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Due to the minimum antenna area constraints of synthetic aperture radar (SAR), high-resolution and wide-swath (HRWS) imaging is difficult to be achieved using classical modes, such as scan, spotlight, and so on. Range ambiguity is one of the main technical challenges limiting HRWS imaging in spaceborne SAR. Inspired by the cocktail party effect, an advanced scheme based on blind source separation (BSS) is put forward to suppress the range ambiguity of spaceborne SAR in this article. In the scheme, multiple subantennas are used to collect multiple echo datasets, which are given different patterns. In this way, the echo signal of the desired region and that of the ambiguous region are weighted using different coefficients. During postprocessing, a detailed flow with the BSS algorithm is proposed to separate the desired echoes from those datasets. To verify the scheme, point target and scene simulations based on GF-3 are carried out. The results indicate that the range ambiguity can be reduced by more than 15 dB using the proposed scheme, the processing flow is effective, and the scheme has a good performance without increasing the complexity of the system design under the current SAR system conditions. The scheme has the potential to be adopted in future spaceborne SAR missions. [ABSTRACT FROM AUTHOR]

Hybrid-polarimetric architecture may become a new option for spaceborne quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) mainly due to the range ambiguity improvement without loss of polarization information. To inherit the analytical methods developed for the conventional quad-pol SAR data, it is necessary to transform the hybrid quad-pol SAR data into a linear polarimetric basis. However, this operation will result in deterioration of azimuth ambiguity performance. The theoretical analysis on range and azimuth ambiguities in quad-pol SAR has been relatively complete, which is also summarized in this article. Yet, there is no real data to verify the equivalence of polarization information and to compare the ambiguity levels. Based on P-band airborne experiment, real hybrid and conventional quad-pol SAR data are used for comparative verification. In addition, considering that polarimetric target decomposition is often used in the practical applications, we suggest a new perspective from target decomposition for the ambiguities in quad-pol SAR. Identical polarimetric processing results for two sets of comparative data verify that hybrid quad-pol SAR data are indistinguishable from conventional quad-pol SAR data. The demonstrated intensity images and decomposed RGB images with azimuth ambiguities, acquired by the two quad-pol modes, completely conform to the theory. [ABSTRACT FROM AUTHOR]

Polarimetric synthetic aperture radar (SAR) has been extensively used in various remote sensing applications. In this article, a unified framework is designed to compare the classification performance of different polarimetric systems, which include quad-polarimetric (QP), compact-polarimetric (CP), and dual-polarimetric (DP). To avoid problems, such as the lack of uniform standards in feature extraction, the classification algorithm is directly based on the statistical characteristics of the coherency/covariance matrix and is implemented by extending the Wishart mixture model (WMM). The GF-3 data set in San Francisco and the AIRSAR agricultural data set in Flevoland are used in the experiment, and the following conclusions are generated. QP can achieve the highest classification accuracy in all classification tasks. When distinguishing three typical classes (water, urban, and vegetation) with very different scattering characteristics, the performance of different polarimetric systems is similar, and QP has only a slight advantage. For classification tasks of different classes with similar scattering characteristics, CP performs better in agricultural scenes, and the overall accuracy (OA) is only reduced by 3%–4% compared with QP. DP performs better in urban scenes, and OA is only reduced by 1%–3% compared with QP. These conclusions can provide guidance for future payloads’ design and the choice of polarimetric operation mode for existing multi-polarimetric SAR systems to achieve the purpose of giving full play to the advantages of different polarimetric systems. [ABSTRACT FROM AUTHOR]

This article deals with the synthesis and analysis of quasi-orthogonal nonlinear frequency modulation (NLFM) waveforms to mitigate the impairments of ambiguous returns in quadrature-polarimetric (quad-pol) synthetic aperture radars (SARs). To this end, focusing on signals with a continuous piecewise linear instantaneous frequency, the design of a waveform pair exhibiting both a low cross correlation energy (CCE) and low peak to sidelobe ratios (PSLRs), is considered. To handle the resulting nondeterministic polynomial (NP) hard problem, a coordinate descent (CD) method is employed, where, at each step, the marginal minimization is tackled via a MATLAB optimization toolbox. Hence, transmission/reception schemes jointly capitalizing quasi-orthogonal NLFM waveforms and azimuth phase coding (APC) techniques are proposed to suppress ambiguity interference. Moreover, a systematic framework for the evaluation of the resulting azimuth ambiguity-to-signal ratio (AASR) and range ambiguity-to-signal ratio (RASR) is provided. Finally, detailed simulation experiments based on the LuTan (LT-1) parameters are carried out to verify the practicability and effectiveness of the newly proposed transceiver schemes. [ABSTRACT FROM AUTHOR]

