The Spaceborne ATI-GMTI system is made up of several satellites equipped with Synthetic Aperture Radar.The space between satellites must be big enough to acquire high sensitivity and avoid collision.However,the long baseline brings on blind velocity and worsens the GMTI performance.It is an important problem to determinate the length of the along track baseline for an ATI-GMTI system.Basing on splitting the whole aperture into multiple sub-apertures,this paper investigates the design method of optimum velocity response.Firstly,the output signal-to-clutter-plus-noise ratio expression of the system is derived,and then the way of designing optimum velocity response is presented.Finally,computer simulation results validate its effectiveness.