Spaceborne Synthetic Aperture Radar Processing Research Articles

Development of China’s spaceborne SAR satellite, processing strategy, and application: take Gaofen-3 series as an example

ABSTRACT After mastering the key technologies of manufacturing spaceborne Synthetic Aperture Radar (SAR), China’s SAR satellites have been successfully launched into space. As the only civil microwave satellite listed in the “National High-resolution Earth Observation System Major Project,” the Gaofen-3 (GF-3) 01 satellite is the first C-band multi-polarization SAR satellite with a resolution of 1 m. GF-3 series satellites stand out among civil SAR satellites worldwide because of their high resolution, wide swath, high geometric radiation quality, multiple imaging modes, and long operation time. Taking GF-3 series satellites as an example, this study introduces the development of China’s civil SAR satellites, as well as their processing strategies and applications. The success of the GF-3 series satellites shows that China’s SAR remote sensing technology has stepped into a new era of high-quality and high-precision Earth observations.

An Efficient Full-Aperture Approach for Airborne Spotlight SAR Data Processing Based on Time-Domain Dealiasing

The two-step approach (TSA) is an efficient full-aperture method to solve the Doppler spectrum aliasing when processing data acquired by a synthetic aperture radar (SAR) system operating in beam steering mode. This simple approach solves the spectral aliasing by implementing an azimuth convolution between the raw data and a chirp signal, thus avoiding complex subaperture division and recombination of subaperture approaches. However, as aliasing reoccurs in the slow time-domain after the convolution, the estimation and compensation for residual motion error by autofocus cannot be performed immediately, which is crucial for airborne and high-resolution spaceborne SAR processing. The extended TSA solves this problem by implementing full-aperture azimuth scaling, but it is inefficient when the scaling factor deviates from unity severely, which is common in the data acquisition geometry of an airborne SAR. Aiming at addressing the existing shortcomings, this article combines the TSA with autofocus by the time-domain dealiasing and provides an efficient full-aperture processing framework of airborne spotlight SAR data. Simulated and experimental airborne spotlight SAR data with the transmission signal bandwidth of 1.2 GHz and a coherent integration angle of 15<inline-formula><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> are processed by the proposed algorithm and the clarity of the processed results shows its effectiveness.

An Extended Two Step Approach to High-Resolution Airborne and Spaceborne SAR Full-Aperture Processing

The processing of synthetic aperture radar (SAR) echoes collected during radar antenna beam steering, such as in the staring spotlight, sliding spotlight, and TOPS operating modes, requires additional efforts due to the limited pulse repetition frequency (PRF). Subaperture processing is mostly used in these cases, but the complexity arising from subaperture dividing and recombination is preferably avoided. The two-step approach (TSA) is an elegant solution to full-aperture processing. Nevertheless, for high-resolution airborne and spaceborne SAR processing, the TSA is still faced with a number of difficulties, which does not occur, however, in subaperture processing. For instance, the high range bandwidth induces Doppler spectrum aliasing during the spectral analysis (SPECAN) stage of the original TSA. Moreover, the Doppler spectrum obtained in TSA, when inverse Fourier transformed back to the slow time domain, might also suffer from aliasing, which precludes the estimation and correction of the unknown residual motion error or tropospheric disturbance. In this article, we extend the TSA to address these problems and present a full-aperture processing framework to precisely focus the high-resolution airborne and spaceborne SAR data. Simulation of X-band spaceborne SAR echoes with transmission signal bandwidth of 3.6 GHz and coherent integration angle of 12.5° has validated the extended TSA (ETSA). Meanwhile, experimental X-band airborne SAR data with the same range and azimuth resolution are also processed using the ETSA, where the conventional autofocusing techniques have been smoothly incorporated.