The bistatic synthetic aperture radar (BiSAR) plays an important role in remote sensing. However, the deviation between the two oscillators in BiSAR systems will cause a residual modulation of the echo signal. Therefore, the phase synchronization is an important issue that must be addressed in the BiSAR system. An advanced noninterrupted phase synchronization scheme is used for LuTan-1. The synchronization pulses are exchanged immediately after the ending time of the radar echo receiving window and before the starting time of the next pulse repetition interval, which will not interrupt the normal SAR operation. In order to evaluate the accuracy of the phase synchronization scheme, the model of phase synchronization is introduced at first. The hardware design and processing flow of LuTan-1 are introduced in detail. An innovative internal calibration strategy is also described. Then, the test data acquired by the ground validation system are analyzed to verify the effectiveness of the phase synchronization scheme. The signal-to-noise ratio (SNR) and the synchronization rate are the two most important factors to influence the accuracy in phase synchronization. The conclusions have guiding significance for the synchronization module design of LuTan-1 and the future BiSAR system. [ABSTRACT FROM AUTHOR]

More and more synthetic aperture radar (SAR) satellites in orbit provide abundant data for remote sensing applications. In August 2016, China launched a new Earth observation SAR satellite, Gaofen-3 (GF-3). In this paper, we utilize a small stack of GF-3 differential interferograms to map land subsidence in Beijing (China) using the time-series SAR interferometry (InSAR) technique. The small stack of differential interferograms is generated with 5 GF-3 SAR images from March 2017 to January 2018. Orbit errors are carefully addressed and removed during differential InSAR (DInSAR) processing. Truncated singular-value decomposition (TSVD) is applied to strengthen the robustness of deformation rate estimation. To validate the results of GF-3 data, an additional deformation measurement using 26 Sentinel-1B images from March 2017 to February 2018 is carried out using the persistent scatterer interferometry (PSI) technique. By implementing a cross-comparison, we find that the retrieved results from GF-3 images and Sentinel-1 images are spatially consistent. The standard deviation of vertical deformation rate differences between two data stacks is 11.24 mm/y in the study area. The results shown in this paper demonstrate the reasonable potential of GF-3 SAR images to monitor land subsidence. [ABSTRACT FROM AUTHOR]

This letter first attempts to simultaneously reduce range ambiguities and sidelobes in spaceborne synthetic aperture radar (SAR). Two of the limitations of SAR are the range ambiguity phenomenon and the sidelobes effect. The range ambiguity appears with long delayed echoes. The current transmitted signal does not contribute to the reflected echo that is received after the transmission of one or more new pulses. Moreover, the conventional radar commonly uses linear frequency modulated (LFM) waveforms, producing a peak sidelobe ratio (PSLR) of −13.25 dB. Unfortunately, the sidelobes in SAR imagery contribute to speckle and interfere with nearby scatterers. This letter investigates the opportunity of designing the transmitted signals using orthogonal non-linear frequency modulated (ONLFM) waveform. The results demonstrate that ONLFM signals outperform conventional LFM signal. Using the proposed ONLFM waveforms, the PSLR is reduced to −50.25 dB, meanwhile the range ambiguities can be greatly suppressed, despite broadening the mainlobe. [ABSTRACT FROM AUTHOR]

Synthetic aperture radar (SAR) imagery classification is considered as one of the most significant SAR-based application. Developing SAR imagery classification applications based on new datasets requires considerable amount of work, such as feature extraction and validation. The classification processing flow is complex and should be specially designed for different SAR images. To reduce the complexity and improve the processing efficiency, a unified processing scheme based on the deep neural networks (DNN) is proposed. The scheme can be applied to most SAR imagery classification tasks by simply adjusting the model parameters. The proposed scheme is employed to extract building areas from different high resolution SAR images obtained by different sensors based on fully connected feedforward deep network (FDN) and convolutional neural network (CNN). The study results indicate that the proposed classification scheme has high accuracy and efficiency. [ABSTRACT FROM AUTHOR]

