High-resolution Wide-swath Research Articles

Fusion of dynamic predictive block adaptive quantization and vector quantization for staggered SAR data compression

ABSTRACT Staggered synthetic aperture radar (SAR) is an advanced concept for high-resolution wide-swath (HRWS) imaging, which overcomes the problem of blind ranges by continuously varying the pulse repetition frequency (PRF). For better performance of information recovery in blind ranges, data are typically highly oversampled in azimuth for staggered SAR system. Thus a huge data volume onboard is required and the demands for internal data storage and downlink capacity are increasing. In this paper, we investigate a novel method, named Dynamic Predictive Block Adaptive Vector Quantization (DP-BAVQ), to reduce the volume of downlinked data for staggered SAR. It firstly compresses the difference of the raw data and their predictions with the dynamic predictive block adaptive quantization (DP-BAQ). Then a secondary compression is performed with vector quantization (VQ). The simulation results and the experiments with real data show that the proposed method achieves an improvement in the signal-to-quantization noise ratio (SQNR) compared with DP-BAQ. Moreover, DP-BAVQ at 2 bits/sample provides a higher SQNR compared with standard block adaptive quantization (BAQ) at 3 bits/sample. Thus the proposed method allows for a significant reduction of data volume.

A Novel Compressive Sensing-Based Multichannel HRWS SAR Imaging Technique for Moving Targets

When high-resolution wide-swath (HRWS) multichannel synthetic aperture radar (MC-SAR) system is used for ocean observation, vast amount of redundant data is generated, significantly limiting its applications. Though compressive sensing (CS)-based method has been applied to the traditional single-channel or dual-channel SAR imaging system, it is no longer applicable for MC-SAR due to the existence of channel error when using the space-time equivalent sampling technique for azimuth signal reconstruction. By analyzing such periodic channel error, i.e., frequency-dependence phase mismatch (FD-PM), in this article, a novel dictionary is constructed for CS-based HRWS MC-SAR imaging after an improved range cell migration correction method is applied. As a result, a novel CS imaging mode is proposed for the ocean moving target based on the sparsity of the target scattering centers, by which the amount of data in MC-SAR can be reduced by sampling below the Nyquist sampling rate and the swath width can be further increased. Experimental results show that the proposed method clearly eliminates the azimuth defocus and blur caused by low sampling rate and FD-PM, and significantly reduces the amount of data to about one-third when compared to sampling at the Nyquist rate.

A Novel Motion Compensation Scheme for 2-D Multichannel SAR Systems With Quaternion Posture Calculation

The displaced phase center antennas in azimuth and digital beamforming (DBF) in elevation are two state-of-the-art techniques for achieving high-resolution wide swath (HRWS) imaging in the multichannel synthetic aperture radar (SAR) systems. However, due to the atmospheric turbulence, airborne HRWS-SARs inevitably suffer trajectory disturbances, which will consequently defocus the SAR image. Although various motion compensation (MoCo) methods have been proposed, they are mostly designed for the traditional single-channel SAR and, therefore, ignore the channel-dependent posture error. The posture motion error will introduce residual motion and time-variant channel errors, which not only defocuses the image but also causes azimuth ambiguity and degrades the SNR improvement of the final images of the azimuth multichannel and DBF-SAR, respectively. To solve these problems, a novel MoCo scheme for 2-D multichannel SAR systems is proposed. The posture error is described and calculated in matrix form through the use of quaternions. Then, the posture error is transformed into the translational motion error for each receiving channel and is subsequently compensated precisely. To address the residual aperture-variant motion error, a modified aperture-dependent MoCo is integrated into the proposed scheme. Simulations and airborne experiments, including processing the data of a C-band SAR system with four azimuth channels and an X-band DBF-SAR system with 16 elevation channels, have been implemented to validate the effectiveness of the proposed MoCo scheme.

A Novel MIMO-SAR System Based on Simultaneous Digital Beam Forming of Both Transceiver and Receiver.

