Ground moving target indicator (GMTI) using space-based sparse array is attractive.How to estimate the clutter rank simply and accurately is an essential issue in space time adaptive processing (STAP) Space-based sparse arrays usually appear as subarrays or arrays with non-isotropic elements.The 2D space time signal can be viewed as 1D space signal received from an equivalent synthetic space array according to space time equivalence theory.Then simple and accurate clutter rank estimating formulas are derived from the synthetic space array according to these two classes of sparse array.The simulations verify the accuracy of the formulas.Basic design constraints of the distributed small satellite based GMTI radar are derived from the clutter rank.