Multichannel in azimuth synthetic aperture radar (SAR) operating in the terrain observation by progressive scans (TOPS) acquisition mode has attracted much attention recently for its capability to achieve ultrawide-swath imaging with a high spatial resolution. In order to verify the feasibility and operability of this newly developed remote sensing concept, a C-band airborne azimuth dual-channel TOPS SAR has been designed by the Institute of Electronics, Chinese Academy of Sciences, as a test bed for future spaceborne realizations. This paper introduces the experimental SAR system and reports the data processing results of an outfield experiment conducted in late September 2014. The importance of the experiment resides in its potential to validate several important technical aspects of this novel SAR operation with real experimental data, including channel mismatch cancellation and unambiguous signal reconstruction. Besides, two kinds of processing methods are proposed to calibrate the influence of antenna phase center fluctuation occurred in the dual-channel TOPS SAR. Finally, the experimental results obtained, including the phase mismatch cancellation and the focused imageries, are presented and analyzed. [ABSTRACT FROM PUBLISHER]

This letter presents a methodology for urban area mapping with density-based spatial clustering of applications with noise (DBSCAN) using the Advanced Synthetic Aperture Radar (ASAR), Sentinel-1A, and HuanJing-1C data. Urban areas have a diversity of shapes, including circles, squares, strips, and other irregular shapes, and the DBSCAN clustering algorithm is suitable for identifying clusters of arbitrary shapes. Exploiting DBSCAN to extract urban areas is a key aspect of this method, and improvements via the incorporation of synthetic aperture radar data preprocessing and postprocessing also play important roles in optimizing the extractions. Different test site sizes were chosen to demonstrate the effectiveness and feasibility of the proposed method, and the validation results showed that the method is efficient and accurately extracts urban areas ranging from small towns to super metropolitan areas. [ABSTRACT FROM PUBLISHER]

In this paper, a novel Doppler rate (DR) estimation-based two-dimensional (2-D) subaperture motion compensation (MOCO) approach is proposed for high-resolution scanning synthetic aperture radar (ScanSAR) mode. It can estimate space-variant phase errors in different subswaths simultaneously by exploiting data of multiple subswaths. High-resolution ScanSAR images, produced by full-aperture processing algorithm, are often defocused by uncompensated phase errors. As a result of the periodic gapped data in each subswath in ScanSAR mode, the performance of the commonly used stripmap MOCO approaches will be degraded or even not work when using full-aperture processing algorithm. The effectiveness of the proposed approach is demonstrated by an airborne ScanSAR real data set containing different types of terrain, with a high geometric resolution of about 3.5 m (mid or far range) or 5 m (near range) in azimuth working at C band. [ABSTRACT FROM PUBLISHER]

In this letter, we propose an approach for multichannel spaceborne/stationary synthetic aperture radar (SAR) interferometry based on maximum a posteriori (MAP). Spaceborne/stationary SAR is a typical bistatic SAR configuration. In order to solve the phase disconnection problem while working with a low signal-to-noise ratio and a limited number of baselines as well as having large look angle variations, we use the height estimation results derived from iterative multibaseline unwrapping as the initial heights for the MAP estimation. The method presented here is highly experimental. An experiment is carried out to verify the effectiveness of the proposed approach. In this experiment, TerraSAR-X, working in the high-resolution spotlight mode with a 300-MHz bandwidth, acts as the transmitter. The receivers with three echo channels are placed on the ground to receive the reflected waveform. As proof of concept, we demonstrate the height estimation of several buildings. [ABSTRACT FROM PUBLISHER]

A fully polarimetric response of landslide areas at X-band was studied by a Chinese high-resolution airborne synthetic aperture radar system. Polarimetric decompositions, including the Yamaguchi four-component decomposition and the Cloude decomposition, are used to analyze the scattering mechanisms of several typical landslides caused by the 2008 Wenchuan Earthquake in southwestern China. The experimental results indicate that areas affected by large-scale landslides show complicated scattering mechanisms at X-band, which are a mixture of surface, double bounce, and volume scattering. Simple classification results based on supervised Wishart classifier and polarimetric scattering similarity parameters are also presented, which can distinguish landslide areas from others, such as forest and water, very well. However, it does not perform well for urban areas. Additional information, such as prelandslide imagery, is needed to distinguish landslide areas from urban areas or bare soil. From these results, we can conclude that landslide mapping using fully polarimetric data has great potential for rapid response and management of landslide disasters. [ABSTRACT FROM PUBLISHER]

