A Comprehensive Method of Ambiguity Suppression for Constellation of Geostationary and Low Earth Orbit SAR

ConGalSAR is a novel SAR constellation that has an outstanding performance of revisit and economy. In the system, with the MirrorSAR technology, one geostationary illuminator and several economical low earth orbit transponders work together to achieve a short revisiting interval. Because of the huge difference in the distances of transmitting and receiving, the spatial weighting of the antenna patterns on the ground is quite different, which leads to the deterioration of ambiguity to signal ratio. The multiphase center and digital beamforming antennas can suppress the ambiguity energy; however, they are too expensive and heavy to be equipped on the dozens of transponding nanosatellites. In this article, a comprehensive ambiguity suppression method, including the reflector antenna design and signal processing, is proposed to reduce the ambiguity ratio, which well solves the ambiguity problem under the condition of the low manufacturing costs and lightweight transponders. In terms of the hardware design, the multifeed reflector antenna is applied to the transponder, in which the feeds receive an echo together. Based on this, the low sidelobes beamforming is achieved to well suppress the ambiguity. Meanwhile, the amplitude-modulated chirps are also used here aiming at the residual ambiguity in the large dynamic scene. Through spectral selection and extrapolation processing, range ambiguity energy is suppressed more while preserving the resolution. The digital simulations show that the comprehensive method can realize -20 dB ambiguity ratios at least, which conforms to the ambiguity-to-signal ratio design of ConGaLSAR.