High-resolution and wide-swath (HRWS) imaging serves as an important task for synthetic aperture radar (SAR). However, the trade-off between high azimuth resolution and wide unambiguous swath coverage has not been well optimized in traditional SAR system. Meanwhile, the ultra-wideband signal is required to obtain ultra-high range resolution imaging. It is not easy to increase the bandwidth directly due to the difficulty of system design. To this end, this work makes notable contribution on a sub-band frequency diverse array framework to realize range ambiguous echoes separation and wideband signal synthesis from narrowband signals. By introducing a small frequency increment across the elements within each subarray, the transmit steering vector within each subarray is range-angle-dependent, making it feasible to resolve range ambiguity in the spatial frequency. In addition, the transmitted waveforms of different subarrays occupy different frequency bands in range frequency domain, which play a pivotal role in achieving high resolution imaging. With phase compensation and sub-band frequency spectrum splicing technique, the wideband signal can be obtained. Simulation results have verified the effectiveness of the proposed approach.

By adopting the transmit minimum redundancy array (MRA) in frequency diverse array multiple-input multiple-output (FDA-MIMO), extra degrees-of-freedom (DOFs) in the transmit spatial domain can be obtained owing to enlarged virtual elements. In this paper, an interference suppression method is developed with the MR-FDA-MIMO radar using data-dependent beamforming based on virtual samples. In our designed method, both the transmit and receive beamforming are explored via a two-stage approach, including: i) at the receive beamforming stage, sidelobe interferences are firstly mitigated with minimum variance distortionless response (MVDR)-based beamforming after blocking the true target and mainlobe interferences. ii) at the transmit beamforming stage, the samples from the physical array are divided into multiple subsets, while a difference co-array procedure is applied to each subsets and multiple virtual samples are obtained to construct the interference-plus-noise covariance matrix. Furthermore, such a matrix is reconstructed based on the Capon spectrum. In Addition, the errors for both the virtual samples and virtual covariance matrix are analyzed. The proposed approach outperforms the traditional sparse smooth strategies in terms of suppressible interference number regardless of the number of physical array element. The effectiveness of the proposed method for interference suppression is verified through numerical simulations.

In the conventional array radar, the suppressible interference is generally limited by the array element number which is referred to as the degree-of-freedom (DOF) of the array. Similarly, the suppressible number of deceptive interference is limited by the transmit element number of frequency diverse array (FDA) multiple-input multiple-output (MIMO) radar. In this paper, an interference suppression method based on data-independent beamforming is proposed with FDA-MIMO radar by using minimum redundancy array (MRA) design in its transmit port, referred to as MR-FDA-MIMO radar. This breakthroughs the limitation on the number of suppressible deceptive interferences. The MRA is employed to obtain extended DOF by using difference co-array, resulting in suppressible number of order O(2M) with M transmit element number. The beamforming method consists of two manifolds: i) in the receive beamforming stage, the possible target is preserved by using orthogonal subspace projection and the sidelobe barrage interference is suppressed; ii) in the equivalent transmit beamforming stage, possible non-ideal factors are taken into consideration for constructing the covariance matrix to widen the notches corresponding to the predicted deceptive interference. The proposed method outperforms in terms of number of the suppressible deceptive interferences and the signal-to-interference-plus-noise ratio, which is demonstrated with simulation examples.

Frequency diversity array (FDA) is capable of suppressing mainlobe deceptive interference with delays exceeding one pulse relative to the true target, known as interpulse interference. However, current FDA-based interference suppression methods are ineffective when both interpulse and intrapulse interferences coexist. To this end, we propose a two-stage mainlobe deceptive interference suppression method with planar FDA (PFDA) multiple-input–multiple-output radar: First, horizontal transmit spatial frequency compensation is applied using the range information of each range bin to differentiate between interpulse interferences and target. Subsequently, data-independent beamforming is performed to suppress interpulse interferences in the horizontal spatial frequency domain. Second, to distinguish the target from intrapulse interferences, vertical transmit spatial frequency compensation is applied utilizing the initial range of the range bin containing target. Afterward, an adaptive beamforming approach based on transmit–receive spatial smoothing is applied to suppress intrapulse interference while avoiding target cancelation. Furthermore, the frequency increments in both rows and columns of PFDA are elaborately designed to effectively distinguish between true target and interferences. Extensive simulations are conducted to evaluate the excellent suppression effect of the proposed scheme against the mainlobe deceptive interference.

It is a challenge to realize wide swath imaging due to the conflict between Doppler ambiguity and range ambiguity for hypersonic vehicle (HSV) radar. In addition, there are many conditions requiring the forward-looking imaging. In a forward-looking synthetic aperture radar (SAR) system, left–right ambiguity arises, since two symmetrical targets have the same Doppler frequency magnitude. After selecting an appropriate pulse repetition frequency (PRF) to avoid Doppler ambiguity, we only need to solve the range ambiguity and left–right ambiguity. To handle these issues, this paper proposes an approach to resolve the range ambiguity and left–right ambiguity using the frequency diverse array (FDA). With the range-angle-dependent property of the transmit steering vector, FDA can distinguish the range ambiguous echoes in the spatial frequency domain. By performing transmit beamforming after range compensation, the echo from the desired range region can be extracted from ambiguous echoes. Then, the back projection (BP) algorithm is used to achieve imaging. Next, the echoes of all channels are processed by two receive beamformers, which are designed for the right and left sides, respectively. With the aforementioned procedures, an unambiguous image can be obtained. Simulation results have verified the effectiveness of the proposed approach.

In this letter, a novel method based on multifrequency pulse diversity (MFPD) is proposed to achieve high-resolution and wide-swath (HRWS) imaging by utilizing the displaced phase center antenna (DPCA) technique. In the MFPD mode, multiple waveforms from different frequency bands are transmitted through a single channel. Thus, within the same receive window, the echoes from different range regions correspond to different frequency bands, making it possible to separate the range ambiguous echoes in the range frequency domain. However, the azimuth sampling rate will be reduced in the MFPD mode, leading to the Doppler ambiguity. To this end, the MFPD-DPCA technique is utilized, which is capable of separating the range ambiguous echoes without loss of azimuth sampling rate. Moreover, the MFPD-DPCA technique can achieve high range resolution by spectrum splicing, which enhances the feasibility of super-high-resolution imaging. Finally, the HRWS imaging can be obtained by performing the traditional synthetic aperture radar (SAR) algorithm on the reconstructed unambiguous wideband echoes. The proposed method offers an alternative in system implementation but does not necessarily offer improved swath width over current classical HRWS-SAR methods. Numerical results corroborate the effectiveness of the considered HRWS imaging strategies in ambiguous scenarios.