Interferometric inverse synthetic aperture radar (InISAR) can achieve 3-D imageries of maneuvering target. However, the quality of 3-D imageries suffers from the noise and sidelobes seriously. In addition, the defocusing effect due to the nonlinear moving of the target can also bring into a bad effect for 3-D imaging. To tackle these issues, a 3-D InISAR imaging approach via compressed sensing (CS)-based super-resolution (SR) ISAR imageries is proposed. First, we establish the target sparsity-constraint optimization model containing both amplitude and phase information of scatterers. Second, a Bayesian CS approach is adopted to get high-quality SR ISAR image. The quasi-Newton solver guarantees both high amplitude and phase information recovery precision. At last, a high-quality 3-D view of the target is achieved via the conventional ISAR imagery pair interferometry technique. Computer simulation and real data experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

Both in quality-guide phase unwrapping algorithms and weighted minimum-norm phase unwrapping algorithms, quality maps play a crucial role in obtaining the absolute phase from the wrapped ones. In this letter, a new technique for generating quality maps based on the gray level co-occurrence matrix (GLCM) is proposed. GLCM is a classical second-order statistics method for analyzing the texture features of images. Through exploring the second-order statistics of GLCM, much useful information in the image can be exploited. According to the characteristics of the interferogram, the second-order statistic of GLCM called “difference of entropy” is used to generate the quality maps. Besides, we modified the definition of “difference of entropy” to make the statistic more suitable for the problem. Finally, the new algorithm is compared with other conventional algorithms both in the simulated and real data experiments and the results show its better performance. [ABSTRACT FROM PUBLISHER]

To simultaneously achieve two-dimensional high resolution and wide swath in synthetic aperture radar (SAR), azimuth MIMO structure combined with stepped-frequency chirp signals was developed via splitting the antenna into N subapertures. During transmitting each subaperture transmits a chirp pulse at a different carrier frequency, while during receiving every subaperture receives the N scattered pulses at the same time. Separating the N scattered pulses received by each subaperture and downlinking them to the ground yield N² different signal paths. Due to the dedicated network in the SAR system, the channel imbalance is inevitable. To correct the channel imbalance, this paper presents an external calibration method, where the channel characteristics are estimated from the peak value of a strong point target for each channel. Simulation and real raw data experiments are performed to validate the proposed method. [ABSTRACT FROM AUTHOR]

Interferometric synthetic aperture radar (InSAR) multichannel system, which produces more than one interferogram in a multifrequency or multibaseline configuration, allows us to reconstruct highly sloped and discontinuous terrain height profiles. In this letter, a novel method based on a closed-form robust Chinese remainder theorem (CRT) is presented to solve the height reconstruction problem. Proper reference remainder selection and remainder differential process, which do not exist in the traditional CRT, are adopted to improve the robustness of the height reconstruction. We also modify the method by exploiting statistical characteristics of interferometric phase and the combined information of InSAR interferograms. Moreover, a special weighted mean filtering method is adopted to get a better result. Experimental results prove the effectiveness of the method. [ABSTRACT FROM PUBLISHER]

This letter presents a motion compensation solution for high-resolution automobile frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR). The motion error model is proposed, and the characteristics of the motion error in automobile FMCW SAR are analyzed by comparing with that of airborne SAR. Furthermore, an effective compensation approach is presented, which can handle motion error while keeping the data well focused in the case of high resolution. Finally, we validate the formulated motion error model and compensation method by using simulated FMCW SAR data as well as the real data that were acquired by the Institute of Electronics, Chinese Academy of Sciences. The results show that the proposed method can work well for automobile FMCW SAR. [ABSTRACT FROM PUBLISHER]

