Yuanhao Li

Performance Analysis of L-Band Geosynchronous SAR Imaging in the Presence of Ionospheric Scintillation

An L-band geosynchronous synthetic aperture radar (GEO SAR) will be inevitably affected by ionosphere scintillation because of its low carrier frequency. Meanwhile, compared with the low Earth orbit (LEO) SAR, a higher orbit of GEO SAR makes it have a longer integration time and a longer operation time within the susceptible regions of ionospheric scintillation. Thus, its imaging is more sensitive to ionospheric scintillation, and the corresponding degradation will have a different pattern. However, few works are focused on the quantitative analysis of the ionospheric scintillation impacts on L-band SAR. Moreover, the parameters of ionospheric irregularities utilized in the analyses are hard to be determined. In this paper, we first deduced the azimuth point-spread function with the consideration of both the amplitude and phase scintillation. Then, based on the measurable statistical parameters of ionospheric scintillation, performance specifications, including azimuth resolution, azimuth peak-to-sidelobe ratio (PSLR), and azimuth integrated sidelobe ratio (ISLR) are obtained to fully evaluate the impacts. The analysis suggests that in GEO SAR imaging, the azimuth ISLR severely deteriorates, whereas degradations of the azimuth resolution and PSLR are negligible. Finally, the simulations and a real ionospheric scintillation monitoring experiment by employing Global Positioning System satellites receivers were conducted, verifying the conclusions that the serious degraded contrast and focus quality of the images are brought by the raised azimuth ISLR.

Optimal Data Acquisition and Height Retrieval in Repeat-Track Geosynchronous SAR Interferometry

Geosynchronous synthetic aperture radar (GEO SAR) will move in a high orbit of ~36,000 km with a long integration time of hundreds of seconds. It is obviously impacted by orbital perturbations and the Earth’s rotation, which can give rise to un-parallel repeated tracks and induce a squint-looking angle in the repeat-track SAR interferometry (InSAR). Thus, the traditional data acquisition method using in the zero-Doppler centroid (ZDC) configuration to generate the GEO InSAR pair will bring about the obvious rotation-induced decorrelation. Moreover, the conventional height retrieval model with the broadside mode imaging geometry and the approximate expression of the interferometric baseline will induce large height and localization errors in the GEO InSAR processing. In this paper, a novel data acquisition method is firstly presented based on a criterion of optimal minimal rotational-induced decorrelation (OMRD). It can significantly improve the coherence of the InSAR pair. Then, considering the localization equations in the squint-looking mode and the accurate expression of the interferometric baseline, a modified GEO InSAR height retrieval model is proposed to mitigate the height and localization errors induced by the conventional model. Finally, computer simulations are carried out for the verification of the proposed methods. In a typical inclined GEO InSAR configuration, the averaged total correlation coefficient increases more than 0.4, and height errors of hundreds of meters and localization errors of more than 10 degrees are removed.

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.

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。

Impacts of Temporal-Spatial Variant Background Ionosphere on Repeat-Track GEO D-InSAR System

An L band geosynchronous synthetic aperture radar (GEO SAR) differential interferometry system (D-InSAR) will be obviously impacted by the background ionosphere, which will give rise to relative image shifts and decorrelations of the SAR interferometry (InSAR) pair, and induce the interferometric phase screen errors in interferograms. However, the background ionosphere varies within the long integration time (hundreds to thousands of seconds) and the extensive imaging scene (1000 km levels) of GEO SAR. As a result, the conventional temporal-spatial invariant background ionosphere model (i.e., frozen model) used in Low Earth Orbit (LEO) SAR is no longer valid. To address the issue, we firstly construct a temporal-spatial background ionosphere variation model, and then theoretically analyze its impacts, including relative image shifts and the decorrelation of the GEO InSAR pair, and the interferometric phase screen errors, on the repeat-track GEO D-InSAR processing. The related impacts highly depend on the background ionosphere parameters (constant total electron content (TEC) component, and the temporal first-order and the temporal second-order derivatives of TEC with respect to the azimuth time), signal bandwidth, and integration time. Finally, the background ionosphere data at Isla Guadalupe Island (29.02°N, 118.27°W) on 7–8 October 2013 is employed for validating the aforementioned analysis. Under the selected background ionosphere dataset, the temporal-spatial background ionosphere variation can give rise to a relative azimuth shift of dozens of meters at most, and even the complete decorrelation in the InSAR pair. Moreover, the produced interferometric phase screen error corresponds to a deformation measurement error of more than 0.2 m at most, even in a not severely impacted area.

