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Editor Work2018 Vol. 7, No. 2
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2018, 7(2): 153-166.
Abstract
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Unlike the conventional phased array that provides only angle-dependent transmit beampattern, Frequency Diverse Array (FDA) employs a small frequency increment across its array elements to produce automatic beam scanning without requiring phase shifters or mechanical steering. FDA can produce both range-dependent and time-variant transmit beampatterns, which overcomes the disadvantages of conventional phased arrays that produce only angle-dependent beampattern. Thus, FDA has many promising applications. Based on a previous study conducted by the author, " Frequency Diverse Array Radar: Concept, Principle and Application” (Journal of Electronics & Information Technology, 2016, 38(4): 1000–1011), the current study introduces basic FDA radar concepts, principles, and application characteristics and reviews recent advances on FDA radar and its applications. In addition, several new promising applications of FDA technology are discussed, such as radar electronic warfare and radar-communications, as well as open technical challenges such as beampattern variance, effective receiver design, adaptive signal detection and estimation, and the implementation of practical FDA radar demos.
Unlike the conventional phased array that provides only angle-dependent transmit beampattern, Frequency Diverse Array (FDA) employs a small frequency increment across its array elements to produce automatic beam scanning without requiring phase shifters or mechanical steering. FDA can produce both range-dependent and time-variant transmit beampatterns, which overcomes the disadvantages of conventional phased arrays that produce only angle-dependent beampattern. Thus, FDA has many promising applications. Based on a previous study conducted by the author, " Frequency Diverse Array Radar: Concept, Principle and Application” (Journal of Electronics & Information Technology, 2016, 38(4): 1000–1011), the current study introduces basic FDA radar concepts, principles, and application characteristics and reviews recent advances on FDA radar and its applications. In addition, several new promising applications of FDA technology are discussed, such as radar electronic warfare and radar-communications, as well as open technical challenges such as beampattern variance, effective receiver design, adaptive signal detection and estimation, and the implementation of practical FDA radar demos.
2018, 7(2): 167-182.
The carrier frequencies of array elements in a Frequency Diverse Array (FDA) radar are slightly distinguished, leading to a range-angle-time-dependent transmit beampattern. Thus, an FDA radar carries additional information in a certain range and provides more flexibility in signal processing and new technical issues. FDA is covered by scope of the general waveform diversity concept. This paper overviews the state-of-the-art FDA technology and its radar applications. From the viewpoint of the general radar system theory, we mainly introduce the coherent FDA and orthogonal FDA frameworks. The orthogonal FDA is also referred to as Multiple-Input Multiple-Output (MIMO) radar using FDA or FDA-MIMO radar. Key applications in anti-jamming and issues related with range ambiguity are addressed. We also outline the challenges in FDA radar applications and several interesting research topics.
The carrier frequencies of array elements in a Frequency Diverse Array (FDA) radar are slightly distinguished, leading to a range-angle-time-dependent transmit beampattern. Thus, an FDA radar carries additional information in a certain range and provides more flexibility in signal processing and new technical issues. FDA is covered by scope of the general waveform diversity concept. This paper overviews the state-of-the-art FDA technology and its radar applications. From the viewpoint of the general radar system theory, we mainly introduce the coherent FDA and orthogonal FDA frameworks. The orthogonal FDA is also referred to as Multiple-Input Multiple-Output (MIMO) radar using FDA or FDA-MIMO radar. Key applications in anti-jamming and issues related with range ambiguity are addressed. We also outline the challenges in FDA radar applications and several interesting research topics.
2018, 7(2): 183-193.
To meet the urgent demand of low-observable moving target detection in complex environments, a novel method of Frequency Diverse Array (FDA) radar signal processing method based on Space-Rang-Doppler Focusing (SRDF) is proposed in this paper. The current development status of the FDA radar, the design of the array structure, beamforming, and joint estimation of distance and angle are systematically reviewed. The extra degrees of freedom provided by FDA radar are fully utilizsed, which include the Degrees Of Freedom (DOFs) of the transmitted waveform, the location of array elements, correlation of beam azimuth and distance, and the long dwell time, which are also the DOFs in joint spatial (angle), distance, and frequency (Doppler) dimensions. Simulation results show that the proposed method has the potential of improving target detection and parameter estimation for weak moving targets in complex environments and has broad application prospects in clutter and interference suppression, moving target refinement, etc..