Orthogonal frequency division multiplexing (OFDM) chirp waveform, which is composed of two or more successive identical linear frequency modulated sub pulses, is a newly proposed orthogonal waveform scheme for multi-input multi-output (MIMO) synthetic aperture radar (SAR) systems. However, according to the waveform model, there will be range ambiguity if the mapping width exceeds the maximum unambiguous width determined by the transmitted signal. This greatly limits its application in high-resolution wide-swath (HRWS) remote sensing. The traditional system divides the echoes by digital beam forming (DBF) to solve this problem, but the energy utilization rate is low. A MIMO-SAR system using simultaneous digital beam forming of both transceiver and receiver to avoid range ambiguity is designed in this paper. Compared with traditional system, the novel system designed in this paper obtain higher energy utilization and waveform orthogonality.

Slow Pulse Repetition Interval Variation for High-Resolution Wide-Swath SAR Imaging

In the context of spaceborne synthetic aperture radar (SAR) imaging, high resolution and wide swath are inherently conflicting requirements. These may, however, be simultaneously satisfied by advanced imaging modes with multichannel architectures in elevation and/or azimuth. This article elaborates on a new mode based on multiple elevation beams and a simple pulse repetition interval (PRI) variation scheme which allows high-resolution wide-swath imaging. It is shown to use the illumination time more efficiently than ScanSAR and yet to be simpler than staggered SAR. Good SAR imaging performance is achieved with a rather compact antenna design. The proposed imaging mode is suitable for spaceborne SAR systems with planar and reflector antennas. In order to improve the imaging performance, a reflector antenna architecture with a multichannel feed in both elevation and azimuth is considered.

Advanced high-order nonlinear chirp scaling algorithm for high-resolution wide-swath spaceborne SAR

Spaceborne Synthetic Aperture Radar (SAR) is a well-established and powerful imaging technology that can provide high-resolution images of the Earth’s surface on a global scale. For future SAR systems, one of the key capabilities is to acquire images with both high-resolution and wide-swath. In parallel to the evolution of SAR sensors, more precise range models, and effective imaging algorithms are required. Due to the significant azimuth-variance of the echo signal in High-Resolution Wide-Swath (HRWS) SAR, two challenges have been faced in conventional imaging algorithms. The first challenge is constructing a precise range model of the whole scene and the second one is to develop an effective imaging algorithm since existing ones fail to process high-resolution and wide azimuth swath SAR data effectively. In this paper, an Advanced High-order Nonlinear Chirp Scaling (A-HNLCS) algorithm for HRWS SAR is proposed. First, a novel Second-Order Equivalent Squint Range Model (SOESRM) is developed to describe the range history of the whole scene, by introducing a quadratic curve to fit the deviation of the azimuth FM rate. Second, a corresponding algorithm is derived, where the azimuth-variance of the echo signal is solved by azimuth equalizing processing and accurate focusing is achieved through a high-order nonlinear chirp scaling algorithm. As a result, the whole scene can be accurately focused through one single imaging processing. Simulations are provided to validate the proposed range model and imaging algorithm.

Performance Evaluation of a Modified SweepSAR Mode for Quad-Pol Application in SAR Systems

Efficient earth observation using spaceborne synthetic aperture radar (SAR) requires the analysis of high-resolution wide-swath (HRWS) systems, which have a fine resolution and a short revisit time through wide-area observation, and quadrature-polarimetric (quad-pol) systems, which can obtain a quad-pol image and have many applications. The operational method of SAR generally affects system performance involving the system parameters and antenna patterns, and thus analyzing HRWS quad-pol SAR systems is essential to the system design. In this paper, an extension of the modified SweepSAR technique to a quad-pol observation scenario is performed and a comparison to ScanSAR and conventional SweepSAR is provided for a simulated scenario. On the basis of the system requirements, we selected the optimal parameters and designed a reflector antenna suitable for a wide-swath quad-pol SAR system. As a result of comparing the wide-swath quad-pol SAR operation modes, the modified SweepSAR mode demonstrated advantages in terms of system performance, such as the ambiguity ratio and swath width, and reflector antenna design for a spaceborne SAR system.

