Motion and Doppler Characteristics Analysis Based on Circular Motion Model in Geosynchronous SAR

The Doppler characteristics of geosynchronous synthetic aperture radar (GEO SAR) cannot be explained by the conventional linear motion model (LMM), and the work modes of GEO SAR have not been clearly illustrated. To solve these problems, a circular motion model (CMM) is proposed based on the mathematical concept of curvature circle. GEO SAR is considered to run along the curvature circle whose center is not the earth center at most orbital positions. Then, the velocity of the zero-Doppler plane is derived based on the CMM to correctly express the Doppler parameters and analyze the Doppler characteristics of GEO SAR. Analyses results indicate that the circular motion of GEO SAR causes the sign-variant velocity of the zero-Doppler plane, which then induces the complex Doppler characteristics. Furthermore, GEO SAR is considered to work in two different modes according to the motion and Doppler characteristics. When the Doppler rate is zero or near zero, GEO SAR is similar to a partial aperture circular SAR (CSAR) and has a nonlinear time-frequency relationship, which causes the invalidity of the conventional frequency-domain imaging algorithms. When the Doppler rate is nonzero, GEO SAR is similar to the low-earth-orbit (LEO) SAR. Finally, the validity of this study is verified by computer simulations. The proposed CMM can explain the complex Doppler characteristics of GEO SAR. The derived expressions and the work mode analyses can provide further directions for the system design and imaging.