Real-time echo restoration from signals containing range ambiguities is a technique challenge for high pulse repetition frequency synthetic aperture radar (SAR). In this letter, to ensure real-time processing, a novel azimuth phase coding scheme is utilized to realize the processing of raw data in groups, which is conceived for a conventional SAR system. Meanwhile, an advanced fast algorithm is proposed to further decrease the computational complexity of the restoration processes. The proposed scheme is validated by the simulated point-like and distributed targets SAR data. The quantitative analysis results show that the ambiguous signal can be at least suppressed by 40 dB and the image average range ambiguity separation error is less than -60 dB. Finally, compared with some conventional methods in computational complexity and memory cost, the results illustrate that the processing efficiency has been significantly improved and the memory resource occupation has been significantly reduced. Particularly, the proposed fast algorithm improves the computational efficiency by about 80 times.

Military operations often demand considerable concealment and raid capabilities, particularly at night or in adverse weather conditions. However, the use of synthetic aperture radar (SAR) technology provides early warning and target localization capabilities. While spaceborne or airborne SAR systems can capture expansive SAR scenes, they frequently encounter challenges in delivering timely and high-resolution data, thereby limiting their effectiveness in detecting small ground vehicle targets. To address this issue, our research has developed a low-cost, high-resolution, and real-time monostatic MiniSAR system for the effective detection of small targets, such as vehicles. Furthermore, to enhance the stealthiness of the MiniSAR, a bistatic MiniSAR system has been developed to accomplish detection tasks. Nevertheless, despite the utilization of MiniSAR systems for ground armored target detection, two primary challenges persist: the presence of highly ultracomplex scene interference making accurate target detection difficult; and poor real-time performance resulting in slow detection and tracking. To overcome these challenges, this article proposes a ground vehicle target recognition method based on an improved lightweight anchor-free detection network using monostatic/bistatic SAR images. The method initially leverages the inherent features of SAR targets for localization, embedding these features into SAR images, and then outputs detection results through the improved lightweight anchor-free network. We validate the effectiveness of this method on our self-constructed monostatic/bistatic SAR datasets and verify the algorithm’s robustness on publicly available ship datasets. Experimental results demonstrate that this method outperforms other representative methods in detecting SAR vehicle small targets, exhibiting higher detection accuracy and timeliness.

Multi-input and multi-output (MIMO) radar has drawn much attention in synthetic aperture radar (SAR) due to the possession of more degrees of freedom (DOFs). However, there are some repeated equivalent phase centers (EPCs) caused by the same wave path have no contribution to the improvement of DOFs. To this end, a novel interpulse phase coding and multi-carrier (IPCMC) transmission scheme is investigated to restored repeated EPCs. Furthermore, an advanced range ambiguity separation method is proposed based on the increased efficient EPCs. Finally, distributed targets simulation experiments are performed and the experiment results illustrate that the range ambiguity suppression performance is significantly improved due to the restored EPCs.

Waveform design plays a critical role in ruling the performance of a pulse compression radar system, and keeps being a hotpot for several decades. Unfortunately, some coded waveforms being widely employed in recent years nearly have their limitations. The idealistic phase code waveform has a high spectral sidelobe brought by the instantaneous phase change. The polyphase-coded FM (PCFM) waveform can provide a continuous phase function, but the frequency template error (FTE) metric is indispensable for it to control the spectrum, thereby inducing high design complexity. The nonlinear frequency modulated (NLFM) waveform has a controlled spectral content, while its coding structure is non-parameterized. To this end, we develop a novel parameterized frequency modulated (PFM) waveform and a constant envelope. Simulation and real experimental results verify the superior performance of the proposed waveform in term of autocorrelation sidelobes and energy ratio within bandwidth compared to the phase code and PCFM waveforms.

Multi-input and multi-output (MIMO) radar is an advanced radar system, which grows into a promising candidate for the future synthetic aperture radar (SAR) because the MIMO radar can provide more degrees of freedom (DOFs). Monostatic MIMO SARs (where transceiver channels share the same antenna array) will produce lots of repeated equivalent phase centers (EPCs) due to the same wave path, and these repeated EPCs actually have no improvement to the DOFs of the radar system, resulting in a tremendous waste of the radar resources. To this end, this paper devises a novel radar framework, which is referred to as MIMO SAR with interpulse phase coding and multi-carrier (IPCMC). The IPCMC scheme employs multi-carrier and well-designed phase codes in transmitting channels, which has the advantage of increasing the DOFs in elevation. Concretely, by introducing the carrier information into the transmission-receiving spatial frequency domain, non-overlapped spatial frequency difference curves can be obtained to further increase the DOFs in elevation. Furthermore, an advanced range ambiguity separation method based on the proposed IPCMC MIMO SAR is presented. Compared with SAR systems with different numbers of DOFs, the proposed IPCMC MIMO scheme can precisely separate more ambiguous regions, due to the increased DOFs. Finally, detailed simulation experiments are carried out to verify the efficacy of the proposed IPCMC MIMO SAR.

