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This paper presents the theoretical and experimental performance evaluation of a novel and innovative concept to add a multichannel ground moving target indication (GMTI) capability to a synthetic aperture radar (SAR) reflector antenna with a standard single feed. This approach exploits the capability to achieve a phase center displacement by combining signals obtained from different electromagnetic excitation modes of the feedhorn. While, in the past, this so-called multimode concept has been mainly analyzed in simulation and by various antenna laboratory measurements, several flight experiments have been recently conducted to verify and demonstrate its GMTI performance. The theoretical and experimental analysis provides qualitative and quantitative insights into the effectiveness of multimode SAR-GMTI applications, such as the SAR image-based along-track interferometry (ATI) or post-Doppler GMTI approaches based on space-time adaptive processing. In addition, it introduces practical lessons learned, along with an example of the detection and parameter estimation for a large vessel using ATI. This paper finishes with an examination of the expected performance improvement when the transmitted modes are alternated between subsequent pulses. This toggling results in a time-multiplexed three-channel GMTI system with significantly enhanced estimation performance.