An unambiguous moving target imaging method for maritime surveillance in high-resolution wide-swath mode

ABSTRACT Multichannel synthetic aperture radar (SAR) is an effective method to achieve high-resolution wide-swath (HRWS) imaging simultaneously. Many algorithms have been proposed to reconstruct stationary targets. However, moving targets cannot be well reconstructed through traditional reconstruct algorithm for static scenes and the ghost targets of the moving targets will have a great influence on SAR image interpretation, especially in maritime scenes. This paper proposed an unambitious imaging method for moving targets in maritime scenes. Firstly, the slant-range velocity of the moving target is estimated through maximizing the minimum variance distortionless response (MVDR) beamformer output in the image domain. Secondly, the phase introduced by the slant-range velocity is compensated before reconstruction. Finally, the Chirp Scaling (CS) algorithm is used to image the full scene and the moving targets can be refocused separately after image formation. The proposed method has the advantage of estimating the slant-range velocity without search or redundant channels. The performance of the proposed method is verified by simulation experiments and real data of Chinese Gaofen-3 (GF-3) SAR sensor.

Design of an end-to-end demonstration mission of a Formation-Flying Synthetic Aperture Radar (FF-SAR) based on microsatellites

The paper focuses on space system design aspects related to an end-to-end demonstration mission, aiming at showing the feasibility of a Formation Flying Synthetic Aperture Radar (FF-SAR) with microsatellite class platforms (~100 kg). Trajectory design approaches that can fulfil payload requirements are addressed to enable selected FF-SAR applications. The exploitation of these applications relies on suitable combinations of FF-SAR techniques like Signal-to-Noise Ratio (SNR) enhancement, High-Resolution Wide Swath (HRWS) SAR imaging, and Coherence Resolution Enhancement (CRE). In this framework, a cluster of 3 micro-satellites, working in X-band, flying in a Low Earth Orbit (LEO) close-formation, has been designed as a candidate end-to-end system demonstration mission. One satellite embarks a Transmitting-Receiving (Tx/Rx) radar, i.e. it is a monostatic SAR. The other two satellites are Receiving-only platforms. Critical design aspects related to spacecraft subsystems and formation-flying analysis are addressed to confirm the technical feasibility of the spaceborne distributed system implementing the FF-SAR principle.

On the Frequency Dispersion in DBF SAR and Digital Scalloped Beamforming

Digital beamforming (DBF) with Scan-On-Receive (SCORE) in elevation is a powerful technique for spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide-swath (HRWS) images. DBF in spaceborne SAR should be implemented in real-time to reduce the volume of the downloaded data. Current digital beamformers in SAR systems usually use phase shift instead of a time delay network to ensure real-time implementation. However, conventional phase shift DBF SAR with narrow array bandwidth is not applicable for broadband signals. The beam steering deviation as a function of frequency is obvious for signals with large fractional bandwidth, which appears as a phenomenon of frequency dispersion. If the signal bandwidth is wider than the array bandwidth, frequency dispersion can significantly deteriorate the signal-to-noise ratio (SNR) and must be well relieved. This article analyzes the frequency dispersion in DBF SAR and proposes an innovative DBF-SCORE scheme with specially designed scalloped beam. The proposed digital scalloped beamforming (DSBF) scheme is effective in relieving the influence of frequency dispersion in DBF SAR. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme. The proposed DSBF scheme gives practical solution to the problem of frequency dispersion in broadband DBF SAR, which makes DBF-SCORE a more practical technique for future spaceborne HRWS SAR.

Multiaperture Antenna Architecture Design for Azimuth Uniform Sampling in High-Resolution Wide-Swath SAR

Azimuth multichannel synthetic aperture radar (SAR) is an essential remote sensing device to obtain images with high resolution and wide swath (HRWS). However, using a conventional multichannel phased array antenna, azimuth uniform sampling cannot be ensured, if the operated pulse repetition frequency of the system deviates from the optimum value. A novel multichannel antenna architecture design approach in HRWS SAR is proposed for uniform sampling. Phase center positions of both transmitting aperture and receiving subapertures can be adjusted flexibly by invalidating the elements of the antenna. First, a limited number of elements of receiving subapertures can be closed to adjust receive phase center spacing. The transmit phase center could be further adjusted, if the azimuth nonuniform sampling still cannot be fully compensated for. With this multiaperture antenna architecture, azimuth uniform sampling could be ensured and system performances are obviously improved. Advantages of the proposed azimuth multiaperture antenna architecture adjust method are verified by a simulation experiment.