To meet the urgent demand of low-observable moving target detection in complex environments, a novel method of Frequency Diverse Array (FDA) radar signal processing method based on Space-Rang-Doppler Focusing (SRDF) is proposed in this paper. The current development status of the FDA radar, the design of the array structure, beamforming, and joint estimation of distance and angle are systematically reviewed. The extra degrees of freedom provided by FDA radar are fully utilizsed, which include the Degrees Of Freedom (DOFs) of the transmitted waveform, the location of array elements, correlation of beam azimuth and distance, and the long dwell time, which are also the DOFs in joint spatial (angle), distance, and frequency (Doppler) dimensions. Simulation results show that the proposed method has the potential of improving target detection and parameter estimation for weak moving targets in complex environments and has broad application prospects in clutter and interference suppression, moving target refinement, etc..
2018, 7(2): 194-201.
To accurately identify the range of each target, traditional Multiple-Input Multiple-Output (MIMO) radar techniques not only require designing a shift matrix to describe different range bins but also a large number of snapshots.To alleviate this problem, a multidimensional parameter estimation method based on sparse iteration is proposed for a MIMO radar with Frequency Diverse Array (FDA).The FDA-MIMO radar uses small frequency increments across the array elements, and its transmit steering vector is a function of both range and angle.On the basis of the feature of the FDA-MIMO radar, we consider a weighted lq (0 q 1) minimization problem that is solved using a sparse iterative algorithm.Finally, the target parameters (the amplitude, range, and angle) are obtained using a single snapshot.Moreover, numerical simulations are used to demonstrate the superior performance of the proposed method compared with those of DAS, IAA, and IAA-R.
To accurately identify the range of each target, traditional Multiple-Input Multiple-Output (MIMO) radar techniques not only require designing a shift matrix to describe different range bins but also a large number of snapshots.To alleviate this problem, a multidimensional parameter estimation method based on sparse iteration is proposed for a MIMO radar with Frequency Diverse Array (FDA).The FDA-MIMO radar uses small frequency increments across the array elements, and its transmit steering vector is a function of both range and angle.On the basis of the feature of the FDA-MIMO radar, we consider a weighted lq (0 q 1) minimization problem that is solved using a sparse iterative algorithm.Finally, the target parameters (the amplitude, range, and angle) are obtained using a single snapshot.Moreover, numerical simulations are used to demonstrate the superior performance of the proposed method compared with those of DAS, IAA, and IAA-R.
2018, 7(2): 202-211.
The ground clutter of sidelooking Multiple-Input Multiple-Output (MIMO) bistatic radar has severe problems of range-dependence and clutter spectrum spreading. In this paper, the clutter characteristics of bistatic airborne radars are analyzed and a method of range-ambiguous clutter suppression is proposed. This method employs the range-dependent phase term in the traditional MIMO bistatic radar to provide extra degrees of freedom. The range information of the phase term enables clutter separation with different ranges and consequent suppression. Thus, a range-ambiguous clutter suppression for bistatic airborne early warning radars is achieved. The simulation results show the accuracy of the method. By a comparison with the existing Doppler Warping (DW) and clutter suppression method with the Improved Factor (IF) line, the results showed that the method has better performance, which proved the effectiveness of ambiguous clutter suppression in MIMO bistatic radars.
The ground clutter of sidelooking Multiple-Input Multiple-Output (MIMO) bistatic radar has severe problems of range-dependence and clutter spectrum spreading. In this paper, the clutter characteristics of bistatic airborne radars are analyzed and a method of range-ambiguous clutter suppression is proposed. This method employs the range-dependent phase term in the traditional MIMO bistatic radar to provide extra degrees of freedom. The range information of the phase term enables clutter separation with different ranges and consequent suppression. Thus, a range-ambiguous clutter suppression for bistatic airborne early warning radars is achieved. The simulation results show the accuracy of the method. By a comparison with the existing Doppler Warping (DW) and clutter suppression method with the Improved Factor (IF) line, the results showed that the method has better performance, which proved the effectiveness of ambiguous clutter suppression in MIMO bistatic radars.
2018, 7(2): 244-253.
Synchronization is a key problem in distributed Synthetic Aperture Radar (SAR) systems. In this paper, we perform a complex mathematical deduction and then analyze the influences of time synchronization on the SAR imaging and interferometric process. We discuss the relationship between time and phase synchronization, considering that different oscillators in separated transmitters and receivers lead to both time and phase synchronization errors. With respect to beam synchronization, we present the effects of the accuracies of beam pointing and satellite attitude on the antenna gain, based on the attitude-steering strategy, which involves azimuth weighting of the Doppler spectra for independent zero-Doppler beam steering. We also analyze the influences of beam synchronization on Doppler decorrelation, Signal-to-Noise Ratio (SNR), and overlapping swath error. We conduct simulations to validate the analysis results. Our findings provide guidance for system design.