Analytical Approximation Model for Quadratic Phase Error Introduced by Orbit Determination Errors in Real-Time Spaceborne SAR Imaging

Research on real-time spaceborne synthetic aperture radar (SAR) imaging has emerged as satellite computation capability has increased and applications of SAR imaging products have expanded. The orbit determination data of a spaceborne SAR platform are essential for the SAR imaging procedure. In real-time SAR imaging, onboard orbit determination data cannot achieve a level of accuracy that is equivalent to the orbit ephemeris in ground-based SAR processing, which requires a long processing time using common ground-based SAR imaging procedures. It is important to study the influence of errors in onboard real-time orbit determination data on SAR image quality. Instead of the widely used numerical simulation method, an analytical approximation model of the quadratic phase error (QPE) introduced by orbit determination errors is proposed. The proposed model can provide approximation results at two granularities: approximations with a satellite’s true anomaly as the independent variable and approximations for all positions in the satellite’s entire orbit. The proposed analytical approximation model reduces simulation complexity, extent of calculations, and the processing time. In addition, the model reveals the core of the process by which errors are transferred to QPE calculations. A detailed comparison between the proposed method and a numerical simulation method proves the correctness and reliability of the analytical approximation model. With the help of this analytical approximation model, the technical parameter iteration procedure during the early-stage development of an onboard real-time SAR imaging mission will likely be accelerated.

Multifrequency Subpulse SAR: Exploiting Chirp Bandwidth for an Increased Coverage

Spatial resolution and swath width are fundamental quality parameters for spaceborne synthetic aperture radar (SAR) products. They have driven the research on new spaceborne SAR system concepts in the last decades, and a new generation of SAR systems is emerging. Main feature of these future systems is the use of multiple digital channels and new SAR processing techniques. A further characteristic is that they can resort to a large radar signal bandwidth. In this letter, a novel SAR operational mode is presented, denoted as multifrequency subpulse (MFSP). The MFSP exploits the available radar signal bandwidth to increase the imaged swath extension, without the emergence of range ambiguities. The proposed approach is explained both theoretically and by a design example, based on the future German X-band SAR system, HRWS. Moreover, the achievable SAR imaging quality is investigated.

High-Resolution Spaceborne SAR Processing Using the Decomposed Transfer Function

In this paper, a new analytic decomposed transfer function (DTF) for spaceborne synthetic aperture radar processing is calculated using the stationary phase approximation and Taylor's series expansion. The DTF copes with large Doppler centroid variations and executes range cell migration correction, secondary range compression, azimuth compression, and higher order effects. A fourth-order (DTF4) and fifth-order (DTF5) algorithms have been implemented using segmented block processing. It is shown that the DTF4 yields high quality 3-look target responses at azimuth resolution 0.3 m and range resolution 0.5 m at low squint (yaw = 0.3°) for X -band. At higher squint (yaw = 7.5°) the DTF5 has to be used to obtain adequate image quality.

Geometric Accuracy Evaluation of YG‐18 Satellite Imagery Based on RFM

AbstractYaoGan‐18 (YG‐18) is a Chinese high‐resolution synthetic aperture radar (SAR) satellite, which was launched on 29th October 2013. So far, however, there have been no published reports evaluating the geometric accuracy of its imagery; this is rectified in this paper. A refined rational function model (RFM) with an affine transformation is utilised for the spaceborne SAR processing. By setting corner reflectors as ground control points (GCPs) and independent check points, the orthorectification accuracy of YG‐18 satellite images was better than 2·5 m. Its stereo‐SAR measurement accuracy was subsequently evaluated using a high‐precision digital surface model (DSM), achieving an elevation accuracy of 11·36 m. The feasibility of the orthorectification method based on GCPs selected from corresponding image points of YG‐18 SAR images and ZiYuan‐3 (ZY‐3) optical images was also verified, offering single‐image accuracy better than 4·5 m.

Modifications on Multichannel Reconstruction Algorithm for SAR Processing Based on Periodic Nonuniform Sampling Theory and Nonuniform Fast Fourier Transform

The next generation of space-borne synthetic aperture radar (SAR) systems will emphasize on high-resolution and wide-swath imaging. For these design goals, multichannel technology in azimuth is a promising candidate and 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. In the multichannel SAR system, the nonuniform azimuth signal needs to be reconstructed when the pulse repetition frequency is different from a specific one. A reconstruction algorithm based on periodic nonuniform sampling theory has been proposed in current literature, but it is computationally rather expensive. In this paper, nonuniform fast Fourier transform is employed to reduce the computation load of the original algorithm from $O(N^2) $ to $O(N\log N) $ , where $N$ is azimuth sample number. Furthermore, a modification is implemented by taking the out-of-band energy into account to suppress the azimuth ambiguity when the signal is not band-limited, which is actually the case of the azimuth modulation of SAR with a real antenna pattern. The computation and reconstruction performances validate the proposed method’s effectiveness in multichannel signal processing and its suppression effect on azimuth ambiguity.

