Search

Circular synthetic aperture radar (SAR) imaging on the geosynchronous orbit has many potential advantages, such as high-resolution three-dimensional (3D) imaging and continuous surveillance of broad area. The mission considered is an L-band geosynchronous circular SAR (GEOCSAR) with full aperture measurement. Since the integration time of GEOCSAR can be as long as 24 h, the orbit altitude is about 36 000 km and the coverage may reach 1/3 of the Earth's surface. The spatial and temporal variation of the ionosphere may impose significant effects on 3D imaging performance of GEOCSAR. The effects of ionospheric spatial variation including vertical and horizontal structure, the ionospheric turbulence and the ionospheric temporal variation are discussed. The analysis shows that the temporal variation of the ionospheric electron content during the synthetic aperture and the ionospheric turbulence may severely affect the GEOCSAR focusing performance on the XY plane. The vertical and horizontal structure of the ionosphere will cause image shift, and the dispersion effect is negligible.

Circular synthetic aperture radar (SAR) imaging on the geosynchronous orbit has significant potential advantages, such as high resolution three-dimensional imaging, large coverage, but it also has challenges to overcome. One major problem limiting the high-resolution geosynchronous circular SAR (GEOCSAR) performances is the atmospheric effects including the tropospheric and ionospheric effects. In this study, the authors mainly consider the tropospheric effects on L-band GEOCSAR imaging. Since the integration time of GEOCSAR may be as long as 24 h, and the coverage can reach 1/3 of the Earth's surface, the effects of the spatial and temporal variation of the tropospheric refractive index cannot be ignored if well focused images are to be obtained. The analysis shows that the temporal variation of the tropospheric refractive index may cause severe degradation of the SAR images; the spatial variation of the refractive index and the tropospheric turbulence are not the important factor limiting the GEOCSAR imaging performance.

Circular synthetic aperture radar (SAR) imaging on the geosynchronous orbit is an innovative SAR imaging mode, which can produce high-resolution three-dimensional (3D) images and offer 24-h continuous surveillance of a broad area. In this paper, we use the geosynchronous circular synthetic aperture radar (GEOCSAR) to efficiently produce high-resolution 3D surface displacement maps by repeat pass interferometry. The coverage areas with GEOCSAR imaging can be mapped from different view directions; thus, a 3D displacement can be provided daily. First, we derive the connection between the GEOCSAR interferometric phase and the surface deformation for analytically interpreting its capability to resolve the three components of the deformation. Then, to retrieve the three components of the deformation, a retrieval matrix is deduced. Furthermore, analyses of the errors in the observation phase vector and retrieval matrix are described, and the way these errors propagate into the deformation vector is modeled. To reduce the errors of the deformation measurement, the retrieval matrix is optimized by choosing appropriate parameters and making the condition number of the matrix minimal. Finally, the simulation results validate the correctness of the analysis, and the 3D retrieval deformation is presented with three subapertures and shows an improvement of accuracy when the optimal retrieval matrix is used.

The geosynchronous circular synthetic aperture radar (GEOCSAR) is an innovative SAR system, which can produce high resolution three-dimensional (3D) images and has the potential to provide 3D deformation measurement. With an orbit altitude of approximately 36 000 km, the orbit motion and orbit disturbance effects of GEOCSAR behave differently from those of the conventional spaceborne SAR. In this paper, we analyze the effects of orbit errors on GEOCSAR imaging and interferometric processing. First, we present the GEOCSAR imaging geometry and the orbit errors model based on perturbation analysis. Then, we give the GEOCSAR signal formulation based on imaging geometry, and analyze the effect of the orbit error on the output focused signal. By interferometric processing on the 3D reconstructed images, the relationship between satellite orbit errors and the interferometric phase is deduced. Simulations demonstrate the effects of orbit errors on the GEOCSAR images, interferograms, and the deformations. The conclusions are that the required relative accuracy of orbit estimation should be at centimeter level for GEOCSAR imaging at L-band, and that millimeter-scale accuracy is needed for GEOCSAR interferometric processing.

An interferometric performance analysis for repeat-pass geosynchronous circular SAR (GEOCSAR) is presented. The analysis mainly includes the errors caused by the following sources: radar thermal noise, spatial baseline decorrelation, images’ misregistration and the atmospheric effects. For circular SAR (CSAR) imaging on the geosynchronous orbit, the altitude is very high, the atmospheric effect is severe and the high sidelobe is existed in the focused GEOCSAR signal, the characteristics of the various error sources of GEOCSAR interferometry will behave differently from the conventional interferometric SAR.

The nadir track of geosynchronous satellite can be “figure-8”, circular or ellipse track which could be produced with appropriate orbit parameters. In this paper, the characteristics of the interferometric deformation detection for geosynchronous SAR with circular aperture are analyzed. The connection between the interferometric phase and the deformation of repeat pass geosynchronous SAR is derived, and its potential to get three-dimensional 3D deformation measurement is interpreted.