Synchronization is a key problem in distributed Synthetic Aperture Radar (SAR) systems. In this paper, we perform a complex mathematical deduction and then analyze the influences of time synchronization on the SAR imaging and interferometric process. We discuss the relationship between time and phase synchronization, considering that different oscillators in separated transmitters and receivers lead to both time and phase synchronization errors. With respect to beam synchronization, we present the effects of the accuracies of beam pointing and satellite attitude on the antenna gain, based on the attitude-steering strategy, which involves azimuth weighting of the Doppler spectra for independent zero-Doppler beam steering. We also analyze the influences of beam synchronization on Doppler decorrelation, Signal-to-Noise Ratio (SNR), and overlapping swath error. We conduct simulations to validate the analysis results. Our findings provide guidance for system design.
2018, 7(2): 212-219.
It has been shown that Frequency Diverse Arrays (FDA) exhibit a range-angle dependent beam steering feature by employing a uniform frequency increment across the array elements. However, this beam pattern generates maxima at multiple range values, possibly leading to loss of signal-to-interference-plus-noise ratio when the interferences are located at any of the maxima. Herein, we prove that the beam pattern of FDA is range-periodic and propose the basic criteria for the FDA configuration to decouple the range and angle. In an illuminated space, a single-maximum beam pattern can be obtained by configuring the frequency increment between the elements. Specific examples have been discussed herein, and the simulation results verify the proposed theory.
It has been shown that Frequency Diverse Arrays (FDA) exhibit a range-angle dependent beam steering feature by employing a uniform frequency increment across the array elements. However, this beam pattern generates maxima at multiple range values, possibly leading to loss of signal-to-interference-plus-noise ratio when the interferences are located at any of the maxima. Herein, we prove that the beam pattern of FDA is range-periodic and propose the basic criteria for the FDA configuration to decouple the range and angle. In an illuminated space, a single-maximum beam pattern can be obtained by configuring the frequency increment between the elements. Specific examples have been discussed herein, and the simulation results verify the proposed theory.
2018, 7(2): 220-234.
To solve the bottlenecks associated with traditional Synthetic Aperture Radar (SAR) systems related to the limitations of single working models and core indexes, a new system known as Multi-Input Multi-Output (MIMO) SAR was recently proposed. This novel radar system obtains greater freedom by the use of more equivalent transceivers. As such, it is not constrained by the limitations of traditional SAR systems and can achieve high-resolution wide-swath imaging and mean-time multi-mode operation. In this paper, we analyze the connotations and characteristics of MIMO-SAR in depth, and summarize the current research situation and future development trends. To lay a foundation for China’s future SAR technology, we have also developed the first MIMO-SAR prototype with the same time-frequency coverage. We summarize the key technologies and flight results and analyze the prospects for MIMO-SAR applications.
To solve the bottlenecks associated with traditional Synthetic Aperture Radar (SAR) systems related to the limitations of single working models and core indexes, a new system known as Multi-Input Multi-Output (MIMO) SAR was recently proposed. This novel radar system obtains greater freedom by the use of more equivalent transceivers. As such, it is not constrained by the limitations of traditional SAR systems and can achieve high-resolution wide-swath imaging and mean-time multi-mode operation. In this paper, we analyze the connotations and characteristics of MIMO-SAR in depth, and summarize the current research situation and future development trends. To lay a foundation for China’s future SAR technology, we have also developed the first MIMO-SAR prototype with the same time-frequency coverage. We summarize the key technologies and flight results and analyze the prospects for MIMO-SAR applications.
2018, 7(2): 235-243.
Radio-Frequency Interference (RFI) adversely affects useful signals, thereby seriously affecting image quality. In this study, a modified eigensubspace-based approach for radio-frequency interference suppression of Synthetic Aperture Radar (SAR) images is proposed. In the preprocessing stage of our proposed algorithm, RFI detection is conducted in both the frequency and time domains. Subsequently, we can only deal with the data containing the RFI via the traditional eigensubspace-based approach. Compared with the traditional eigensubspace-based approach, our proposed algorithm can function more efficiently and effectively.
Radio-Frequency Interference (RFI) adversely affects useful signals, thereby seriously affecting image quality. In this study, a modified eigensubspace-based approach for radio-frequency interference suppression of Synthetic Aperture Radar (SAR) images is proposed. In the preprocessing stage of our proposed algorithm, RFI detection is conducted in both the frequency and time domains. Subsequently, we can only deal with the data containing the RFI via the traditional eigensubspace-based approach. Compared with the traditional eigensubspace-based approach, our proposed algorithm can function more efficiently and effectively.