In order to enhance the target recognition probability in the inverse synthetic aperture radar (ISAR) imaging domain, the interferometric ISAR (InISAR) mode is presented to achieve the 3-D information of a target. However, the real data results of a maneuvering target are not enough, due to the difficult signal processing. In this letter, a combined processing approach is proposed to realize high-quality 3-D imagery of a maneuvering target. In the approach, the range alignment and phase adjustment are implemented together on echoes to avoid destroying the coherence of the echoes. Then, the high-quality 3-D InISAR imagery of the maneuvering target can be reached. Real data results are provided to confirm the effectiveness of the proposal. [ABSTRACT FROM PUBLISHER]

This paper presents a double-channel hybrid bistatic synthetic aperture radar (SAR) system. It can be implemented with the available illuminator (e.g., spaceborne and airborne SAR systems) to acquire the 2-D and 3-D microwave images for remote-sensing applications. This system can be used as a test system for the validation of complex bistatic acquisitions, novel synchronization algorithms, and advanced imaging techniques. In this paper, we will demonstrate the time and phase synchronization strategies, fast time-domain imaging algorithm, 2-D bistatic SAR, and 3-D bistatic stereo radargrammetry in SAR images. Based on the proposed bistatic system, the acquired bistatic image has a much better sensitivity than its monostatic counterpart, which will facilitate the detection and recognition of targets based on their characteristic and bistatic scattering behaviors. Finally, the experiment results highlight the differences between monotatic and bistatic SAR images. [ABSTRACT FROM AUTHOR]

The bistatic configuration of frequency-modulated continuous wave (FMCW) synthetic aperture radar (SAR) offers considerable capabilities, reliability and flexibility in designing the compact size and low cost remote sensing systems. Furthermore, the spacial separation between transmitter and receiver avoids the antenna isolation problem. A signal model is proposed here to accurately formulate the bistatic slant range history in FMCW SAR configuration, where the motion during the long pulse duration is taken into account. Moreover, based on the preceding model, an optimization approach is presented to deal with the double square root (DSR) term, and thus an accurate FMCW bistatic point target reference spectrum (BPTRS) is derived. The proposed spectrum can not only address the signal characteristics of bistatic FMCW SAR, but also significantly simplify the bistatic FMCW SAR processing, which will really push the applications of FMCW SAR in the remote sensing field. Furthermore, based on the proposed signal model, the application of an improved range-Doppler algorithm (RDA) is further proposed to focus the bistatic FMCW SAR data. Simulation experiments are carried out to verify the accuracy of this novel formulation and highlight the performance of the proposed focusing approach. [ABSTRACT FROM AUTHOR]

Future spaceborne SAR missions will require complete and frequent coverage of the Earth with a reasonably high geometric resolution. To achieve this imaging capacity, multichannel SAR systems with a planar antenna will be widely adopted in the next generation spaceborne microwave remote sensing missions. However, the high data rate, the increased system complexity, the difficulty of multichannel signal processing, the heavy load, and the wide bandwidth downlink may hamper the demonstration of these imaging modes. With the rapid development of electronic engineering and digital signal processing, these problems will be well resolved. Furthermore, regarding the implementation complexity, multichannel SAR systems with a deployable reflector antenna seem to be a better alternative for HRWS imaging [17, 18]. However, the major drawback of the reflector antenna is the limited beam-steering capacity compared with a planar antenna, which will affect some of the remote sensing applications. Thus, multichannel SAR systems with a planar antenna rather than a reflector are more suitable for the higher geometric resolution, especially for the higher-carrier frequency (such as X-band). [ABSTRACT FROM PUBLISHER]

Frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) systems offer smaller size and lower cost compared with pulsed-mode systems. They are therefore widely used for Earth observation where frequent revisits at low costs or small sizes are desirable. By accurately formulating the instantaneous slant range during the transmitting and receiving operations, an analytical point target reference spectrum has been developed for the FMCW SAR system, where an additional range–azimuth coupling term is found. Based on previous work, this letter presents a modified inverse chirp-Z transform (ICZT) algorithm to deal with the range–azimuth coupling term, which appears to be a scaling factor in range. The new scaling factor is formulated in this letter for the first time. An ICZT is well suited for FMCW SAR since it can handle a dechirped signal without interpolations, thus reducing the computing load. Simulation experiment and real data processing result validate the proposed focusing algorithm. [ABSTRACT FROM PUBLISHER]