공간해상도에 따른 위성 영상레이더 위상간섭기법 긴밀도 분석

최근 고해상도 영상레이더를 탑재한 위성이 성공적으로 발사, 운용되고 있다. 이들 위성에서 획득된 자료를 이용한 위상간섭기법의 활용은 다양한 지구과학적 분야에서 보다 자세한 정보를 제공하고 있다. 위상간섭기법 적용에서 긴밀도는 영상레이더 자료로부터 생성된 위상간섭도 질을 평가하는 매우 중요한 요소이다. 본 연구에서는 미국 서부 텍사스에 위치한 엘파소 지역에 대한 고해상도 X-밴드 TerraSAR-X(TSX), L-밴드 ALOS PALSAR와 중해상도 C-밴드 Envisat ASAR 위성 영상레이더 자료의 긴밀도 특성을 분석 평가하고자 한다. 짧은 시간기선거리(temporal baseline) 조건에서 X-밴드 TSX 자료의 긴밀도는 0.3~0.6으로 L-밴드 ALOS PALSAR 자료와 유사한 정도의 높은 긴밀도를 나타내었다. 이 수치는 C-밴드 Envisat ASAR 자료에 비해서는 상당히 높은 것이며 영상레이더 신호의 파장이 길수록 위상간섭도의 긴밀도 유지에 있어 보다 유리하다는 일반적인 산란 이론을 고려해 볼 때 의미있는 결과라 할 수 있다. TSX 자료가 높은 긴밀도를 갖는 이유는 안정적인 산란 특성을 잘 반영할 수 있는 높은 공간 해상력이 하나의 원인일 것으로 추정된다. 하지만 11~33일 정도의 짧은 시간기선거리에서는 비교적 높은 긴밀도를 유지하는 반면에 시간기선거리가 다소 길어질 경우 긴밀도가 크게 저하된다. 본 연구 결과를 통해 긴밀도가 시간기선거리와 매우 밀접한 관계에 있음을 확인할 수 있었다. Recently high spatial resolution space-borne Synthetic Aperture Radar (SAR) systems have launched and have been operated successfully. Interferometric SAR (InSAR) processing with the space-based high resolution observations acquired by these systems can provide more detail information for various geodetic applications. Coherence is regarded as a critical parameter in the evaluating the quality of an InSAR pair. In this study, we evaluate the coherence characteristics of high-resolution data acquired by TerraSAR-X (X-band) and ALOS PALSAR (L-band) and intermediate-resolution data acquired by Envisat ASAR (C-band) over western Texas, U.S.A. Our coherence analysis reveals that the high-resolution X-band TSX (3.1 cm) data has a high coherence level (0.3-0.6), similar to that of the L-band ALOS PALSAR data (23.5 cm) in short temporal baselines. Further more, the TSX coherence values are significantly higher than those of the C-band (5.6 cm) Envisat ASAR data. The higher coherence of the TSX dataset is a surprising result, because common scattering theories suggest that the longer wavelength SAR data maintain better coherence. In vegetated areas the shorter wavelength radar pulse interacts mostly with upper sections of the vegetation and, hence, does not provide good correlation over time in InSAR pairs. Thus, we suggest that the higher coherence values of the TSX data reflect the data's high-resolution, in which stable and coherent scatters are better maintained. Although, however, the TSX data show a very good coherence with short temporal baseline (11-33 days), the coherences are significantly degraded as the temporal baselines are increased. This result confirms previous studies showing that the coherence has a strong dependency on the temporal baseline.

TerraSAR-X Antenna Calibration and Monitoring Based on a Precise Antenna Model

The high flexibility and tight accuracy requirements of modern spaceborne synthetic aperture radar (SAR) systems require innovative technologies to calibrate and process SAR images. To perform accurate pattern correction during SAR processing, an antenna model can be used to derive the multitude of different antenna beams generated by active antenna steering. The application of such an antenna model could be successfully demonstrated for the TerraSAR-X mission, launched in 2007. The methodology and the results of the in-orbit verification with an achieved accuracy of better than ±0.2 dB are reviewed in this paper in detail, showing its outstanding accuracy. Additionally, the results of the antenna pattern long-term monitoring are described, pointing out the high stability of the system.

