Acta Geodaetica et Cartographica Sinica

Key technologies of TH-2 satellite system

LOU Liangsheng, LIU Zhiming, ZHANG Hao, QIAN Fangming, ZHANG Xiaowei

2022, 51(12): 2403-2416. doi:10.11947/j.AGCS.2022.20210567

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The TH-2 satellite system is the first microwave surveying satellite system based on interferometric synthetic aperture radar (InSAR) technology and the first short-range formation satellite system in China. It is composed of two equal satellites, and the satellites formation in different orbits and the bistatic radar transceiver mode are adopted. By using satellite formations to form the baseline needed for interference, it can measure the global digital surface models by scale of 1∶50000 in a short time and acquire radar orthophotos at the same time. This paper gives a detailed introduction of the InSAR measurement principle and the technical system of TH-2 is also expounded carefully. To ensure the performance of system and the accuracy of product, several key techniques such as satellite formation, cooperative mode of two satellites, high-precision internal calibration, baseline determination, high-precision baseline measurement, high-precision baseline calibration, imaging of high phase fidelity and absolute ambiguity number calculation using dual-frequency need to be solved. These key technologies are analyzed in this paper, and the solutions are proposed. During the development of the TH-2 satellite system, simulation data and semi-physical simulation test were used to verify the feasibility of the main key technology solutions. After the satellites were launched, the on orbit test showed that the system was operating in good condition and the main performance indicators were better than the designed indicators, which further verified the feasibility of the key technology solutions and the correctness of these methods.

Algorithm to changing bistatic imaging geometric model for TH-2 satellite

CHEN Gang, QIAN Fangming, LIU Zhiming, LOU Liangsheng

2022, 51(12): 2417-2424. doi:10.11947/j.AGCS.2022.20210328

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Imaging geometric model of master and slave satellite directly affects many steps in InSAR data processsing, such as complex image rough registration, flat plain effect removing, InSAR location, baseline calibration, block adjustment and ortho-rectification. In order to keep unified algorithm in imageing, both master and slave satellite of TH-2 use bistatic imaging geometric model. For complex image rough registration, flat plain effect removing and InSAR location step, bistatic imaging geometric model just increase algorithm complexity, but for baseline calibration, it brings new chanlleges. On one hand, most current baseline calibration algorithms are based on monostatic imaging geometric model and can not be used in TH-2; on the other hand, pair position combines to form four baselines, and four baselines exist correlation in calibration, which leads to difficulty for baseline calibration. In order to keep accuracy of baseline calibration, the paper presents algorithm to chang bistatic geometric model into monostatic model and anlyses the transformation accuracy for slave satellite. It's proved that the algorithm has high accuracy and the brought error can be ignored by theoretical analysis and test.

Comparison and analysis of two baseline calibration models for TH-2 satellite

QIAN Fangming, CHEN Gang, LOU Liangsheng, LIU Wei, ZHANG Hao, MENG Xin

2022, 51(12): 2425-2432. doi:10.11947/j.AGCS.2022.20210316

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The two equivalent satellites in the TH-2 satellite system formed a flying-around formation in early July 2019, and began to obtain global radar interference data using the one-transmit and double-receiving system. The precise interference baseline is to realize the production of high-precision surveying and mapping products. This article gives the definition of the baseline in the antenna phase center (APC) coordinate system of the main radar. It introduces the single-scene data baseline calibration model based on the slave radar range modification equation and doppler equation under the one-transmit and double-receiving system, and a joint near-far beam positions calibration model of obtaining parallel and effective baseline errors by steps. According to the principle of baseline error intersection, a control points selection strategy is given, that is, the strategy of selecting control points in the near and far sub strips. The 17 times Xinjiang calibration field data acquired by TH-2 was used to carry out single-scene data and near-far beam positions joint baseline calibration experiments, and the ground positioning precision analysis of the two baseline calibration models after calibration was carried out. The experiments show that the average horizontal baseline error is -2.74mm, and the average vertical baseline error is -5.49mm calibrated by the single-scene data model. The average horizontal baseline error is -1.95mm, and the average vertical baseline error is -5.84mm calibrated by the near-far beam positions joint model. The ground positioning precision of the system after the near-far beam positions joint baseline calibration is higher than that of the single-scene data baseline calibration.

