Determination of Key Geometric Parameters for Spaceborne Dual-Beam Along-Track Interferometric SAR under Asymmetric Geometry

SHEN Qingyuan; ZHANG Yangyang; LAI Tao; ZHU Yuting; XIE Zhifeng; WANG Xiaoqing

doi:10.11999/JEIT260870

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SHEN Qingyuan, ZHANG Yangyang, LAI Tao, ZHU Yuting, XIE Zhifeng, WANG Xiaoqing. Determination of Key Geometric Parameters for Spaceborne Dual-Beam Along-Track Interferometric SAR under Asymmetric Geometry[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT260870

Citation:

SHEN Qingyuan, ZHANG Yangyang, LAI Tao, ZHU Yuting, XIE Zhifeng, WANG Xiaoqing. Determination of Key Geometric Parameters for Spaceborne Dual-Beam Along-Track Interferometric SAR under Asymmetric Geometry[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT260870

Citation:

SHEN Qingyuan, ZHANG Yangyang, LAI Tao, ZHU Yuting, XIE Zhifeng, WANG Xiaoqing. Determination of Key Geometric Parameters for Spaceborne Dual-Beam Along-Track Interferometric SAR under Asymmetric Geometry[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT260870

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Determination of Key Geometric Parameters for Spaceborne Dual-Beam Along-Track Interferometric SAR under Asymmetric Geometry

doi: 10.11999/JEIT260870 cstr: 32379.14.JEIT260870

1.
School of Electronics and Communication Engineering, Sun Yat-sen University, Shenzhen 518107, China
2.
Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China

Funds: The National Key R&D Program of China (2023YFB3904905)

Received Date: 2026-06-29
Accepted Date: 2026-07-29
Rev Recd Date: 2026-07-22

Available Online: 2026-08-28

Abstract

Abstract

Objective Spaceborne Dual-Beam Along-Track Interferometric Synthetic Aperture Radar (DBATI-SAR) acquires fore- and aft-looking radial velocities to support two-dimensional ocean surface current retrieval. The stability of the inversion depends on the relative directions of the two Radar Line-Of-Sight (RLOS) projections on the target local tangent plane. Conventional flat-Earth geometry models generally assume symmetric squint angles or time offsets. However, orbital curvature, Earth curvature, Earth rotation, and local projection nonlinearity produce asymmetric spaceborne geometries. Therefore, symmetric squint angles do not necessarily guarantee orthogonal ground-projected RLOS directions. A method is developed to determine the fore- and aft-looking observation times, squint angles, and down-looking angles directly under the ground-projected RLOS orthogonality constraint. Methods A complete Earth-Centered Earth-Fixed (ECEF) geometry model is established from the satellite state and target position. The RLOS is projected onto the target local tangent plane, and a signed ground-projected RLOS angle is defined relative to the zero-squint reference direction. The squint and down-looking angles are calculated from the observation times rather than prescribed independently. A two-dimensional observation geometry matrix is constructed to relate the horizontal current components to the two radial velocities. Under an ideal symmetric geometry used for the analytical derivation, the singular values and condition number show that a one-sided ground-projected RLOS angle of $ {45}^{{^{\circ}}} $ provides the best inversion conditioning. A normalized geometric amplification factor is introduced to quantify the additional error amplification caused by nonorthogonal projections. Near the zero-squint reference point, a closed-form analytical leading term is derived to relate the observation-time offset to the ground-projected RLOS angle. This expression reveals the effects of the reference slant range, down-looking angle, equivalent along-track velocity, and second-order range-history curvature. A local polynomial inverse mapping is then constructed from complete ECEF forward-geometry samples. Target ground-projected RLOS angles of $ {-45}^{{^{\circ}}} $ and $ +{45}^{{^{\circ}}} $ are substituted into the inverse mapping to obtain the fore- and aft-looking observation times, after which the corresponding squint and down-looking angles are calculated. Results and Discussions Numerical experiments are conducted using 500 random orbit-parameter sets. Fifth- and sixth-order polynomials produce relatively large inverse-mapping errors, whereas a seventh-order model substantially improves the accuracy. With 20 samples, the mean ground-projected RLOS angle error is 0.0392°, and further increases in polynomial order or sample number provide limited improvement (Fig. 2, Table 2). The analytical leading term agrees well with the complete ECEF geometry model. For a representative case, the angular root-mean-square error and maximum deviation are approximately $ {0.11}^{\circ } $ and $ {0.31}^{\circ } $, respectively. Across the 500 cases, the root-mean-square errors of the fore- and aft-looking observation times predicted by the analytical leading term are 0.95 s and 0.57 s, respectively. These results indicate that the analytical leading term captures the dominant observation-time scale, while the numerical inverse mapping accounts for asymmetric higher-order effects (Fig. 3). The conventional flat-Earth geometry model produces a squint-angle correction of up to approximately $ {3}^{\circ } $ and a ground-projected RLOS orthogonality error of approximately $ {6.5}^{\circ } $. The proposed method reduces the mean ground-projected RLOS angle error to approximately $ {0.04}^{\circ } $ and decreases the normalized geometric amplification factor from approximately 1.12 to 1.000 7 (Fig. 4). The semimajor axis has the strongest effect on the aft-looking observation time, which varies from approximately 34 s to 76 s, while variations caused by other orbital parameters remain within 7 s (Fig. 5). Under combined squint- and down-looking-angle perturbations of up to 0.2°, most samples retain ground-projected RLOS angle errors below $ {1}^{\circ } $ (Fig. 6). Conclusions A method is proposed to determine key geometric parameters for spaceborne DBATI-SAR under asymmetric geometry. The analytical leading term explains the dominant observation-time scale, whereas the complete ECEF numerical inverse mapping accurately determines the fore- and aft-looking observation times and corresponding squint and down-looking angles. The proposed method provides substantially higher geometric accuracy than the conventional flat-Earth geometry approach. Practical mission design should also consider pulse repetition frequency, azimuth ambiguity, Doppler bandwidth, beam-steering range, and along-track interferometric coherence.

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References(25)

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