Xichao Dong

Theoretical Analysis and Verification of Time Variation of Background Ionosphere on Geosynchronous SAR Imaging

Geosynchronous synthetic aperture radar (SAR) (GEO SAR) has the characteristic of long integration time; thus, the time-freezing model assumption of background ionosphere for traditional low Earth orbit (LEO) SAR no longer holds in GEO SAR. Furthermore, the background ionosphere variation within the integration time cannot be omitted either. In this letter, the variation of total electron content within integration time is analyzed and described in detail by using polynomial approximation, and a new GEO SAR signal model influenced by background ionosphere is also proposed. In view of this novel model, the analytical expression of image shift and defocusing phase error are derived in the first place. Then, a quantitative analysis for the image shift and image defocusing in the range and azimuth directions is conducted, and the performance bounds of time-varying parameters of background ionosphere effects on focusing are obtained. Finally, the U.S. Total Electron Content measured data are used to verify the theoretical results of background ionosphere effects on GEO SAR focusing.

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.

Background Ionosphere Effects on Geosynchronous SAR Focusing: Theoretical Analysis and Verification Based on the BeiDou Navigation Satellite System (BDS)

Due to the ultra-long aperture time and large coverage characteristics of geosynchronous synthetic aperture radar (GEO SAR), the ionosphere freezing model in the traditional low earth orbit synthetic aperture radar (LEO SAR) imaging is assumed to fail in the GEO SAR, thus it should take the effects of the temporal-spatial variation (TSV) background ionosphere on the GEO SAR imaging into account. Meanwhile, owing to the curved trajectory effects caused by the ultra-long synthetic aperture time of GEO SAR, and the complex geometric kinematic relations between the GEO SAR satellite and the Earths rotation, the LEO SAR ionospheric effect analysis method based on the traditional kinematic model will be no longer suited to GEO SAR. Moreover, as there are no GEO SAR satellites operating in orbit, how to obtain the real-time ionospheric TEC values on the signal propagation path of GEO SAR satellite has become a great challenge. In this paper, through the establishment of a curved trajectory GEO SAR signal model for long aperture time under the influence of TSV background ionosphere, it has first derived the two-dimensional (2-D) spectrum of GEO SAR signal in the context of background ionosphere, and probed into the GEO SAR 2-D image shift and image defocusing phenomena caused by the background ionosphere, as well as analyzed and summarized the boundary conditions of relevant effects. Concurrently, this paper proposed an approach to equivalently retrieve TEC values on the GEO SAR propagation path based on the BeiDou Navigation Satellite System (BDS) IGSO satellites and combined with the Klobuchar model, which was utilized to equivalently acquire the GEO SAR ionospheric TEC retrieved data. In addition, the retrieved data were used to perform the GEO SAR imaging simulation in the context of the background ionosphere, and verified the correctness of the proposed analysis method.

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.

Adaptive Secondary Range Compression Algorithm in Geosynchronous SAR

In geosynchronous synthetic aperture radar (GEO SAR), due to larger equivalent squint angle and extremely complicated geometrical relationship between satellite motion and earth rotation at equator, the Doppler parameter space-variance becomes further strained, i.e., its value gets larger and its direction becomes uncertain. Meanwhile, the large imaging area brings in additional difficulties to compensate the Doppler parameter. In addition, since the synthetic aperture time is up to hundred seconds, the assumptions of the linear trajectory model and the Fresnel approximation appear to be decreasingly effective in GEO SAR, and the range cell migration and two-dimensional coupling also become larger. In allusion to the problems mentioned above, this paper proposes an improved secondary range compression (SRC) algorithm. First, special issues of GEO SAR imaging are analyzed, such as the Doppler parameter space-variance, the error of the linear trajectory model and the Fresnel approximation. Then, the effects of Doppler parameter space-variance on GEO SAR imaging are analyzed. Finally, the core issue of GEO SAR imaging at equator, i.e., adaptive phase compensation, is discussed in detail. The direction of Doppler parameter space-variance is determined, and the effects of Doppler parameter space-variance are compensated by sub-block processing, the processing is operated along the direction of Doppler parameter space-variance at an interval of calculated scale. Simulations of point array targets and area targets at equator are performed, and the correctness of this algorithm is validated.

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.

Feasibility Study of C- and L-band SAR Time Series Data in Tracking Indonesian Plantation and Natural Forest Cover Changes

Tropical coverage by Envisat ASAR is sparse in space and time and has limited value for monitoring deforestation. The only available dual-polarized multitemporal dataset over Riau province, Indonesia (nine images in a single year), is used to distinguish and monitor tropical plantations and their dynamics and is compared with annual L-band PALSAR data and land cover maps derived from Landsat data. For the ASAR data, both VV and VH are important in discriminating different types of forest cover; whereas, at L-band, most of the relevant information is in the cross-polarized channel. The ASAR VV (but not the VH) backscatter from acacia plantations is strongly affected by whether the underlying soil is peat or nonpeat, which affects the separability of acacia from oil palm. Maximum likelihood classification of the C-band data gave overall accuracies of 86.2% and kappa coefficient of 0.78 by comparison with land cover maps derived from optical data. This was not improved by combining C- and L-band data. Classification of the C-band time series allows the rotation cycle of acacia plantations to be tracked. The available 4-year annual L-band time series shows potential for monitoring these dynamics, but the 1-year time spacing increases the risk of missing changes masked by the rapid growth of acacia.

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.