Index system study on distributed InSAR formation system

CHEN Li, ZHANG Dexin, CHEN Junli, SHAO Xiaowei

2022, 51(12): 2433-2439. doi:10.11947/j.AGCS.2022.20210375

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TH-2 is the first close-range dual-satellite formation distributed InSAR satellite system in China, which enables China to establish global digital surface model and radar orthophoto acquisition capability. Addressing the need for rapid access to global elevation data (1∶50000), this paper investigates the index system for satellite formation design, control and planning based on an integrated design concept in the context of the TH-2. This method establishes a demonstration model of formation technical index for three types of application requirements: image bypass overlap, interferometric imaging baseline and earth observation effectiveness, based on the full consideration of the coupling effect of technical index on system application performance. The proposed method integrates formation theory into engineering practice, which can achieve the goal of fast and efficient distributed SAR satellite formation system design and support the development of model equipment and ground operation and management system. The in-orbit results of TH-2 show that the proposed method is reasonable and feasible, and the performance indicators are accurately assigned.

An engineering optimization method for distributed spaceborne InSAR formation configuration based on multiple constraints

LI Nan, CONG Lin, CHEN Chonghua, ZHAO Di, HOU Yusheng, CHEN Xizhi

2022, 51(12): 2440-2447. doi:10.11947/j.AGCS.2022.20210381

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The formation configuration design of the distributed spaceborne interferometric SAR is the key issue affects the DEM system performance. Combined with the practical engineering of TH-2 satellite, an optimal method of InSAR formation determination based on multi-engineering constraints is proposed. The relative movement of formation is introduced firstly, and the mathematical description of the engineering bounds is modeled; then by using an improved ant colony algorithm,the parameters of formation are finally achieved with safety function maximization. Through the simulation, it is proved that the formation obtained can not only meets all the constraints, but also has the passive safety. Moreover, the correctness and validity of this method has already been verified in the on-orbit experiments.

Formation-maintaining control strategy for InSAR satellite

CHEN Chonghua, LI Nan, WAN Bei, DU Yaoke, WANG Wenyan, SUN Yingmei

2022, 51(12): 2448-2454. doi:10.11947/j.AGCS.2022.20210382

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The control accuracy of distributed interferometric synthetic aperture radar satellite formation configuration maintenance is one of the key factors affecting the baseline length and thus the elevation measurement accuracy. According to the description and analysis of the relative orbital elements of formation dynamics, an unbiased four-pulse formation maintenance control method with independent in-plane and out-of-plane control is optimized and designed based on the traditional pulse control scheme. In particular, the coupling influence between relative orbital roots in the control process is compensated by combining with engineering practice, also the corresponding control strategy is formulated. The method has been applied to TH-2 system, and the correctness of the method has been proved by ground simulation and in-orbit data, so as to effectively ensure the expected distribution of interference baseline and provide guarantee for the smooth development of land elevation survey task.

Time synchronization error analysis and evaluation of TH-2 satellite

HUANG Yan, ZHANG Xiaowei, CONG Lin, DONG Xiaohuan

2022, 51(12): 2455-2461. doi:10.11947/j.AGCS.2022.20220219

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The TH-2 satellite composed of two equal satellites,it adopts the technical system of satellite formation in different orbits and a bistatic radar transceiver mode.For this technical structure,time synchronization, phase synchronization and space synchronization must be established between two radars.The width of the mapping product,the image quality of the receiving radar,and the echo coherence are greatly affected by time synchronization error.In this paper,The mechanism of satellite time synchronization is described in detail, the synchronization error is analyzed and evaluated from the establishment and maintenance of time synchronization,and the effect of time synchronization error is analyzed systematically,The result of ground test and in-orbit test show that the analysis and evaluation conclusion is reliable.