Optimal 3D deformation measuring in inclined geosynchronous orbit SAR differential interferometry

Three dimensional (3D) deformation can be obtained by using differential interferometric synthetic aperture radar (D-InSAR) technique with the cross-heading tracks data of low earth orbit (LEO) SAR. However, this method has drawbacks of the low temporal sampling rate and the limited area and accuracy for 3D defor- mation retrieval. To address the aforementioned problems, by virtue of a geosynchronous (GEO) SAR platform, this paper firstly demonstrates the expressions of 3D deformation and the corresponding errors in GEO SAR multi-angle processing. An optimal multi-angle data selection method based on minimizing position dilution of precision (PDOP) is proposed to obtain a good 3D deformation retrieval accuracy. Moreover, neural network is utilized for analyzing the accuracy of the retrieved 3D deformation under different orbit configurations and geo-locations. Finally, the proposed methods and the theoretical analysis are verified by simulation experiments. A 3D deformation retrieval accuracy of the order of centimeter-level or even millimeter-level can be obtained by using the selected optimal multi-angle data.

Avoiding the Ionospheric Scintillation Interference on Geosynchronous SAR by Orbit Optimization

L-band geosynchronous synthetic aperture radar (GEO SAR) images will most likely deteriorate in the presence of ionosphere scintillation interference due to the low carrier frequency of GEO SAR. Meanwhile, because of the high orbit and the long working time above the region with the active ionosphere, GEO SAR will experience ionospheric scintillation with a higher probability. To make the GEO SAR avoid being interfered by ionospheric scintillation, we propose an orbit-optimization strategy by utilizing the diurnal and geographical pattern of the ionospheric scintillation occurrence in this letter. As the equatorial region is likely to experience ionospheric scintillation during the specified time window from the early evening after sunset to midnight, the orbit can be optimized by tuning the GEO SAR orbit parameters (e.g., a proper time past perigee) to avoid imaging over the equatorial region during the specified time window. Finally, simulation is conducted to verify the effectiveness of the method under the proposed three types of GEO SAR orbits, and the corresponding effective sets of time past perigee are obtained.

A Sparse SAR Imaging Method Based on Multiple Measurement Vectors Model

In recent decades, compressive sensing (CS) is a popular theory for studying the inverse problem, and has been widely used in synthetic aperture radar (SAR) image processing. However, the computation complexity of CS-based methods limits its wide applications in SAR imaging. In this paper, we propose a novel sparse SAR imaging method using the Multiple Measurement Vectors model to reduce the computation cost and enhance the imaging result. Based on using the structure information and the matched filter processing, the new CS-SAR imaging method can be applied to high-quality and high-resolution imaging under sub-Nyquist rate sampling with the advantages of saving the computational cost substantially both in time and memory. The results of simulations and real SAR data experiments suggest that the proposed method can realize SAR imaging effectively and efficiently.

Three-Dimensional Deformation Retrieval in Geosynchronous SAR by Multiple-Aperture Interferometry Processing: Theory and Performance Analysis

The 3-D deformation retrieval is significant for the accurate evaluation of geologic disasters (e.g., earthquakes and landslides). Multiple-aperture interferometry (MAI) is an effective method to obtain 3-D deformation, combined with the cross-heading tracks synthetic aperture radar (SAR) data. However, because of the limitations of the low earth orbit SAR, a long satellite revisit time, small common areas of the cross-heading tracks data, and the unsatisfied along-track deformation measurement accuracy usually exist in the traditional MAI 3-D deformation retrieval. Geosynchronous SAR (GEO SAR) runs in the geosynchronous orbit, which has the advantages of a large observation area and a short revisit time. This paper focuses on 3-D deformation retrieval by GEO SAR MAI processing. Aiming at the high orbit and the squint looking of GEO SAR, the accurate expressions of the along-track deformation, 3-D deformation, and the errors in GEO SAR MAI processing are given. The distortions and their correction in the MAI interferogram brought by the geometrical difference between the forward- and backward-looking interferograms and the multicycles flat-earth and topographic phases are given. Moreover, an optimal subaperture selection method based on minimum position dilution of precision is proposed. Finally, the effectiveness of the proposed method is validated by simulations and the experiment of BeiDou-2 inclined geosynchronous orbit navigation satellite. The theoretical analysis and the experimental results suggest centimeter-level and even millimeter-level deformation measurement accuracy could be obtained in 3-D by GEO SAR MAI processing.

Numerical Analysis of Orbital Perturbation Effects on Inclined Geosynchronous SAR.

The geosynchronous synthetic aperture radar (GEO SAR) is susceptible to orbit perturbations, leading to orbit drifts and variations. The influences behave very differently from those in low Earth orbit (LEO) SAR. In this paper, the impacts of perturbations on GEO SAR orbital elements are modelled based on the perturbed dynamic equations, and then, the focusing is analyzed theoretically and numerically by using the Systems Tool Kit (STK) software. The accurate GEO SAR slant range histories can be calculated according to the perturbed orbit positions in STK. The perturbed slant range errors are mainly the first and second derivatives, leading to image drifts and defocusing. Simulations of the point target imaging are performed to validate the aforementioned analysis. In the GEO SAR with an inclination of 53° and an argument of perigee of 90°, the Doppler parameters and the integration time are different and dependent on the geometry configurations. Thus, the influences are varying at different orbit positions: at the equator, the first-order phase errors should be mainly considered; at the perigee and apogee, the second-order phase errors should be mainly considered; at other positions, first-order and second-order exist simultaneously.