Abstract: A comprehensive and systematic study of the operational imaging modes for the Spaceborne/Airborne hybrid Bistatic SAR (SA-BiSAR) is made firstly, including imaging geometry, echo equation, scene width, Doppler history, and resolutions and so on. To overcome the shortcomings of the Taylor series expansion in deriving the Bistatic Point Target Reference Spectrum (BPTRS) in conventional BPTRS-resolving methods, an exact analytical BPTRS, which is derived from the one-to-one correspondence between time and azimuth frequency with no Taylor series expansions, is proposed, and the derived result is free of constraints. Finally, the validity of the proposed solution is verified by point targets simulations.