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One of the challenges of the low-frequency spaceborne synthetic aperture radar (SAR) is that propagation through the ionosphere will introduce nonnegligible errors in the final SAR product. In the low-frequency bistatic SAR (BiSAR) system, the ionosphere will degrade the imaging performance and cause nonnegligible phase errors in the single-pass SAR interferometry application, which results in undesired errors of digital elevation model (DEM). In this article, a method that embedded into the focusing procedure is proposed, which aims to estimate and remove ionospheric effects on L-band spaceborne BiSAR system. First, the impacts of ionospheric effects on the BiSAR system are demonstrated, including the deterioration of imaging performance and the geometric distortion. Then, a method is proposed to correct ionospheric effects. Afterward, the simulations, including point targets and distributed targets, are carried out to verify the effectiveness of the proposed method. The imaging results and the DEM reconstruction results show that the proposed method can effectively estimate and remove ionospheric effects on spaceborne BiSAR systems. [ABSTRACT FROM AUTHOR]

Multichannel SAR system operating in the sliding spotlight mode has the capacity of imaging with very high resolution as requested by future spaceborne synthetic aperture radar (SAR) mission; hence, it has been attracted more and more attention in the SAR community. In a multichannel sliding spotlight SAR system with $N$ channels, the overall Doppler bandwidth of the data acquired by each channel is greater than $N \cdot \text {PRF}$ ; thus, the preprocessing algorithms (channel mismatch calibration and signal reconstruction) are no more available. To address this issue, a full-aperture imaging method for azimuth multichannel sliding spotlight mode is proposed in this article. First, the raw data of each channel need to be preprocessed based on a deramping operation, which can obtain an unambiguous Doppler spectrum. Afterward, the algorithm proposed for single-channel sliding spotlight mode can be performed. Finally, to avoid possible back-folded SAR images, postprocessing after focusing is demanded in some cases. The imaging results of the data acquired by GaoFen-3 (GF-3) are given for the first time in this article, which proves the effectiveness of the proposed imaging method and demonstrates the capacity of high-resolution wide-swath imaging of the multichannel sliding spotlight mode. [ABSTRACT FROM AUTHOR]

In recent years, the multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) concept has been widely researched because it can provide more degrees of freedom to dramatically improve SAR system performance. However, the echo separation issue of different transmit antennas is the most challenging issue and it stirs up extensive discussions. In this letter, a novel MIMO-SAR imaging scheme based on the improved orthogonal frequency-division multiplexing (OFDM) waveforms is proposed. The main contributions of this work are that: 1)Improved generation method for $M$ OFDM waveforms is presented. This method can compensate the extra carrier frequency deviation and compared with the other compensation methods, this method is more general and suitable for $M$ OFDM waveforms. 2)Based on the proposed OFDM waveforms, a novel and low-cost spaceborne MIMO-SAR imaging scheme is proposed. In this scheme, a simple time-shift weighting process and a bandpass filter bank are employed to separate the echoes from the close arrival angles. Then, the digital beamforming (DBF) on receive in elevation is employed to separate the echoes from far arrival angles. Furthermore, the distributed scene simulation results are presented to verify the practicability of the proposed scheme. [ABSTRACT FROM AUTHOR]

The echo separation issue associated with different transmit antennas is the most technical challenge in realizing the multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) system. In this article, a novel MIMO-SAR imaging scheme based on an advanced radar waveform, namely, segmented-phase-code (SPC) waveform, is proposed. Compared with the state-of-the-art short-term shift-orthogonal (STSO) waveform beamforming schemes, this scheme relieves the short-term shift-orthogonality condition of transmitted waveforms without losing the imaging performance, which extends the optional waveform space. In this scheme, the separation of the echoes from close arrival angles is ensured by a simple time-shift weighting processing. Furthermore, a range bandpass filter bank and the digital beamforming (DBF) technique are employed to ensure that the echoes from far arrival angles are separable. Finally, detailed simulation experiments are performed to verify the feasibility of the proposed scheme, and in-depth discussions of different waveforms and MIMO-SAR imaging schemes are presented. [ABSTRACT FROM AUTHOR]

The minimum antenna principle gives rise to the extremely complex engineering implementation and data processing of spaceborne high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR). To address this problem, a multiswath jump SAR with sound range ambiguity performance is proposed in this letter, which continuously switches multiple radar signals with nonoverlapping spectrum to radiate different areas during the transmitting time. To further obtain high-resolution images, the echo signals are processed by the methods of range spectrum splicing and azimuth multichannel reconstruction. Besides, some simulation experiments are executed to verify the capability of HRWS imaging of multiswath jump SAR and the theoretical analysis. [ABSTRACT FROM AUTHOR]