TerraSAR-X SAR Processing and Products

The TerraSAR-X mission was launched in June 2007. After successful completion of the commissioning phase, the mission entered its operational phase in January 2008. Since that time, TerraSAR-X provides the scientific remote sensing community and commercial customers with high-quality spaceborne synthetic aperture radar (SAR) data products. The intention of this paper is to present the SAR data processing concept and the comprehensive portfolio of products reflecting the instrument's diverse imaging capabilities together with options of processing and achieved product quality as well as the essentials of SAR processing. Furthermore, it shall also provide details on how to fully exploit the precision of the TerraSAR-X products.

Accurate Antenna Pattern Modeling for Phased Array Antennas in SAR Applications—Demonstration on TerraSAR-X

The high flexibility and tight accuracy requirements of today's spaceborne synthetic aperture radar (SAR) systems require innovative technologies to calibrate and process the SAR images. To perform accurate pattern correction during SAR processing, an Antenna Model is used to derive the multitude of different antenna beams generated by active antenna steering. The application of such an Antenna Model could be successfully demonstrated for the TerraSAR-X mission, launched in 2007. The methodology and the results of the inorbit verification with an achieved accuracy of better than dB is reviewed in this paper in detail showing its outstanding accuracy.

Tight formation flying for an along-track SAR interferometer

While space-borne synthetic aperture radar (SAR) has evolved into a mature technology over the past two decades, there is a growing interest in interferometric SAR applications. Especially along-track interferometry with its capability to resolve the velocity of on-ground objects and ocean currents is of high interest for scientific applications. The accuracy of the resolved velocity on ground scales directly with the along-track separation between adjacent SAR antennas. Since space vehicles are quite limited in size, a formation flying approach with two SAR instruments distributed onto two spacecraft thus appears to be an innovative approach to along-track SAR interferometry. In the framework of an ESA study, this paper discusses the potential benefits, drawbacks and problems associated with a close formation flight for an along-track interferometry SAR mission. To this end, the absolute and relative orbit reconstruction requirements for the SAR processing chain are derived from basic interferometric principles as well as appropriate baselines of the satellite formation in L-Band and X-Band. A discussion of potential space-borne navigation sensors is presented along with the accuracy of state-of-the-art relative orbit reconstruction. Finally, appropriate thrusters for formation acquisition and control are discussed together with approaches to formation flying guidance and control as well as fuel consumption.

Ultra high resolution spaceborne SAR processing

The accuracy requirements of the azimuth signal for spaceborne synthetic aperture radar (SAR) processing is analysed. Degradations of simulated extended scenes and point targets are studied at high squint or if erroneous Doppler parameters are used. At a resolution less than 1 m for X-band it is shown that the azimuth signal for spaceborne SAR geometry should be used. It is found that the fourth-order extended exact transfer function (EETF4) is a good candidate for demanding SAR processing.

On-board medium resolution SAR processing for fast image generation

Spaceborne Synthetic Aperture Radar (SAR) instruments are becoming a very important source of information for Earth observation. These systems produce a great amount of data to be stored and extensively processed on ground. However, recent technological developments concerning microprocessors and solid state memories will enable the possibility of on-board SAR data processing. In this scenario, efficient algorithms for generating SAR images are demanded. This new situation requires specially designed algorithms for the processing stages because high-resolution approaches to SAR image generation are still too computationally expensive. We compare and analyse a very efficient SAR processing method for reduced resolution images called spectral analysis (SPECAN) and its variants. This work compares for the first time SPECAN techniques and their variants with other possible candidates. Overall processing evaluation is validated in a simulation environment using ERS-1 raw data. This study compares the computational cost of the algorithms. The conclusion obtained from this study is that SPECAN and especially SPECAN combined with Chirp-Z Transform is a very appropriate candidate for processing SAR data not only in azimuth but also in range. The results show that these new developments are very promising for obtaining fast and simple SAR image generation and they could be suitable candidates for potential on-board implementation.