High-precision estimation and verification for the centroid of satellite based on error correction

CHEN Ting, LI Wu, WU Yuanbo, LI Shizhong, SHAO Long

2022, 51(12): 2462-2469. doi:10.11947/j.AGCS.2022.20210388

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The centroid deviation of the receiving antenna is usually necessary to be modified during satellite precise orbit determination, and to correct the measurement data of the reference point to the centroid of the satellite. The deviation of the centroid in different directions has significant influence on orbit determination. Due to the large flat deployable antenna and the weight of the solar cell array as well as the ground test equipment, test methods, and the environmental factors, the centroid of the satellite under the state of on-orbit is difficult to be directly measured on the ground. At the same time, the centroid of satellite can be affected by other deployable components of satellite. All these factors directly increase the difficulties for predicting the centroid of the satellite. Aiming at the influence of the satellite centroid error on the baseline measuring accuracy of distributed SAR satellite, a satellite centroid prediction technology based on the existing centroid measurement technology to improve the estimation accuracy of satellite centroid is proposed in this paper. Based on the theory of error correction, the method of calculating the centroid of a satellite with second-order precision is realized by combining the calculation of the centroid with the measured data based on enough mass and centroid information of measurable parts. The errors of the centroid a on-orbit satellite are analyzed by this method, and the results show that the method achieves the expected results after the on-orbit test calibration.

Key technologies for on-orbit azimuth antenna pattern measurement of TH-2 dual satellites

GAO Jingkun, WANG Zhilong, CHENG Jiasheng, CONG Lin, FAN Weikang, HU Zhenlong

2022, 51(12): 2470-2480. doi:10.11947/j.AGCS.2022.20210373

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On-orbit (inflight) measurement of the antenna pattern of space-borne synthetic aperture radar (SAR) is an indispensable and important procedure for SAR calibration, performance tuning and index test. Usually, in practical on-orbit (inflight) measurement of the SAR antenna pattern, it is separated into the vertical (elevation) and the horizontal (azimuth) directions respectively. The TH-2 satellite set is made up of two satellites to enable bistatic interferometry SAR (InSAR) observation. At the beginning of the on-orbit tests, the two satellites adopted a “following” flying formation in which one satellite flew about 40km away after the other one. In this phase, the two satellites worked independently in SAR observation model to facilitate calibration, tuning and test of the satellites individually. When conducting the azimuth antenna pattern measurement, the collected signals from the two satellites in the ground receiver were overlapped (interleaved) and superposed, leading to the difficulty of measurement with the method designed for only one satellite. Aiming at solving this problem, an efficient and fully automated signal separation method for azimuth antenna pattern measurement is proposed. Based on the modeling and analysis of the sampled signal in the ground receiver, a novel signal separation method based on three main parts: key pulse rising edge extraction, pulse position estimation and pulse amplitude estimation is proposed to realize the separation of the signals from different satellites. After signal separation, the subsequent antenna pattern measurement tasks can be done using the traditional single-satellite method. Experimental results sufficiently show the effectiveness of the proposed method. This method escorted the on-orbit tests of the satellites smoothly, and the test efficiency can be doubled compared with the individual test of each single satellite. The proposed method can be used in the on-orbit tests of similar satellites in the future.

A brief analysis of the positioning accuracy for the TH-2 satellite system

LI Shizhong, YE Yu, FAN Weikang, CONG Lin, GAO Jingkun, SHAO Long

2022, 51(12): 2481-2492. doi:10.11947/j.AGCS.2022.20210397

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The TH-2 satellite system is China's first microwave surveying satellite system based on distributed interferometric technology. In this paper, the positioning accuracy of the satellite system is studied. According to the InSAR mechanism model, the error sources that affect the positioning accuracy are analyzed, and the InSAR data processing flow is designed. The satellite positioning accuracies in plain and mountain area are tested and verified by using the baseline measurement accuracy and ground processing accuracy of ground design and on-orbit test. The tests show that the plane and elevation accuracy of the satellite system after processing the measured data on orbit is better than that of the original simulation analysis at the baseline length of 700~1050m in formation configuration, it can satisfy the surveying and mapping precision of 1∶50000 scale topographic map in plain and mountain area for China.