The challenging problem to realize high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) imaging is the ambiguity suppression in the azimuth and range directions. According to the spatial angle difference of each ambiguity component, the current technical approach is to design the spatial filter for achieving the ambiguity suppression based on the 2-D multichannel system. Along with the increasing of HRWS imaging requirements, the number of system channels also gradually increase and further result in the complex structure design of the phased array antenna system. Meanwhile, the traditional phased array antenna cannot effectively control the direction of the transmit beampattern in range. Unlike the traditional phased array, frequency diverse array (FDA) employs a small-frequency increment across the whole array elements and forms the range-angle-dependent S-shaped transmit beampattern, which can be utilized to separate the different range ambiguous region. Considering the above-mentioned characteristics and the range periodicity problem of transmit beampattern, this letter devises a scheme for spaceborne SAR HRWS imaging mode in the view of transmit beampattern utilizing 2-D planar array, i.e., the FDA in azimuth for removing the range nonperiodicity ambiguity and the conventional phased array in elevation for removing the range periodicity ambiguity. Simulation results have been presented to validate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

For synthetic aperture radars (SARs), an alternate transmitting mode based on waveform diversity is widely discussed for suppressing range-ambiguity in many letters, because it is easy to implement and there is no need to improve the pulse repetition frequency (PRF). These studies mainly focus on the discussion of pseudo-orthogonal waveform design, such as up–down chirp waveforms and orthogonal-frequency-division-multiplexing (OFDM) waveforms; however, the effect on imaging caused by waveform diversity is ignored. This letter, for the first time, provides a demonstrative derivation of imaging for the alternate transmitting mode, which will deepen the understanding of this mode and be helpful for the future research. In this letter, we point out that transmitting different waveforms will introduce a phase-amplitude periodic modulation in the azimuth domain; furthermore, it will cause the aliasing of the azimuth spectrum. In addition, the simulation experiment is performed for verifying the correctness of the theoretical analysis. [ABSTRACT FROM AUTHOR]

LuTan-1 [(LT-1), i.e., TwinSAR-L] mission is an innovative spaceborne bistatic synthetic-aperture radar (BiSAR) mission focusing mainly on differential interferometry, which will be launched in 2020. This article introduces some important aspects of the LT-1 mission for the first time, including the formation configuration, imaging mode, application scenarios, and the efficient baselines between the master satellite and the slave satellite. To realize the high accurate topography and deformation measurements, a wide swath BiSAR focusing algorithm with phase reserving ability should be developed. This article proposes a modified bistatic range-Doppler algorithm based on 2-D principle of stationary phase spectrum, which reduces the phase error introduced by the root term expansion and has an excellent focus performance and a good phase reserving ability. Finally, the spaceborne bistatic simulation experiments using orbital parameters and imaging mode of the LT-1 mission, including point targets and scene targets, are utilized to illustrate the validity and accuracy of the proposed algorithm. [ABSTRACT FROM AUTHOR]

The next generation of spaceborne synthetic aperture radar (SAR) remote sensing systems will emphasize on high-resolution and wide-coverage imaging. For these design goals, variable pulse repetition interval (PRI), which is also called staggered SAR, is a promising candidate to deal with the inherent problem of blind range effect in conventional SAR with constant PRI. This letter presents an improved work of signal processing to cope with two unavoidable problems in staggered mode: the loss of echo signal and nonuniform sampling in azimuth. Since the existing signal processing methods are mostly based on a high oversampling ratio, which would increase the range ambiguity to signal ratio and the amount of data to be downlinked, this letter focuses on signal processing with low oversampling factors. A two-step processing scheme—recovery and reconstruction—is proposed to recover the missing sampled data using the spectral-estimation techniques first and then reconstruct the in-band energy and minimize the ambiguity energy, which is from band unlimited SAR raw data, to obtain uniform sampling in azimuth. Point target simulations and artificially gapped data generated from China’s S-band spaceborne SAR system HJ-1-C are shown to validate the proposed scheme. [ABSTRACT FROM PUBLISHER]

Modified full-aperture imaging algorithm for sliding Mosaic mode synthetic aperture radar (SAR) is presented in this paper, which includes scalloping correction and spikes suppression. The full-aperture imaging algorithm is introduced into Mosaic mode and validated by real C-band airborne SAR imaging experiments. The main idea is to substitute zeros between bursts with linear-predicted data extrapolated from adjacent bursts to suppress the spikes caused by multibursts processing. Furthermore, scalloping correction for sliding Mosaic mode is integrated with this algorithm. It is innovational to correct the azimuth beam pattern weighting altered by radar antenna rotation in azimuth with deramping preprocessing operation. Finally, experiments performed by the C-band airborne SAR system with a maximum bandwidth of 200 MHz validate the effectiveness of the approach. [ABSTRACT FROM PUBLISHER]

