Weiming Tian

Extended NLCS Algorithm of BiSAR Systems With a Squinted Transmitter and a Fixed Receiver: Theory and Experimental Confirmation

This paper proposes an extended nonlinear chirp scaling (CS) image formation algorithm for the bistatic synthetic aperture radar systems with the squinted transmitter and a fixed receiver. Since the transmitter with the squint mode was adopted in the system, two main problems, i.e., the spatial variance of the frequency-modulation rate and cubic phase terms, were introduced in the image formation algorithm. The former problem was solved by the linearity approximation of parameter p and deduced q (the second- and third-order coefficients of CS factors in range, which could be used to remove the spatial variation and high-order phase in the range direction) along the range domain while the latter one was compensated by a cubic analytical phase term in the frequency domain. A corresponding experimental hardware system and the bistatic experiments were also described in this paper. Both the simulation and experimental results validated the proposed algorithm.

Experimental Study of Ionospheric Impacts on Geosynchronous SAR Using GPS Signals

The L-band geosynchronous synthetic aperture radar (GEO SAR) is very susceptible to ionosphere as the significant increases of its integration time and wide swath, leading to image drifts and degradations. This paper demonstrates an experimental study of analyzing ionospheric impacts on GEO SAR, including both background ionosphere and ionospheric scintillation. The experiment consists of two parts. One is the global positioning system (GPS) data recording in which we employ GPS satellites to probe ionosphere and collect the transionosphere GPS signals. Then the recorded signals are used to create the data basis on which simulations are based. The other is the reconstruction of the signal distortions based on the GPS data. Then the two parts are combined to generate the ionosphere-impacted GEO SAR signals. But GEO SAR has very different orbit trajectories from GPS. Thus, in the real operation, the transformation of the temporal-spatial frame between GPS and GEO SAR should be first performed before the focusing and the evaluation are carried out. In cases of current GEO SAR configurations, the background ionosphere will induce image drifts but can be corrected through image registration techniques. The image is also likely to get defocused in azimuth when the second and higher derivatives of total electron content exceed thresholds which are dependent on GEO SAR configurations and the corresponding integration time. Comparatively, scintillations will mainly affect the focusing in azimuth, especially for integrated sidelobe ratios (ISLRs). But scintillations rarely occur over China mainland, and it is suggested to avoid the GEO SAR working during its occurrence.

Multiangle BSAR Imaging Based on BeiDou-2 Navigation Satellite System: Experiments and Preliminary Results

This paper analyzes the multiangle imaging results for bistatic synthetic aperture radar (BSAR) based on global navigation satellite systems (GNSS-BSAR). Due to the shortcoming of GNSS-BSAR images, a multiangle observation and data processing strategy based on BeiDou-2 navigation satellites was put forward to improve the quality of images and the value of system application. Twenty-six BSAR experiments were conducted and analyzed in different configurations. Furthermore, a region-based fusion algorithm using region-of-interest (ROI) segmentation was proposed to generate a high-quality fusion image. Based on the fusion image, typical targets such as water area, vegetation area, and artificial targets were compared and interpreted among single/multiple-angle images. The results reveal that the multiangle imaging method was a good technique to enhance image information, which might extend the applications of GNSS-BSAR.

A novel subsidence monitoring technique based on space-surface bistatic differential interferometry using GNSS as transmitters

Synthetic Aperture Radar Differential Interferometry (DInSAR) technique is an effective tool with large coverage and high spatial accuracy for subsidence monitoring. Nevertheless, the temporal resolution is usually poor so that rapid deformation cannot be measured due to the long revisit time of radar satellites. Bistatic SAR Differential Interferometry technique using Global Navigation Satellite System (GNSS) as illuminator has a shorter revisit time, whereas the measurement accuracy is constrained by low signal power, narrow bandwidth and atmospheric delay error. To cope with these problems, in this paper, we propose a novel subsidence monitoring technique based on Space-Surface bistatic Differential Interferometry (SS-DI) with GNSS transmitters, where two stations consist of a reference one and a measurement one that are deployed on the ground. First, we applied a space differential processing between two stations to cancel identical errors such as tropospheric and ionospheric errors etc. Then we used a long time coherent integration to improve the signal noise ratio. Subsequently, we also utilized a time differential processing to construct double differential equations with respect to unknown deformation variables. Finally, we solved the equations to obtain a highly accurate estimation of three dimension deformation. Furthermore, an SS-DI experiment using Beidou-2 as transmitters was carried out to validate the proposed method, where a high accuracy (0.01 mm) device was utilized to simulate subsidence deformation. The experimental results reveal that the proposed method has better performance of spatial measurement accuracy of 0.53 mm compared with differential GPS method.