High-Resolution Wide-Swath IRCI-Free MIMO SAR

A frequency-domain system identification-based multiple-input multiple-output (FDSI-MIMO)-SAR algorithm using multiple phase center multiple azimuth beams is proposed to obtain high-resolution wide-swath (HRWS) imaging. Frequency-division multiple access is used in such a way that each transmitter emits a conventional linear frequency-modulated (LFM) waveform modulated by a different carrier frequency and the obtained range resolution corresponds to the total transmitted bandwidth. In this article, an MIMO-SAR problem is modeled, using the principle of displaced phase center, as multiple but separate multiple-input single-output (MISO) system identification problems. The channel impulse responses of the individual MISO problems are identified in the range dimension using an FDSI-based estimation algorithm in such a way that the estimated range profile is free of interrange cell interference. In addition, the proposed algorithm does not require separating the subband waveforms at the receiver as they are processed jointly without a need to add guard bands between the adjacent subbands, which would allow utilizing the available bandwidth to the maximum efficiency. The method of synthesizing a wide transmit antenna beam from a narrow antenna beam applied in single-input multiple-output (SIMO) SAR is extended to MIMO SAR to remove the sidelobes effects of the receive beams and hence leading to the desired signal-to-noise ratio. A pulse repetition frequency lower than the Doppler bandwidth is used to obtain wide swath without experiencing aliasing in the Doppler spectrum. Finally, both simulated and constructed raw data are used to validate the effectiveness of the proposed algorithm.

Channel Error Effect Analysis for Reconstruction Algorithm in Dual-Channel SAR Imaging

Dual-channel synthetic aperture radar (SAR) system is promising in high-resolution wide-swath (HRWS) imaging. However, gain and phase unbalance in the dual-channel SAR system will severely degrade the performance. To make clear the channel error effect on the image of the reconstruction algorithm, a precise analysis of the channel error effect is proposed in this letter. Based on the channel error effect analysis, the amplitude and position of the ambiguity caused by the channel error can be calculated, which is significant for understanding the image quality degradation caused by channel unbalance. Finally, based on the Chinese first dual-channel spaceborne SAR system—GF-3, the validity of this letter is verified by the simulated and real data, respectively.

Processing of Spaceborne Squinted Sliding Spotlight and HRWS TOPS Mode Data Using 2-D Baseband Azimuth Scaling

This article presents an efficient 2-D baseband azimuth scaling (2-D BAS) approach for the focusing of data acquired in the spaceborne squinted sliding spotlight and high-resolution wide-swath (HRWS) terrain observation by progressive scan (TOPS) imaging modes. The existing approaches can become inefficient or invalid due to the coexistence of central “absolute” and marginal “relative” squint in the above-mentioned modes. In this article, the signal properties of spaceborne squint synthetic aperture radar (SAR) with the antenna electronically steering in the azimuth dimension are analyzed first, based on which a new parity-decomposition-based range equation (PDRE) is presented to dedicatedly model the range histories of targets illuminated by the rotating beam. A novel 2-D BAS approach is then developed to remove the odd asymmetric part but preserve the even symmetric part of PDRE, which means a “true derotation” for eliminating the effect of both absolute and relative squints caused by the beam rotation. An even-order-multinomial perturbation function is applied and integrated into a range-Doppler-based SAR processing kernel to efficiently compensate the azimuth variation of high-order range cell migration (RCM) and azimuth phase modulation caused by the 2-D BAS. The proposed processor is efficient, since none of the data extension, the postprocessing for resolving focused-domain folding, or the 2-D frequency-domain interpolation at high squint is needed. Simulations with point targets in the squinted sliding spotlight and HRWS TOPS modes are used to validate the developed algorithm.

Azimuth Phase Center Adaptive Adjustment upon Reception for High-Resolution Wide-Swath Imaging.