Bistatic InSAR interferometry imaging and DSM generation for TH-2

XIANG Jianbing, Lü Xiaolei, FU Xikai, XUE Feiyang, YUN Ye, YE Yu, HE Ke

2022, 51(12): 2493-2500. doi:10.11947/j.AGCS.2022.20210323

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TH-2 is a bistatic synthetic aperture radar (SAR) satellite system in formation flight. Compared with traditional InSAR systems, it can eliminate decoherent sources such as time and atmosphere, besides, it can generate highly coherent SAR image pairs. This paper firstly describe the extended chirp scaling (ECS) imaging algorithm based on the hyperbolic equivalent method, and also introduces pre-filtering to deal with problems such as reduced coherence and interference phase errors caused by mixed baselines. Secondly, it introduces the interference processing method and the technical process of DSM reconstruction in the bistatic mode. Finally, an interference imaging experiment is performed using the original echo data of a certain mountainous experimental area, and the 3D reconstruction experiment is performed by using the generated SAR image pair, which analyzes the coherence of the image, the phase unwrapping results and the DSM reconstruction results. The experimental results verify that the interference imaging algorithm in this paper has good focusing effect and phase preservation capacity. At the same time, the interferometry and 3D reconstruction capabilities of the data are verified as well.

Target location performance evaluation of single SAR image of TH-2 satellite system

WANG Sai, MENG Xin, LOU Liangsheng, CHEN Gang, QIAN Fangming, LIU Zhiming

2022, 51(12): 2501-2507. doi:10.11947/j.AGCS.2022.20210329

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In order to evaluate the target location performance of a single SAR image of TH-2 satellites, this paper uses the theory and method of target location using the Range-Doppler model, Earth's ellipsoidal model and digital elevation model in the positioning experiments, and analyzes the experimental results from two aspects of predicting image point coordinate and positioning the ground target. Using three TH-2 SAR images in different regions to conduct experiment, it is concluded that the predicting image point accuracy is about 1 pixel and the ground target positioning accuracy is less than 2m. The results show that the TH-2 satellite system has reliable system parameters and high precision positioning capability, which can provide reliable data for the ground target locating, the radar orthophoto image generation and other tasks.

SAR image quality assessment of satellite TH-2-01

WANG Wei, LI Chunyan, SUN Tao, YANG Jianfeng, YU Xiaqiong, WU Yukuan

2022, 51(12): 2508-2516. doi:10.11947/j.AGCS.2022.20210420

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The satellite TH-2-01 was successfully launched on April 30, 2019, equipped with the high-resolution synthetic aperture radar(SAR) and the high-precision inter-satellite relative state measurement equipment and other payloads, which was able to obtain all-day and all-weather InSAR image data and auxiliary measurement data for scientific experimental research, land and resources census, geographic information mapping and other fields. In order to measure the imaging capability of the satellite, we comprehensively evaluated and analyzed the satellite A and satellite B of TH-2-01 by employing objective quality evaluation indexes based on point target and area target. The analysis results prove that the imaging quality of both satellite A and satellite B are able to meet the requirements of the satellite design index, which means that the images of satellite TH-2-01 can meet high-accuracy in the application of target area selection and analysis, target identification, target feature extraction, etc. Most of the objective indexes of satellite A and satellite B are similar, and the spatial resolution in the range direction is better than that in the azimuth direction, and the values of the range direction integral side lobe ratio and the peak side lobe ratio are better than that in the azimuth direction, that is, the more concentrated the energy of the point targets is in the range direction, the better ability to detect the dark target and the weak target from the radar image. The experimental results show, the images of satellite A has lower peak side lobe ratio in the range direction, that is, satellite A is slightly better than satellite B in range direction imaging quality.

Layover and shadow regions detection based on superpixel segmentation and multi-information fusion

LIU Peng, LI Zhenfang, LOU Liangsheng, YANG Weiming, WANG Zhen

2022, 51(12): 2517-2530. doi:10.11947/j.AGCS.2022.20220111

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In InSAR signal processing, the continuity of InSAR phase maps is seriously destroyed by layover and shadow regions which results in errors in elevation inversion. In this paper, a multi-information fusion-based superpiexl detection algorithm is proposed to make up for the shortcomings of traditional pixel-by-pixel detection methods based on amplitude and coherence coefficient threshold detection, constant false alarm(CFAR) detection and local frequency detection. Due to the lack of digital elevation model of scene, the accuracy verification of the proposed algorithm in practical applications become impossible. Thus, a frequency domain echo simulation technique is proposed. The proposed algorithm is verified based on an advanced integral equation approximation model(AIEM) for simulating SAR echo data. Simulation data and TH-2 data experiments show that the algorithm can distinguish more than 98% of layover and shadow regions in SAR images. Finally, the DEM products precision and the operation efficiency of phase unwrapping are improved significantly by masking layover and shadow regions during the phase unwrapping process.