For synthetic aperture radar imaging, stepped-frequency chirp signals are widely used to obtain high range resolution. One advantage of the approach is the reduction of the instantaneous bandwidth and sampling rate requirements of the radar system. To reconstruct the spectrum of a wideband signal, the amplitude and phase discontinuity between contiguous pulses should be removed. Otherwise, the discontinuity will defocus the range profile and degrade the range resolution. In this letter, a detailed internal calibration technique combined with stepped-frequency mode is proposed to eliminate the discontinuity and thus improve the focusing quality of the final image. The simulation and real raw data experiments are performed to validate the proposed method. [ABSTRACT FROM PUBLISHER]

The combination of frequency-modulation continuous-wave (FMCW) technology and synthetic aperture radar (SAR) promises a lightweight, cost-effective, and high-quality imaging sensor for remote sensing. However, the long signal duration time leads to the failure of the conventional start/stop approximation of the pulsed SAR. In this paper, a signal model is proposed to address the effects of the continuous motion during the transmit time on the echoed signal. Based on the model, an analytical point target reference spectrum is derived. From the spectrum, it will be seen that the continuous motion introduces an additional range-azimuth coupling term and a range walk term compared with the conventional pulsed SAR. The range walk term is well known, whereas the foregoing range-azimuth coupling term is formulated for the first time in the FMCW SAR community. For the squint and spotlight modes, these range walk and range-azimuth coupling terms might significantly degrade the image quality. In this paper, based on the proposed analytical signal model, we further discuss the application of the wavenumber domain algorithm for the FMCW SAR data. In addition, different approximations of the Stolt mapping are made to highlight the effect of the range-dependent higher-order range-azimuth coupling terms on the 2-D impulse responses. Finally, X-band simulated experiments and Ka-band real FMCW SAR data are used to validate the signal model and the processing method. [ABSTRACT FROM AUTHOR]

This paper discusses the processing method for bistatic SAR data in the constant-offset configuration. The constant-offset configuration is also known as the azimuth stationary or invariant configuration where transmitter and receiver follow each other, moving on identical velocity vector. In this paper, the proposed processing method for bistatic SAR data is based on Loffeld's bistatic formula that consists of two terms, i.e., the quasi-monostatic (QM) term and bistatic-deformation (BD) term. Our basic idea is to linearize the aforementioned two terms and then incorporate the BD term into the QM term to obtain an analogous monostatic spectrum. Based on the new spectrum, any efficient 2-D frequency or range-Doppler domain processor can easily be employed to process the bistatic data, where the Doppler phase parameters of the processor need to be adjusted. In this paper, we concentrate on the application of chirp-scaling-algorithm (CSA) processor. In addition, a bistatic-motion error model is developed where the position deviations of the two platforms are simplified as the bistatic slant-range displacement in the zero Doppler plane. Using this model, the monostatic motion-compensation technique is applied and integrated into CSA to compensate the trajectory deviations of transmitter and receiver. Finally, real and simulated data are used to validate the proposed processing method. [ABSTRACT FROM AUTHOR]

A correction to the article "Error Analysis of Bistatic SAR Imaging and Stereoscopy Bistatic SAR" that was published in the August 2013 issue is presented.

Sampling plays a critical role in remote sensing and signal analysis. In conventional sampling theory, the signal is sampled at a uniform rate at a minimum of twice the signal bandwidth. However, in many multichannel systems such as analog-to-digital converters, synthetic aperture radar (SAR), and synthetic aperture sonar (SAS), it requires that multidimensional signals or digital images be reconstructed from their recurrent samples, and the signals may not be bandlimited in the traditional Fourier domain. In this paper, a reconstruction algorithm for two-dimensional (2D) recurrently sampled signals is proposed in the fractional Fourier domain. This algorithm can handle the situations where the signal is nonbandlimited, and it is extended to the case of undersampling, where the traditional reconstruction algorithms might fail. The algorithm is based on the nonuniform fractional spectrum, and a method to speed up the computation of the nonuniform fractional spectrum is introduced. Reconstruction from recurrent samples in the case of undersampling is illustrated using numerical examples, and an application to multichannel SAR imaging is included to illustrate these results. [ABSTRACT FROM AUTHOR]