A spaceborne azimuth multichannel synthetic aperture radar (SAR) system can effectively realize high resolution wide swath (HRWS) imaging. However, the performance of this system is restricted by its two inherent defects. Firstly, non-uniform sampling is generated if the pulse repetition frequency (PRF) deviates from the optimum value. Secondly, multichannel systems are very sensitive to channel errors, which are difficult to completely eliminate. In this paper, we propose a novel receive antenna architecture with an azimuth phase center adaptive adjustment which adjusts the phase center position of each sub-aperture to improve multichannel SAR system performance. On one hand, the optimum value of the PRF can be adaptively adjusted within a certain range by adjusting receiving phase centers to obtain uniform azimuth sampling. On the other hand, false targets introduced by residual channel errors after azimuth multichannel error compensation can be further suppressed. The effectiveness of the proposed method to compensate for non-uniform sampling and suppress false targets is verified by simulation experiments.

Study on spaceborne HRWS TOPSAR mode based on azimuth multichannel

This article addresses a high-resolution wide swath (HRWS) Terrain Observation by Progressive Scans (TOPSAR) mode which integrates the azimuth multichannel HRWS imaging technique and TOPSAR technique. More equivalent phase centres are obtained through the azimuth single-input and multiple-output technique, which would provide more azimuth samples. Hence, a more reliable and accurate imaging results of the large width terrain profile would be obtained. The performance of this HRWS TOPSAR mode is evaluated theoretically. A new processing method for this mode is provided and the results of simulation data processing validate the analysed results and the new method.

First demonstration of airborne multi‐channel InSAR processing based on measured data

Azimuth multi-channel interferometric SAR (AMC-InSAR) mode integrates the azimuth multichannel high-resolution wide swath (HRWS) imaging technique and high-precision InSAR digital elevation model (DEM) generation technique. More equivalent phase centres are obtained through the azimuth single-input and multiple-output technique, which would provide more azimuth samples. Hence, the reliable and accurate DEM measurement of more wide-swath terrain profile would be obtained. Here, the performance of this AMC-InSAR mode is evaluated theoretically. A new processing method based on measured data for this mode is provided and the results of measured data-processing validate the analysed results and the new method.

High-Resolution Wide-Swath SIMO and MIMO SAR Imaging Using Short-Term Shift Orthogonal Code

Conventional single antenna synthetic aperture radar (SAR) has a conflict between wide imaging swath width and fine cross-range resolution. Single input multiple output (SIMO) and multiple input multiple output (MIMO) radars are the two methods to overcome this limitation. In order to separate the signals of MIMO radar, the codes are expected to be orthogonal for all delays. This is impossible in SAR. Due to the special imaging geometry of SAR, short-term shift orthogonal (STSO) codes were proposed to be used in MIMO SAR. This letter compares the SIMO SAR structure and MIMO SAR structure using STSO codes. We draw a conclusion that the number of receivers of MIMO structure is less than that of SIMO structure.

Estimation and Removal of Strong Range Ambiguities in Multistatic Synthetic Aperture Radar With Multiple Elevation Beams

Bistatic and multistatic synthetic aperture radar (SAR) can greatly improve system performance in various aspects, especially in achieving high-resolution wide-swath (HRWS) images. However, the specific system structure may lead to a higher degree of range ambiguities. This letter studies the structure of multistatic multiple elevation beams (MMEB) system which achieves HRWS images with the drawback of more serious range ambiguities compared with the conventional SAR which must be suppressed to get an acceptable system performance. The character that the receivers form, a prospective distributed multichannel system in azimuth, can give a guide in estimating the strong range ambiguities and remove them. An innovative method in estimating and removing (not just smearing) the strong range ambiguities based on this character is proposed. Theoretical analysis and experimental results show the effectiveness of the method. The proposed method considerably improves the range ambiguous performance of the MMEB system without using digital beamforming or pulse coding and makes full use of the system character, and thus improves the applicability of the system.

Spaceborne Azimuth Multichannel BiSAR With Geosynchronous Transmitter and LEO Receiver: High-Resolution Wide-Swath Image Formation

Spaceborne Azimuth Multichannel BiSAR With Geosynchronous Transmitter and LEO Receiver: High-Resolution Wide-Swath Image Formation