Correg-YOLOv3: a method for dense buildings detection in high-resolution remote sensing images

CHEN Zhanlong, LI Shuangjiang, XU Yongyang, XU Daozhu, MA Chao, ZHAO Junli

2022, 51(12): 2531-2540. doi:10.11947/j.AGCS.2022.20210177

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The exploration of building detection plays an important role in urban planning, smart city and military. Aiming at the problem of high overlapping ratio of detection frames for dense building detection in high-resolution remote sensing images, we present an effective YOLOv3 framework, corner regression-based YOLOv3 (Correg-YOLOv3), to localize dense building accurately. This improved YOLOv3 algorithm establishes a vertex regression mechanism and an additional loss item about building vertex offsets relative to the center point of bounding box. By extending output dimensions, the trained model is able to output the rectangular bounding boxes and the building vertices meanwhile. Finally, we evaluate the performance of the Correg-YOLOv3 on our self-produced data set and provide a comparative analysis qualitatively and quantitatively. The experimental results achieve high performance in precision (96.45%), recall rate (95.75%), F₁ score (96.10%) and average precision (98.05%), which were 2.73%, 5.4%, 4.1% and 4.73% higher than that of YOLOv3. Therefore, the proposed algorithm effectively tackles the problem of dense building detection in high-resolution images.

Calibration of multi-head panoramic camera using 2D checkerboard under rotation photography

HUANG Mingyi, WU Jun, PENG Zhiyong

2022, 51(12): 2541-2556. doi:10.11947/j.AGCS.2022.20210337

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Multi-head panoramic camera (MPC) with ultra-wide field of view and high resolution is widely used, but its calibration depends on high-precision 3D control information. In this paper, a MPC calibration method is proposed using one common 2D checkerboard and rotatable platform. Proposed method is distinctly divided into two stages: firstly, side camera calibration using rotation sequence images. With elaborate consideration of setup of coordinate reference frame of checkerboard and rotatable platform, one strict rotating photography equation is first established for the side-camera of MPC. Based on it, initial external parameter values of the image sequence outputed from any side camera of MPC in rotatable platform are given, and further optimized by taking the corner of the chessboard image and a small number of tie points between the adjacent views of side camera as the observation values. Secondly, center camera calibration based on multi-view geometry. The MPC space coordinate system is first established by moving the origin of the rotatable platform coordinate system to the geometric centroid of center of all side cameras. Based on it, the solid multi-view geometry relationship between the center camera and the side camera are used as the constrain as well as a large number of tie points obtained by rotating photography are used as the observation values to optimize the parameters of the center camera through bundle adjustment. The experimental results show that the proposed method can achieve high-precision calibration of MPC, only using one 2D checkerboard and a rotatable platform. It has a good application value because of its high-precision, high-degree of automation and low requirements for environment (independence on 3D control field).

Research on local feature descriptors of multi-source remote sensing images based on structure information

FU Zhitao

2022, 51(12): 2557-2557. doi:10.11947/j.AGCS.2022.20210021

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Research on measurement properties and application of RSSI for indoor localization

XUE Weixing

2022, 51(12): 2558-2558. doi:10.11947/j.AGCS.2022.20210114

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Investigation of the spectral characteristics and the inversion of Spartina alterniflora chlorophyll content under interspecies competition

LIU Pudong

2022, 51(12): 2559-2559. doi:10.11947/j.AGCS.2022.20210142

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Research on water storage change and hydrological loading deformation in the middle and upper reaches of the Yellow River

XIE Xiaowei

2022, 51(12): 2560-2560. doi:10.11947/j.AGCS.2022.20210178

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