The problem of echo separation using digital beamforming (DBF) on receive for multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) is of notable importance to allow for practical systems. Regrettably, current DBF-MIMO-SAR schemes, such as the short-term shift-orthogonal (STSO) scheme, are computationally cumbersome, increasing the required hardware complexity. To alleviate this problem, we here propose an improved echo separation solution for realizing a low-cost MIMO-SAR system. We detail a generic waveform design scheme as well as optimized monostatic radar waveforms (e.g., nonlinear frequency modulation (NLFM) signal) showing how these can be directly adopted in the proposed scheme to improve the imaging performance. The proposed scheme enables the number of the interference segments generated by unmatched waveforms to be halved by the use of the fast time spectrum preprocessing and segment synthesis, dramatically simplifying the array configuration and reduces the system complexity. By exploiting inter-pulse phase coding techniques, the proposed method can provide a reconfigurable waveform transmitting scheme, allowing the system resources in range frequency, elevation space, and Doppler domains to be jointly exploited for the separation of aliased signal returns. The proposed scheme is evaluated using extensive numerical and measured data sets, demonstrating the feasibility and potential of the proposed method for resource-limited spaceborne/airborne MIMO-SAR systems.

This paper deals with a novel transmitted scheme for a multi-subcarrier frequency MIMO-SAR system, which aims at suppressing the intrapulse repeater mainlobe-jamming and immensely improving the dynamic range of the receiver. To this end, due to the multi-subcarrier frequency transmission scheme, the mixed signal with true target signal and repeater jamming can be separated by a well-designed spatial-frequency filter. Thus, the range and direction of arrival (DOA) information is accurately estimated without interrupting the normal work of the radar. Furthermore, to further improve the orthogonality of transmitted waveform, the design of a phase-coded LFM waveform pair exhibiting both low cross-correlation energy (CCE) and low peak to sidelobe ratios (PSLRs), is considered. Besides, to handle the resulting nondeterministic polynomial (NP) hard problem, an alternating direction multiplier method (ADMM) based optimization method is employed. Compared with the traditional jamming suppression method, the proposed method improves the degree of freedom (DOF) from the carrier frequency domain, and it has the capability to suppress the intrapulse repeater mainlobe-jamming. Finally, detailed simulation experiments are carried out to verify the practicability and effectiveness of the newly proposed transceiver schemes.

The range ambiguity issue is a technical challenge in the radar community and has been widely discussed over the years. Researchers have given special attention to multiple-input and multiple-output (MIMO) radar to address the range ambiguity because this radar system can employ more equivalent degrees of freedom. Open studies on MIMO radar are generally based on the assumption of orthogonal waveforms, whereas radar performance is seriously limited by distributed targets due to mismatched energy. To this end, this paper deals with a novel MIMO radar transmission scheme called transmitter interpulse phase coding (TIPC) without using orthogonal waveforms. First, a set of well-designed TIPC codes are employed to modulate transmitter subarrays with the same modulated signal. Second, in the case of high pulse repetition frequency (PRF)1, aliased echoes from different transmitted channels are directly separated by a group of simple Doppler filters; for normal PRF2radar systems, a technique called digital beamforming in azimuth is exploited to ensure an effective multiple waveform separation. Third, a decoding processing is performed for a further derivation of the residual TIPC matrix that is related with ambiguity order. Next, the desired and ambiguous echoes are separated by a specifically designed spatial filter that absorbs the residual TIPC matrix. Particularly, the separated signal can be used for some further applications such as increasing the observation swath. Finally, point-like target and distributed targets simulation experiments are performed to verify the feasibility of the proposed TIPC MIMO radar.

The echo separation issue for multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) is usually regarded as a more technical challenge. Spotlighted as a promising solution to the echo separation, the well-known short-term shift-orthogonal (STSO) beamforming scheme has become increasingly popular. However, for airborne MIMO SAR systems, the digital beamforming (DBF) involved in the STSO scheme usually encounters more issues, e.g., the direction of arrival (DOA) mismatch induced by topography variation. Up to now, relatively less research on robust DBF processing has been conducted for airborne MIMO SAR systems. In this respect, an adaptive DBF technique, based on interference plus noise covariance matrix (IPNCM) reconstruction and desired signal steering vector estimation, has been proposed in this paper. IPNCM reconstruction and steering vector estimation can not only cope with the DOA mismatch problem, but also remove the desired signal component in the training data cells to increase the beamformer convergence rates. Consequently, the proposed approach really improves the array output signal-to-interference-plus-noise ratio (SINR). Moreover, numerous discussions and simulations are carried out to prove the effectiveness of proposed DBF technique under various disturbance environments. Compared with the current DBF techniques, the proposed method provides a bright application prospect for the STSO scheme.

Interrupted FMCW (IFMCW) SAR mode, which only employs a single antenna from a single micro-satellite, was proposed by Ahmed et al., recently. However, periodical data gaps will occur in the interrupted mode, caused by the process of switching the radar transmitter on and off. To solve this problem, in this paper, a modified aperture-interpolation -based approach is proposed to fill the data gaps. The approach can recover the gapped data with excellent performance and thus significantly suppress the artifacts (or ghosts) induced by the periodical data gaps. Processing results of real data acquired with an experimental airborne FMCW SAR system, demonstrate the effectiveness of the proposed approach.