Impacts of ionospheric scintillation on geosynchronous SAR focusing: preliminary experiments and analysis

创新点地球同步轨道合成孔径雷达工作在L波段, 轨道较高, 成像过程极易受到电离层闪烁的影响。本文考虑到上述问题, 设计了基于GPS系统实测电离层闪烁数据的GEO SAR点目标和面目标成像仿真实验, 并对方位向成像质量进行评估。实验结果表明电离层闪烁主要使GEO SAR成像目标的方位向积分旁瓣比严重下降, 同时使GEO SAR图像模糊并含有沿方位向分布的条纹, 但对成像目标方位向分辨率和峰值旁瓣比的影响较小。

Space-Surface Bistatic SAR Image Enhancement Based on Repeat-Pass Coherent Fusion With Beidou-2/Compass-2 as Illuminators

Low signal power density limits the performance of space-surface bistatic synthetic aperture radar (SS-BiSAR) using Global Navigation Satellite System (GNSS) satellites as illuminators. To tackle this problem, in this letter, a novel bistatic SAR image enhancement technique based on repeat-pass coherent fusion is proposed. The works in this letter include three aspects. First, repeat-pass experiments are designed to ensure the best resolution. Second, a modified CLEAN technique is applied to remove the direct signal interference from the focused BiSAR images. Third, a coherence-processing method is proposed to implement coherence of each repeat-pass BiSAR image and then they are coherently fused to obtain a quality-improved BiSAR image. Twenty-two days of repeat-pass BiSAR experiments with Beidou-2/Compass-2 inclined geosynchronous orbit satellites as illuminators have been designed and conducted. The data were processed by the proposed method. The results show that the method can obtain better image quality compared with the traditional noncoherent fusion method and the single-day imaging result, which validates the proposed method and proves the huge potential in realizing local area monitoring with SS-BiSAR using GNSS satellites as illuminators.

Experimental Results and Algorithm Analysis of DEM Generation Using Bistatic SAR Interferometry With Stationary Receiver

This paper presents the theory, algorithm, and results of a new bistatic interferometry synthetic aperture radar (InSAR) method. It employs the data acquired in an innovative bistatic configuration, which uses the orbital sensors as transmitters of opportunity and the stationary receivers on the ground, to generate a digital elevation model (DEM). In the bistatic spaceborne/stationary InSAR configuration, the interferometric phase only depends on the target-receiver range, which could not be obtained directly from the measured bistatic range. Therefore, the conventional transforming relationship between the interferometric phase and the topographic height is no longer practical. In order to solve the problem, we introduce a new conversion relationship between the interferometric phase and the topographic height, which is derived by the model of the ellipsoidal projection in the bistatic configuration. Meanwhile, the error analysis of the new conversion is carried out through a simulation. Both the simulated and measured data are used to test and verify the feasibility of the new bistatic InSAR method. In the spaceborne/stationary InSAR experiment, YaoGan-3 (an L-band spaceborne SAR system launched by China) was selected as the transmitter and two stationary receivers were mounted on the top of a tall building. The generated DEM of high quality shows that the presented method performs very well in the bistatic InSAR data process.

Space-surface BiSAR based on GNSS signal: Synchronization, imaging and experiment result

This paper presents a novel synchronization method for space-surface BiSAR (SS-BiSAR) illuminated by navigation satellites. Direct signal is utilized to obtain theoretical Doppler and navigation data. According to the navigation data and receiver position, theoretical Doppler history can be calculated. Comparing the tracking result and theoretical result, phase synchronization error could be estimated. After phase synchronization error is estimated and compensated, echo of SS-BiSAR is focused by bistatic back-projection algorithm. The proposed method has been verified by SS-BiSAR imaging experiment based on BeiDou signal.

Feasibility study of inclined geosynchronous SAR focusing using Beidou IGSO signals

创新点地球同步轨道合成孔径雷达(GEO SAR)的超长积累时间保证了其具有较高分辨率。但相反地, 多种非理想因素, 如弯曲轨道误差、大气传播效应和轨道摄动等, 也会不断积累, 导致成像质量下降, 甚至失效。本文给出了GEO SAR成像的验证实验。实验采用天地双基地SAR(SS-BISAR)配置。其中, 北斗IGSO卫星作为机会发射源;接收机由一个直达波天线和回波接收天线组成。实验分别对转发器和自然场景进行了成像处理, 结果良好, 验证了GEO SAR在几百到几千秒的积累时间下成像的可行性。

Bistatic SAR imaging processing and experiment results using BeiDou-2/Compass-2 as illuminator of opportunity and a fixed receiver

Global Navigation Satellite System (GNSS) provides opportunities for passive radar applications. This paper reports progress in space-surface bistatic synthetic aperture radar (SS-BiSAR) using Beidou-2 as transmitters of opportunity and a fixed ground-based receiver. Experiment is described and the obtained SAR image is presented and interpreted in detail. Results show that BiSAR using beidou-2 as illuminator is able to reflect different types of targets and SS-BiSAR using GNSS as illuminator of opportunity is potential to realize local area monitoring.