Free space optical communication usually experience channel fading caused by turbulence in the atmosphere. The effect of turbulence in both time and space on the laser beam propagation results in beam broadening, beam wandering, beam jitter and wave front distortions. All these effects ultimately affect the acquisition time required to acquire and locate the space borne target using narrow laser beam. In this paper, performance of FSO system in weak atmospheric turbulence has been analyzed in terms of acquisition time for non-coherent and coherent modulation schemes. It has been observed that there is increase in the acquisition time with the increase in the turbulence level for both coherent and non-coherent modulation schemes. The BPSK gives lowest acquisition time than the other schemes. Amongst non-coherent modulation schemes A/-PPM gives better performance than OOK i.e., with the increase in the level of M, acquisition time decreases. This occurs at the cost of increase in the system complexity.