Due to the light weight and small size, synthetic aperture radar (SAR) sensors with light-small aircraft are very sensitive to atmospheric turbulences, which cause serious trajectory deviations. Limited by the cost or payloads, light-small aircraft sometimes cannot carry high-accuracy inertial navigation systems (INS)/global positioning systems (GPS). For the very high resolution (VHR) case, residual range cell migration (RCM) often exceeds several range resolution cells during the course of the synthetic aperture, thus degrading the image quality substantially. In this paper, a robust scheme for the correction of residual RCM is presented. The scheme consists of three principle steps. First, the range-compressed data in the signal domain are divided into subapertures in azimuth. For each subaperture, a sliding window is used to select the subblock data with the highest signal-to-clutter ratio. Second, for each subaperture, residual RCM is estimated by using the selected subblock data based on a nonparametric entropy minimization technique. Third, the estimated residual RCM of all subapertures are filtered and coherently combined to perform the correction for the full-aperture data. Processing the results of VHR SAR raw data shows that the proposed scheme is effective for highly precise imaging with light-small aircraft equipped with only low-accuracy INS/GPS. [ABSTRACT FROM PUBLISHER]

This paper addresses some important aspects for the spaceborne/stationary bistatic synthetic aperture radar (SAR) (SS-BiSAR) imaging with the transmitter, TerraSAR-X, operated in staring spotlight (ST) mode. With the large integration time reaching 7.5 s and the azimuth steering span reaching \pm 2.2^0, several significant effects occur, including troposphere delay, precision phase and time synchronization, the curved orbit effect, azimuth spectrum aliasing problem, and efficient frequency domain focusing algorithm. To circumvent the main effects, corresponding solutions are proposed, including a precise synchronization strategy with troposphere delay correction based on the direct signal from the transmitter and a modified and integrative bistatic polar format algorithm (PFA). This paper covers the theoretical development, implementation, and analysis of the SS-BiSAR PFA based on 2-D fast Gaussian gridding nonuniform fast Fourier transform with wavefront curvature correction. Furthermore, the high-resolution ST-mode SS-BiSAR image processed by the proposed algorithm is acquired, and the differences of scattering behaviors between monostatic and bistatic SAR images are analyzed in detail. [ABSTRACT FROM AUTHOR]

This paper proposes a synchronization algorithm for bistatic synthetic aperture radar (BiSAR) imaging in a spaceborne/stationary configuration. In real bistatic systems, synchronization errors are generally introduced into the received data. Additionally, the lack of precise imaging parameters, such as the position of the transmitter and the accurate sampling time, could affect the imaging quality greatly. Fortunately, the image could be well focused by the proposed algorithm in the case of lack of the accurate position of a transmitter and the sampling time. First, a preprocessing step is employed to remove synchronization errors through matching an echo signal with a direct signal. Then, a modified chirp scaling factor containing an error phase term is constructed, and the accurate position of the transmitter and the sampling time can be acquired by the phase extraction of the direct signal and the searching method based on contrast optimization. After that, the corresponding imaging process can be implemented. Finally, the proposed algorithm is validated by the simulation and experimental results, where TerraSAR-X is used as the illuminator. [ABSTRACT FROM PUBLISHER]

The next generation of spaceborne synthetic aperture radar (SAR) remote sensing systems will emphasize on high-resolution and wide-coverage imaging. For these design goals, digital beamforming (DBF) in elevation is a promising candidate. DBF-SAR can provide global monitoring capacity for the continuous observation of a highly dynamic and rapidly changing world with high spatial resolution and short repeat intervals. A spaceborne experiment regarding a real complex scene and real spaceborne wave propagation channel effects remains a necessary step to complete the experimental verification of this advanced technique. Fortunately, the spaceborne–stationary bistatic configuration offers a potential chance to validate the advanced technique. The aforementioned experiment can be considered as a test bed for the development and implementation of DBF radar techniques applicable to Earth observation science and planetary measurements. The DBF experiment based on spaceborne–stationary bistatic configuration with TerraSAR-X as an illuminator has been successfully conducted in June 2013 by the Department of Space Microwave Remote Sensing System, Institute of Electronics, Chinese Academy of Sciences. [ABSTRACT FROM PUBLISHER]

