-noise ratio was improved by over 20 dB. By using the two-dimensional pulse coding method, the SAR signal ambiguity was removed, and the imaging swath of over 90 km was realized. The theoretical and test results verify the significant advantages of the pulse coding method in improving radar performance, by which a new technical approach for the realization of high performance radar system is provided.

Electromagnetic waves are transmitted by radars and reflected by different objects, and radar signal processing is highly significant as its analyses can lead to the acquisition of important information such as the situation and radial movement speed. Moreover, deep learning has gained much attention in several fields, and it can be utilized to implement radar signal processing. Compared with the traditional methods, deep learning can realize automatic feature extraction and yield highly accurate results; hence, in this paper, the application of deep learning algorithm in radar signal processing is studied. In addition, the study directions in radar signal processing are summarized into overfitting and interpretation. Thus, these two issues are being considered.

In this paper, we review the recent developments on information metamaterials, including digital metamaterials, coding metamaterials, and programmable metamaterials; furthermore, we discuss their applications in the terahertz (THz)-frequency region. In addition their flexibility to manipulate the electromagnetic waves, the physical principle, numerical simulation, fabrication, and application of information metamaterial are discussed in detail. Moreover, we developed and applied a coding metasurface that works in the THz band. Furthermore, the principle of real-time programmable metamaterials and their application in novel imaging systems and radar systems are illustrated. Information metamaterials and metasurfaces can be used for various functional devices such as beam splitting and low radar cross section, which open up a novel route to manipulate THz radiations.

Aircraft wake vortex is a pair of strong counter-rotating vortices and has attracted considerable attention in various fields including aviation safety and atmospheric physics. The characteristics and detection of wake vortex act as the basis for both behavior prediction as well as hazard assessment. This paper provides a short survey of the characteristics and detection researches. Initially, the wake vortex is classified as clear-air wake vortex (i.e., wake turbulence and contrail) and precipitation wake vortex (i.e., under rainy, foggy or snowy condition). Subsequently, the dynamics and scattering are introduced, and the main verdicts are: the radar (radio detection and ranging) scattering of wake vortex is relatively weak under clear air conditions, but the Lidar (Light detection and ranging) scattering is appreciable owing to the presence of particles such as aerosols. Wake vortices under precipitation conditions and contrails possess relatively good radar reflectivity owing the strong scattering characteristics of precipitation droplets and ice crystals. Furthermore, we have introduced a joint detection scheme of Lidar and radar for wake vortex along with parameter-retrieval algorithms. Finally, we have presented our conclusions and intended future research.

To obtain an accurate polarization scattering matrix, simultaneous full polarization radar systems must transmit two signals. The performance of orthogonal polyphase codes designed by the traditional method is limited by the code length and is sensitive to Doppler frequency. In this paper, we propose a design method for orthogonal polyphase codes that have good Doppler tolerance. We consider the peak sidelobe level and isolation and transform the signal design problem into a nonlinear optimization problem, which we solve using a genetic algorithm. Our simulation results show that our proposed orthogonal polyphase codes have better Doppler tolerance and their peak sidelobe levels and orthogonal performances are 1.5～2 dB better than the codes designed by Deng or Khan. As such, the new design can improve the measurement accuracy of simultaneous full polarization radar systems.

In this study, a new type of Continuous Wave (CW) signal modulated by pseudocode family is designed, solving the problem of more quantity of warhead dynamic fragments, larger speed variation, larger distribution and more difficult resolution during the measurement of warhead dynamic fragments. This signal has thumbtack ambiguity function and multiresolution characteristics. It can meet the measurement needs very well. Herein, correlation properties and ambiguity function characteristics of this signal are analyzed. Moreover, the signal’s limitations are reported. Recommendations pertaining to signal selection, number option, and usage are presented. The analysis results show that this signal can be used for dynamic fragments measurement of warhead. This signal is also of great importance to improve complex waveform design and radar performance.

This study uses time domain terahertz radar system to discuss systematic imaging studies on the scaled models based on the improved Back-Projection (BP) algorithm. We image the scaled models with different shapes and are able to distinguish spatial gaps as small as 6 mm. The Theoretical calculation predicts that the lateral resolution and the axial resolution can be as high as 0.125 mm. Center enhancement and background rings caused by the algorithm in the imaging results are also qualitatively analyzed and are proposed methods to overcome this problem.