2018, 7(2): 254-262.
In this paper, we address target tracking problems by the use of multiple sensors via the Dynamic Programming (DP)-based Track-Before-Detect (TBD) method. Generally, DP-TBD is a grid-based method that estimates target trajectories by searching all the physically admissible paths in a determinate discrete state space. However, this multi-frame detection algorithm provides plot sequences without filtering or smoothing. With the growing complexity of the battle field environment, single radar based on DP-TBD cannot achieve satisfactory results when the Signal-to-Noise Ratio (SNR) is low. Besides, it is very difficult to fuse plot sequences from different radars because they contain no state error covariance matrix. Furthermore, various radars always contain asynchronous data due to the diversity of sampling times and communication delays. To alleviate these problems, we propose a distributed asynchronous recursive filtering fusion (Dynamic Programming Fuison, DPF) algorithm based on DP-TBD, which is divided into two steps. In the first step, we propose an iterative filter algorithm via DP-TBD. Then, we convert the asynchronous evaluation data into synchronous data and implement several distributed fusion algorithms to estimate the target state. Simulation results show that the proposed algorithm can correctly estimate target trajectories and significantly enhance tracking accuracy compared to solo radar. In addition, this algorithm can decrease the track loss rate and calculation burden.
In this paper, we address target tracking problems by the use of multiple sensors via the Dynamic Programming (DP)-based Track-Before-Detect (TBD) method. Generally, DP-TBD is a grid-based method that estimates target trajectories by searching all the physically admissible paths in a determinate discrete state space. However, this multi-frame detection algorithm provides plot sequences without filtering or smoothing. With the growing complexity of the battle field environment, single radar based on DP-TBD cannot achieve satisfactory results when the Signal-to-Noise Ratio (SNR) is low. Besides, it is very difficult to fuse plot sequences from different radars because they contain no state error covariance matrix. Furthermore, various radars always contain asynchronous data due to the diversity of sampling times and communication delays. To alleviate these problems, we propose a distributed asynchronous recursive filtering fusion (Dynamic Programming Fuison, DPF) algorithm based on DP-TBD, which is divided into two steps. In the first step, we propose an iterative filter algorithm via DP-TBD. Then, we convert the asynchronous evaluation data into synchronous data and implement several distributed fusion algorithms to estimate the target state. Simulation results show that the proposed algorithm can correctly estimate target trajectories and significantly enhance tracking accuracy compared to solo radar. In addition, this algorithm can decrease the track loss rate and calculation burden.
2018, 7(2): 263-274.
Optical system and detection range of Synthetic Aperture Ladar (SAL) are analyzed. According to the imaging characteristics of SAL, the concept that SAL uses non-imaging diffractive optical system are proposed, meanwhile, the phased array model is introduced to analyze its performance. In the condition of using binary optical element on the feeder and primary mirror, the phaser parameters and beam pattern are presented using simulation. The signal of 2° view field is introduced into fiber with the 300 mm aperture telescope and compressed optical path. The radar detection range equation of SAL is introduced, coherent detection and signal accumulation gain are analyzed, the conclusion is SAL has good ability of detecting weak signal. Aiming at application requirement, system parameters and working modes of airborne SAL are given with high resolution and long detection range. With 5 cm resolution, the airborne SAL can achieve 5 km detection range with 1.5 km swath in strip-map imaging mode and 10 km detection range with 1 km swath in sliding spotlight imaging mode.
Optical system and detection range of Synthetic Aperture Ladar (SAL) are analyzed. According to the imaging characteristics of SAL, the concept that SAL uses non-imaging diffractive optical system are proposed, meanwhile, the phased array model is introduced to analyze its performance. In the condition of using binary optical element on the feeder and primary mirror, the phaser parameters and beam pattern are presented using simulation. The signal of 2° view field is introduced into fiber with the 300 mm aperture telescope and compressed optical path. The radar detection range equation of SAL is introduced, coherent detection and signal accumulation gain are analyzed, the conclusion is SAL has good ability of detecting weak signal. Aiming at application requirement, system parameters and working modes of airborne SAL are given with high resolution and long detection range. With 5 cm resolution, the airborne SAL can achieve 5 km detection range with 1.5 km swath in strip-map imaging mode and 10 km detection range with 1 km swath in sliding spotlight imaging mode.
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