A promising candidate for ultra–wide-swath spaceborne synthetic aperture radar (SAR) imaging with a fine resolution is the multiple-elevation-beam (MEB) SAR. Conventional MEB SAR suffers from deterioration of noise-equivalent sigma zero (NESZ) and range ambiguity–to–signal ratio (RASR) performances in the far end of the imaging swath and disturbances between different subbeam elevations. Another inherent problem of previously proposed MEB SAR systems is the blind range effect, which blocks continuous mapping. In this paper, we introduce a potential modified MEB SAR system based on space-time-frequency waveform encoding on transmit and on digital beamforming (DBF) on receive that provides a much improved performance of NESZ and RASR, a lower requirement for the transmit peak power, and a more flexible signal energy distribution. Furthermore, to overcome the blind range problem, a periodic sweep of pulse repetition interval (PRI) design strategy is employed. In this paper we are developing the idea of sweep-PRI mode into a complete detailed processing scheme. The explicit signal model, the instructive system design guideline, and the corresponding signal processing scheme for this operational mode are described in detail. A recurrent nonuniform reconstruction algorithm is adopted first to cope with the problems of nonuniform sampling and periodic gaps in the azimuth preprocessing. Additionally, an in-depth analysis of system performance for the presented MEB SAR system is carried out. Finally, an example system with a 400-km-wide swath and 3-m resolution is designed based on the proposed strategies. The simulation results demonstrate that the presented SAR system shows satisfactory performance. [ABSTRACT FROM AUTHOR]

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]

This letter introduces the innovative multichannel sliding spotlight synthetic aperture radar (SAR) system for the acquisition of ultrahigh resolution radar images with wide swath coverage. The instructive system design guideline and the signal processing scheme for the proposed operational mode are exhibited. When deriving the azimuth reconstruction algorithm for the multichannel sliding spotlight mode, a novel squinted geometric model is employed, in which the variable antenna steering direction is taken into account. In addition, an in-depth quantitative analysis of system performance for the presented mode is carried on. Finally, a typical exemplary spaceborne SAR is designed based on the presented strategy, from which numerical simulation results are obtained to justify our derivations. [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]

In this letter, we suggest using multiple azimuth channels (MACs) for synthetic aperture radar in the sliding spotlight mode. First, we analyze the scene size constraints of conventional sliding spotlight mode, showing that a large scene size requires long scene illumination time. However, while using multiple channels in azimuth, this could be changed. Dividing a long antenna into small antennas, we can increase the azimuth resolution improvement fact, where the range size will not be reduced. Therefore, with the same illumination time, the azimuth imaging width can be enlarged for a given azimuth resolution. However, using the MAC sliding spotlight mode, the nonuniform sampling problem occurs. Unlike the processing in the MAC stripmap mode, in the MAC sliding spotlight mode, the unambigous reconstruction should be performed after the deramping in azimuth. Finally, an example is given to validate the proposed new configuration and the corresponding algorithms. [ABSTRACT FROM PUBLISHER]

A synthetic aperture radar (SAR) raw data simulator of distributed target scenes is an important tool for testing the system parameters, imaging algorithms, and so on, particularly for these recently proposed novel imaging modes. The Terrain Observation by Progressive Scans (TOPS) mode achieves the wide-swath imaging coverage and overcomes major drawbacks in ScanSAR by electronically steering the azimuth beam along track during the whole acquisition time. In this letter, a novel TOPS mode raw data generation approach from conventional wide-beam SAR imaging modes is proposed, by invoking the fact that the TOPS raw data can be considered as a part of the stripmap/spotlight raw data with the exploited wide azimuth beam interval. In this approach, an azimuth-varying filter is introduced to obtain the desired TOPS raw data and set others to zero. Moreover, the conventional stripmap and spotlight mode raw data can be efficiently generated in the 2-D Fourier transformed domain. Therefore, a fast raw data simulator can be easily achieved by taking this approach. Simulation results validate the proposed simulation approach. [ABSTRACT FROM PUBLISHER]

Narrow-band interference (NBI) suppression is an important technique in the low-frequency synthetic aperture radar (SAR). NBI sources either intentional or unintentional can mask the SAR signals and cause image degradation. According to the different characteristics of the spectra between the NBI and the SAR signals, a new method using subband spectral cancellation is applied to suppress the NBI. In contrast to the conventional suppression method performed on the raw data, this letter deals with the focused SAR image. By subtracting different range subband spectra of the SAR image, the NBIs are obtained and cancelled. The proposed method does not need to detect and estimate the parameters of the NBI and is easy to put in practice. Its performance is tested on the real data acquired by an experimental SAR system at P-band. [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]