The earth rotation has two main effects on the distributed spaceborne SAR-GMTI. Firstly, the baseline must orient the same direction of radar's footprint on the rotating earth to mitigate the fluctuant terrain clutter and satisfy ATI, DPCA or STAP condition, which means that proper relationships must be maintained between satellites' orbit parameters. Secondly, Doppler is dependent on range, so yaw steering technique is required to mitigate the warping or extended clutter notch. Based on the two effects, a distributed spaceborne SAR system composed of 3 small satellites was designed to perform GMTI function. Performance simulation was made to scale the system capability through SCNR loss metric for both X and L bands.